Method and apparatus for rapid NMR imaging of nuclear densities within an object

Densities of resonant nuclei within elemental volumes along a line are measured using the nuclear magnetic resonance phenomenon called "spin echo". A first planar volume of nuclei is selectively excited to nutate spins by approximately 90.degree.. Thereafter a second planar volume of nuclei, transverse to the first planar volume, is selectively excited to nutate spins by approximately 180.degree.. The nuclei in the line volume common to both of the planar volumes thereafter generate characteristic spin echo signals. A magnetic gradient is established along this line volume during the spin echo read out so that the resultant spin echo signals can be processed to determine the respective densities of resonant nuclei along the line volume. Appropriate phasing of the excitations enables interference with the spin echo signals by the free induction decay to be eliminated. To enable rapid development, successive line volumes are read out which do not lie in previously excited planes.

FIELD OF THE INVENTION 
The present invention relates to apparatus and method for producing a 
cross-sectional image of relative nuclear densities inside an object. More 
particularly, it involves the use of nuclear magnetic resonance techniques 
for in vivo mapping of lines of resonant nuclei densities in a human or 
other animal. 
BACKGROUND OF THE INVENTION 
Presently used techniques for non-invasive examination of a body use X-ray 
(e.g., computerized tomography) and ultrasound procedures. Another 
non-invasive procedure uses nuclear magnetic resonance (NMR) to obtain a 
cross-sectional image of the nuclei densities within a body. The average 
atomic number (Z) of nuclei in tumors tends to be significantly different 
from that of normal tissue. Hydrogen nuclear densities detected by NMR 
techniques are presently considered a strong indicator of changes in the 
average atomic number Z in tissues since hydrogen is the most abundant odd 
mass numbered nucleus present in living tissues. NMR is ideally suited for 
mapping relative hydrogen nuclear densities within living tissues. 
In general, the principles of NMR are well known. All nuclei with an odd 
number of protons or neutrons behave, in effect, like small magnets. When 
placed in a steady external magnetic field, the magnetic axes of such 
nuclei (and hence of the atoms) precess at an angle about the imposed 
field axis at the so-called Larmor frequency. The Larmor frequency 
(f.sub.o) is related to the magnetic field (B.sub.0) at the nucleus by the 
equation f.sub.o =.gamma.B.sub.0 where .gamma. is a constant, the 
magnetogyric ratio characteristic of a particular type of nuclei. 
Where a magnetic field gradient exists through an object, or if 
non-homogeneities exist in the imposed magnetic field, nuclei having the 
same magnetogyric ratio .gamma. will have different Larmor frequencies in 
accordance with their positions within the object. A volume of nuclei in 
the object can thus be conceptualized as having a range of precession 
frequencies centered about a given Larmor frequency. 
It is convenient to view the nuclear process about to be described from a 
rotating frame of reference which rotates at the Larmor frequency such 
that a nuclear magnetic moment precessing at exactly the Larmor frequency 
appears to be substantially stationary. In this frame of reference, the 
macroscopic magnetization M is aligned with the direction of the imposed 
magnetic field B.sub.0. This is illustrated in FIG. 1A. 
As is well known, the direction of net angular momentum or "spin" of a 
group of nuclei (and thus their net magnetic axis) can be reoriented with 
respect to the external magnetic field by electromagnetic signals having a 
frequency equal to the Larmor frequency. The electromagnetic signal 
produces a stationary magnetic field in the rotating frame of reference to 
nutate (reorient) the net spin of resonant (Larmor frequency) nuclei by an 
amount in accordance with its amplitude and duration. The direction of 
nutation is a function of the phase of the electromagnetic signal with 
respect to the nuclear precession about the imposed magnetic field. Thus, 
as illustrated in FIG. 1B, magnetic moment M has been nutated away from 
the Z axis. 
Over a period of time, after removal of the electromagnetic signal, the 
many magnetic moments .mu. will realign parallel to external field 
B.sub.0. As nuclear realignment occurs, the relative phases of the 
individual spins (where phase is defined as the angle between the 
projection of the spin on a plane in the rotating frame of reference and 
an axis in this plane, which passes through the axis of rotation) begin to 
diverge as some nuclei precess faster and some slower than the central 
Larmor frequency. Thus, there is a gradual "dephasing" of the individual 
nuclear spins and a consequent loss of phase coherence. In a perfectly 
uniform magnetic field, such "dephasing", as illustrated in FIG. 1C, 
results from natural processes that cause nuclei to exchange energy with 
each other. The length of time that such "dephasing" takes to occur is 
related to the spin-spin, or transverse, relaxation time constant T.sub.2. 
In a non-uniform field, additional dephasing is caused by the different 
frequencies at different positions within the object. The time constant 
associated with the combination of this effect and T.sub.2 is known as 
T.sub.2 *. 
During realignment, as illustrated in FIG. 1B, the nuclear moments .mu. 
also lose energy to their surroundings and thus relax, reorienting 
parallel to B.sub.0. This process is illustrated in FIG. 1D. The 
spin-lattice, or longitudinal, relaxation time constant T.sub.1, is 
related to this time of relaxation. Thus, as illustrated in FIG. 1E, after 
full relaxation, the macroscopic magnetization has reached its equilibrium 
value M.sub.0 and is aligned parallel to the imposed magnetic field 
B.sub.0. 
Assuming the nuclear spins to be initially aligned as in FIG. 1A, and then 
reoriented transverse to the initial direction, as in FIG. 1B, the net 
nuclear magnetization in the X-Y plane (see FIG. 1B) will induce a 
characteristic RF signal in an appropriately oriented coil connected to an 
RF signal receiver. Initially upon reorientation, a relatively strong 
voltage is induced in the receiver coils which gradually decreases in 
amplitude due field inhomogeneity and to energy exchange between spins 
(the net relaxation time constant is T.sub.2 *). This signal is called the 
free induction decay (FID). 
As is also well known, a "spin echo" or subsequent representation of the 
FID can be generated by bringing the respective spins back into phase 
coherence. 
For example, if, at a time .tau. after the nuclear spins are reoriented 
(for example 90.degree. with respect to an initial direction) by a first 
electromagnetic pulse of appropriate frequency, magnitude and duration 
(hereinafter referred to as a 90.degree. pulse), another electromagnetic 
signal of appropriate frequency, magnitude and duration is applied to 
effect a 180.degree. nutation of the nuclear spins (hereinafter referred 
to as a 180.degree. pulse) each individual spin is effectively rotated by 
180.degree. (in the rotating frame of reference). This means that the 
phase is now the negative of the phase accumulated before the 180.degree. 
pulse. The accumulation of further phase deviations for individual nuclear 
spins is the same as before and therefore, at time 2.tau. (after the 
initial disturbance) all of the individual spins again come into phase 
coherence (the negative phase cancels the further accumulated phase). In 
this manner, a so-called "spin echo" of the FID is generated. The peak 
amplitude of the spin echo is dependent upon the transverse relaxation 
time constant T.sub.2. The spin echo, in effect, comprises a mirror image 
and echo of the FID centered about a time 2.tau. after the initial 
disturbance. 
It should be noted that the spin echo is always peak at a time period after 
the application of the 180.degree. pulse which is equal to the time 
interval between application of the initial disturbance (90.degree. pulse 
generating the FID) and the application of the 180.degree. pulse. This 
phenomenon shall hereinafter be referred to as the "rule of equal times." 
Depending on the spin-spin relaxation time (related to T.sub.2), the FID 
following a selective 90.degree. RF pulse may continue for a relatively 
long period of time. In fact, at least part of the FID will continue 
through the spin echo following a selective 180.degree. RF pulse. Thus, 
the amplitude of the spin echo signal is in fact composed of two 
components: the spin echo from the unit volume, and part of the FID. Thus, 
since the amplitude of the spin echo signal is employed to determine the 
density of nuclei in the excited volume, the FID component will distort 
the spin echo signal, and therefore the measure of relative density 
related thereto. 
For a more detailed description of the basic principles of NMR, reference 
is made to Farrar and Becker "Pulse and Fourier Transform NMR Introduction 
to Theory and Methods," Academic Press, New York, 1971. 
While NMR techniques have long been utilized in the measurement of magnetic 
fields and in chemical analysis, NMR has only recently been applied to 
medical imaging applications. In general, NMR imaging techniques are based 
upon the premise that by purposefully disposing a specimen within a 
position-variant magnetic field (a field having an intensity which varies 
in accordance with position), the Larmor frequencies of the nuclei 
disposed at different positions are made to differ accordingly. Thus, a 
frequency discriminant is provided as between spins from atoms at 
differing positions, and the spin-density of a unit or element of volume 
within the excited volume of nuclei is represented by a particular 
frequency. 
Imaging techniques utilizing NMR typically fall within five categories: 
imaging from projections; FONAR; sensitive point imaging; Fourier imaging; 
and imaging by selective irradiation. 
The imaging from projections technique entails producing a multiplicity of 
projections from many different orientations by, for example, generating a 
linear field gradient within the object and recording a one dimensional 
projection of nuclear density in the direction defined by the gradient. An 
image is then reconstructed from the projections by mathematic techniques 
similar to those used in X-ray tomography. Such a method is described, for 
example, by Lauterbur, Nature, 242:190, March 1973. 
The FONAR technique utilizes shaped magnetic fields applied across the 
object such that only a small resonant window within the sample produces 
an NMR signal. The sensitive region is then scanned through the object, 
for example, by physical movement. For a description of the FONAR 
technique, reference is made to Damadian et al., "Focusing Nuclear 
Magnetic Resonance (FONAR)" Visualization of a Tumor in a Live Animal, 
Science, Vol. 194, pp. 1430-1432, December 1976 and to U.S. Pat. No. 
3,789,832 issued Feb. 5, 1974 to Damadian. 
The sensitive point imaging technique, also known as spin mapping, is a 
method whereby the NMR signals from particular unit volumes are recorded 
in sequence. A magnetic field gradient, alternating at a predetermined low 
frequency (on the order of 50 Hz) is generated along one axis of the 
object. The NMR signals from all elements in the object are thus modulated 
at the frequency of the gradient change, with the exception of the protons 
located in the null plane (zero plane) of the gradient. Similar 
alternating gradients can be applied at asynchronous frequencies along 
transverse axes to, in effect, define a null point in the object at the 
intersection of the gradient null planes. Appropriate lowpass filtering 
thus provides an indication of the NMR signal from the point of 
intersection of the three null planes. Raster-type scanning of the object 
is provided by varying the relative gradients. Such a sensitive point 
imaging technique is described in Hinshaw, Journal of Applied Physics, 
Vol. 47, No. 8, August 1976. 
A multiple sensitive point method utilizing two orthogonal alternating 
gradients to define a null line and a string of coherent, equally spaced 
phase alternated resonant radio frequency pulses, is alluded to in Andrew 
et al., "NMR Images by Multiple Sensitive Point Method, Application to 
Larger Biological Systems," Phys. Med. Biol. 1977, Vol. 22, No. 5, 
971-974, 1977. It is stated that discrete Fourier transformation of the 
signal received between RF pulses is utilized to provide indicia of the 
proton density along the line of intersection of two alternating gradient 
null planes. 
Fourier imaging techniques generally employ an initial RF pulse to reorient 
the spins of the protons in the object by 90.degree.. During the resultant 
FID signal, the object is subject to successive gradients applied 
consecutively in quick succession along the three principal Cartesian axes 
of the system. The FID signal is sampled in the presence of the last 
applied gradient, and a three dimensional Fourier transform is performed 
to develop a three dimensional image. Two dimensional Fourier transform 
methods are also known. For a discussion of the Fourier NMR techniques, 
reference is made to Kumar et al., "NMR Fourier Zeugmatography" Journal of 
Magnetic Resonance 18:69-83 (1975). 
Imaging by selective irradiation techniques entails use of a sequence of 
electromagnetic pulses with predetermined frequency spectrums. A first 
magnetic gradient is applied along a given axis and the object is 
irradiated by a sequence of electromagnetic pulses having a combined 
frequency spectrum having equal intensity at all Larmor frequencies across 
the object with the exception of a narrow band. As a result of the 
irradiation, all of the nuclei within the object, with the exception of a 
narrow plane, will be saturated. The saturated atoms are thereby rendered 
non-responsive to further electromagnetic signals for a period of time on 
the order of the spin-lattice relaxation time constant T.sub.1. The first 
magnetic gradient is replaced by a gradient along an orthogonal direction 
and the object again irradiated by a sequence of electromagnetic pulses, 
this time having a bandwidth corresponding to a particular elementary 
strip within the unsaturated plane. The second sequence of pulses nutates 
the spins of the atoms within the predetermined strip by 90.degree., 
resulting in generation of an FID. The FID is then recorded in the 
presence of a magnetic gradient in the third orthogonal direction (along 
the direction of the strip) and a Fourier transform taken to provide the 
nuclear density distribution along the line. For a more detailed 
description of the imaging by selective irradiation, reference is made to 
U.S. Pat. No. 4,021,726 issued May 3, 1977 to Garroway et al. 
For further descriptions of the above noted NMR imaging techniques, and 
other techniques, reference is made to the following articles: 
P. C. Lauterbur et al., "Magnetic Resonance Zeugmatography" 18th Amper. 
Conf. 1974; P. Mansfield, P. K. Grannel & A. A. Maudsley, "Diffraction and 
Microscopy in Solids by NMR," 18th Amper. Conf. 1974, pp. 431-432; P. C. 
Lauterbur, "Magnetic Resonance Zeugmatography"; P. C. Lauterbur, "Flow 
Measurements by NMR Zeugmatography," Oct. 24, 1973; P. C. Lauterbur, 
"Stable Isotope Distributions by NMR," Proc. First International Conf. on 
Stable Isotopes Conf. 730525, May 9-18, 1973, pp. 255-260; P. C. 
Lauterbur, "Image Formation by Induced Local Interactions: Examples 
Employing Nuclear Magnetic Resonance,"Nature, Vol. 242, Mar. 16, 1973, pp. 
190-191; P. C. Lauterbur et al., "ESR Zeugmatography--Distributions of 
Unpaired Electrons Within Objects," Gordon Conf. Aug. 12-16, 1974; P. C. 
Lauterbur et al., "In Vivo Studies of Cancer by NMR Zeugmatography," 
Gordon Conf. Aug. 12-16, 1974; P. C. Lauterbur, "Reconstruction in 
Zeugmatography--The Spatial Resolution of Magnetic Resonance Signals," 
Intl. Workshop on 3-D Image Reconstruction Techniques, July 16-19, 1974; 
A. N. Garroway, "Velocity Profile Measurements by NRM," 18th Amper. Conf. 
1974, pp. 435-436; W. S. Hinshaw, "The Application of Time Dependent Field 
Gradients to NMR Spin Mapping," 18th Amper. Conf. 1974, pp. 433-434; J. M. 
S. Hutchison, J. R. Mallard & C. C. Goll, "In Vivo Imaging of Body 
Structures Using Proton Resonance," 18th Amper. Conf. 1974, pp. 283-284; 
P. Mansfield & A. A. Maudsley, "Line Scan Proton Spin Imaging in 
Biological Structures by NMR," Phys. in Medicine and Biology 21 No. 5 
(1976), pp. 847-852; P. K. Grannel, "NMR Body Images," Physics Bulletin, 
March 1976, pp. 95-96; P. C. Lauterbur, D. M. Krammer, W. V. House, C. 
Chen, "Zeugmatographic High Resolution NMR Spectroscopy, Images of 
Chemical Inhomogeneity Within Macroscopic Objects," American Chemical 
Society Journal, 97:23, Nov. 12, 1975; P. Mansfield & P. K. Grannel, 
"Diffraction and Microscopy in Solids and Liquids by NMR," Physical Review 
B, Vol. 12, No. 9, Nov. 1, 1975, pp. 3618-3634; P. Mansfield, A. A. 
Maudsley & T. Baines, "Fast Scan Proton Density Imaging by NMR," J. of 
Physics E, Vol. 9, 1976, pp. 271-278; P. C. Lauterbur, "Bibliography on 
Magnetic Resonance Zeugmatography," June 3, 1975; A. N. Garroway, P. K. 
Grannel & P. Mansfield, "Image Formation in NMR by a Selective Irradiative 
Process," J. Phys. C: Vol. 7, 1974, pp. 457-462; A. Kumar, D. Welt & R. 
Ernst, "NMR Fourier Zeugmatography," J. Mag. Res. 18, 69-83 (1975); P. 
Mansfield & A. A. Maudsley, "Medical Imaging by NMR," British Journal of 
Radiology 50, 188-194 (1977); D. I. Hoult, "Zeugmatography: A criticism of 
the Concept of a Selective Pulse in the Presence of a Field Gradient," J. 
Mag. Res. 26, 165-167 (1977); P. Mansfield & A. A. Maudsley, "Planar Spin 
Imaging by NMR," J. of Physics C., Vol. 9, 1976, pp. L409-412; P. 
Mansfield, "Proton Spin Imaging by Nuclear Magnetic Resonance," 
Contemporary Physics, Vol. 17, No. 6, 1976 pp. 553-576; R. Damadian et 
al., "Field Focusing Nuclear Magnetic Resonance (FONAR): Visualization of 
a Tumor in a Live Animal," Science, Vol. 194, 24 December 1976, pp. 
1430-1431, E. R. Andrew, "Zeugmatography," IVth Amper. Summer School, 
September 1976; W. S. Hinshaw, "Image Formation by Nuclear Magnetic 
Resonance: The Sensitive-Point Method," J. of Applied Physics, Vol. 47, 
No. 8, August 1976; R. Damadian, M. Goldsmith & L. Minkoff, "NMR, in 
Cancer: XVI FONAR Image of the Live Human Body," Physiol. Chem. and Phys. 
9, (1977), pp. 97-108; G. N. Holland & P. A. Bottomley, "A Colour Display 
Technique for NMR Imaging," J. of Physics E: 10 (1977), pp. 714-716; T. 
Baines & P. Mansfield, "An Improved Picture Display for NMR Imaging," 
Journal of Physics E: Scientific Instruments 9 (1976), pp. 809-811; E. R. 
Andrew et al., "NMR Images by the Multiple Sensitive Point Method: 
Application to Larger Biological Systems," Physics in Medicine and Biology 
22, No. 5, 917-974 (1977); L. Minkoff, R. Damadian, T. E. Thomas, N. Hu, 
M. Goldsmith, J. Koutcher & M. Stanford, "NMR in Cancer: XVII. Dewar for a 
53-Inch Superconducting NMR Magnet," Physiol. Chem. and Phys. 9 (1977), 
pp. 101-109, Ros Herman, "NMR Makes Waves in Medical Equipment Companies," 
New Scientist, Jan. 12, 1978; L. E. Crooks, T. P. Grover, L. Kaufman & J. 
R. Singer, "Tomographic Imaging with Nuclear Magnetic Resonance," 
Investigative Radiology, 13, 63 January -February 1978; W. S. Hinshaw, P. 
A. Bottomley & G. N. Holland, "Radiographic Thin-Section Image of the 
Human Wrist by Nuclear Magnetic Resonance,"Nature, Vol. 270, No. 22, 29 
December, 1977, pp. 722-723; and T. C. Farrar & E.D. Becker, "Pulse and 
Fourier Transform NMR--Introduction to Theory and Methods," Academic 
Press, 1971, New York, pp. 1-33. 
Further reference is made to U.S. Pat. Nos. 3,975,675 issued to Dunand et 
al. on Aug. 17, 1976; 4,021,726 issued to Garroway et al. on May 3, 1977; 
4,015,196 issued to Moore et al. on Mar. 29, 1977; 4,034,191 issued to 
Tomlinson et al. on July 5, 1977; 3,789,832 issued to Damadian on Feb. 5, 
1974; 3,932,805 issued to Abe et al. on Jan. 13, 1976; 3,651,396 issued to 
Hewitt et al. on Mar. 21, 1973; and 3,999,118 issued to Hoult on Dec. 21, 
1976. 
It should be appreciated that each of the above described techniques is 
disadvantageous in various aspects. For example, techniques developing an 
image from projections require extensive mathematical processing of the 
data. The FONAR technique apparently requires either an extremely complex 
system to scan the magnetic field, or some means for generating relative 
movement between the field and the subject. 
The three dimensional Fourier transform techniques require that all planes 
be scanned simultaneously a multiplicity of times in order to develop 
sufficient data so that data from the various planes can be mathematically 
separated. In two dimensional Fourier transform techniques the repetition 
rate is limited by the T.sub.1 spin-lattice relaxation time of the nuclei 
since each irradiation affects the entire spin system. Further, large 
amounts of computer storage are required. 
Imaging techniques utilizing selective irradiation wherein the entire 
object is saturated with the exception of a single plane are 
disadvantageous in that such systems cannot readily be adapted for rapid 
sequential scanning of multiple planes. That is, before a second plane can 
be addressed a sufficient time must pass for the object to become 
unsaturated. 
The present invention is directed to a technique utilizing selective 
irradiation of the object by electromagnetic pulses to generate spin 
echoes (as opposed to the detection of the free induction decay signals 
(FID)) to readily provide for rapid multiple plane scanning. 
In general, as noted above, the phenomenon of spin echoes is well known. In 
the past, however, the spin echo has been used primarily for measurement 
of the transverse relaxation time constant T.sub.2 of a specimen. An 
example of a system utilizing spin echoes for the measurement of the 
relaxation time T.sub.2 in logging earth formations traversed by a bore 
hole, is described in U.S. Pat. No. 3,128,425 issued Apr. 7, 1964 to 
Codrington. Similarly, U.S. Pat. No. 3,213,355 issued to Woessner on Oct. 
19, 1965 describes a system for measuring the dimensions of a container 
utilizing spin echoes to determine the transverse relaxation time T.sub.2. 
Mansfield and Maudsley, "Planar Spin Imaging by NMR," J. Phys. C: Solid 
State Physics, Vol. 9, 1976 (noted above), appears to indicate that after 
an FID has decayed, various signal-refocusing arrangements (selective 
180.degree. pulses, 90.degree. pulses and various combinations with field 
gradient reversals) may be employed to recall the signal for signal 
averaging purposes. The specific mechanism of the refocusing arrangements, 
however, is not described in the article. In a similar vein, U.S. Pat. No. 
3,781,650 issued Dec. 25, 1973 to Keller appears to describe an NMR 
spectrometer wherein FIDs and spin echoes are combined for purposes of 
interference reduction. 
Also, in the aforementioned communication by Hoult (Journal of Magnetic 
Resonance, 26: 165-167 (1977)), it is stated that the selective 
irradiation techniques violate the "Uncertainty Principle" unless 
non-linearities present in the NMR system are exploited. Hoult states that 
a square selective pulse contains a wide spectrum of frequencies, and that 
during the time the pulse is applied the "flipped spins" dephase. However, 
he further contends that the situation is not irretrievable in that if the 
field gradient is reversed after the pulse, an echo of the pulse is 
formed, and in the middle of the echo for small phase angle pulses, all of 
the spins are in phase. 
Hoult's communication implies that the shape of the selected region is 
essentially identical to the shape of the spectrum of the selective 
irradiation. However, the present inventors have noted that since the 
response of nuclear spins to RF magnetic field is non-linear that the 
shape of selected volume does not exactly correspond to the shape of the 
spectrum of the exciting RF magnetic field. For example a spectrum with a 
perfectly square block of frequencies will excite a volume which covers a 
frequency range slightly wider than the block of frequencies and the edges 
of the excited volume are sloped rather than vertical. The shape of the 
excited volume can be calculated using the Block equations, described in 
Farrar and Becker pages 7 and 8, with an appropriate time dependent RF 
magnetic field having the frequency spectrum being considered. The use of 
the Block equations also determines the spin dephasing which occurs during 
the RF pulse. It can be shown, thus, that Hoult's gradient reversal 
suggestion works even for flip angles that are not small. For a 90.degree. 
flip angle the gradient reversal needed to achieve a maximum signal from 
the spins in the selected volume is a reversed gradient with the same 
strength as the original but with a duration which is about half the 
duration of the irradiation. The exact duration of the reversal gradient 
is dependent on the shape of the RF pulse. The effect of this reversed 
gradient is to recluster most of the spins which dephased during the 
selective irradiation. Since the dephasing during the selective 
irradiation is not linear the rephasing is not perfect, but it is 
substantial. After the termination of the reversed gradient one has a 
signal which we will consider to be an FID although Hoult calls it an 
echo. The other large flip angle which will be frequently used is 
180.degree.. A selective irradiation of this value requires no phase 
correction. The reason is that spin dephasing during the first 90.degree. 
of the flip is cancelled by rephasing during the second 90.degree. of the 
flip. 
The gradient reversal after a selective irradiation is one of several types 
of phase corrections necessary to the operation of the line mapping 
techniques described herein. Application of a reversed gradient for a 
period about half the duration of the selective irradiation will be 
hereinafter referred to as Type I phase correction. In practice the area 
under the correction gradient versus time waveform is the critical factor. 
If the correction gradient were twice as strong it would only have to be 
applied for half the time. This applies to all the types of phase 
corrections and the descriptions use the example of equal strength only 
for simplicity. A second type of phase correction, hereinafter referred to 
as a Type II phase correction, is immediate correction for a gradient 
pulse that has just ended; the phase spreading which occurs in parts of 
the object not subject to a selective irradiation during the application 
of this gradient pulse is corrected by the immediate application of an 
equal and opposite gradient for the same amount of time as the original 
gradient pulse. An extension of a Type II phase correction is to allow a 
delay time before the correction gradient is applied. Events such as spin 
echoes may be observed during this delay time. Such a phase correction 
wherein an opposite polarity gradient is applied after a delay time will 
be referred to as a Type III phase correction. A further type of phase 
correction (Type IV) is similar to a type III correction, except that a 
180.degree. RF pulse is applied to the volume of interest during the 
period between the application of the original and correction gradients, 
such that the polarity of the correction gradient will be the same as that 
of the first gradient. The correction gradient has the same polarity as 
the first gradient because the intervening 180.degree. RF pulse makes the 
phases negative. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method of selective irradiation 
imaging wherein direct analysis of the spin echo (as opposed to analysis 
of the FID) provides for rapid sequential scanning of plural planes 
(planar volumes) within the object. A predetermined number of parallel 
planes are sequentially selectively excited such that the spins of the 
atoms (nuclei) disposed therein are reoriented by approximately 
90.degree.. A predetermined number of transverse planes within the object 
are then selectively excited to reorient the spins of the atoms therein by 
180.degree.. The nuclei disposed in the respective intersections of the 
90.degree. reoriented planes and 180.degree. reoriented planes 
subsequently produce spin echo signals at times in accordance with the 
rule of equal times. The spin echo signals are thus generated during a 
sequence of respective time periods. By measuring the spin echoes in the 
presence of a position variant magnetic field along the line of 
intersection, the spin densities of unit volumes within each individual 
intersection can be provided by Fourier transforming the respective spin 
echo signals. 
However, if the sets of repeated read outs of a particular line occur too 
often T.sub.1 effects will decrease the signal strength from the unit 
volumes. Thus the signal strength indicates the combined effects of spin 
density and T.sub.1. Volume elements with long T.sub.1 s would have lower 
than normal "apparent" spin densities. Volume elements with short T.sub.1 
s will show very little change from their true spin density. Therefore, it 
is necessary to wait until a particular line to be read out is fully 
relaxed. After five T.sub.1, relaxation has occurred to the extent of 
99.3%. It is possible to use this T.sub.1 effect on the spin echo to 
produce T.sub.1 images by varying the repetition rates of repeated read 
outs. One can also use a "fast" repetition rate to enhance the image 
contrast between tissues with similar nuclear densities but different 
T.sub.1 s. Incidentally, by varying .tau., the spin echo signal can be 
effected to produce T.sub.2 images. One can use varying .tau. periods to 
enhance the image contrast between tissues with similar nuclear densities 
but different T.sub.2 s. 
When the spin echo is preferably independent of T.sub.1, it is desirable to 
delay reading out a line until it has fully relaxed. However, this greatly 
delays the time necessary to develop a full image. 
To speed the process, a number of transverse planes may be reoriented in 
one procedure by applying a number of 90.degree. pulses and 180.degree. 
pulses, and then reading out all of the spin echoes as they occur after 
application of all of the 180.degree. pulses. However, the time between 
application of the 90.degree. pulse and 180.degree. pulse for any given 
line must be less than the spin-spin relaxation time (related to T.sub.2). 
After relaxation, no spin echo can be obtained. In some samples, the 
spin-spin relaxation time is very short so that it is difficult, if not 
impossible, to apply a large number of 90.degree. and 180.degree. pulses. 
For such objects, it is necessary to repeat the 90.degree. 
pulse-180.degree. pulse-read out sequence a large number of times, thus 
slowing the time necessary to develop an image, particularly since a 
particular line must be fully relaxed (in the spin-lattice sense) prior to 
reorientation. 
To hasten the development of an image in the present invention, a 
90.degree. RF pulse and a 180.degree. RF pulse are applied to a sample to 
generate a spin echo from a particular line in the same manner as 
described above. While the planes which have been excited are relaxing, 
other lines are read out by applying 90.degree. RF pulses and 180.degree. 
RF pulses to other planes. To eliminate the above-described T.sub.1 
effects, these lines to be read out do not lie within any of the 
previously excited planes. After a number of additional lines have been 
read out, the first planes that had been excited will be relaxed and 
another line in one of those planes can be read out by re-exciting one of 
the first planes and a transverse plane which intersects the first plane 
in the line to be read out. 
If a planar image is desired, the planes to be excited to read out lines of 
the planar image can be selected transverse to the plane of the image. In 
this manner, additional lines in the desired image plane can be read out 
while previously excited lines relax. 
To eliminate the interference of the FID with the spin echo signal, the 
phases of the 90.degree. and 180.degree. RF pulses can be changed. By 
changing the phase of the pulses, the FID and/or the spin echo signal can 
be inverted. By scanning a line of plurality of times, with RF pulses 
having different phases, particular arithmetic combinations of 
corresponding portions of the spin echo signals will cause the FID 
components of the spin echo signal to cancel, or subtract out, while the 
actual spin echo will not cancel out. Thus, the result of any of the 
arithmetic combinations will be a signal related solely to the spin echo 
so that the relative density of nuclei in the line that is read out can be 
determined without interference from the FID.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS 
Referring now to FIGS. 2A-2D and 3, a spin echo scanning sequence in 
accordance with the present invention will be described for determining 
the normal, T.sub.1 modified or T.sub.2 modified density of nuclear spins 
(hereinafter termed the "spin density") in a unit volume 102 within an 
object 100. Suitable apparatus for practicing such spin echo scanning 
sequence will hereinafter be described in conjunction with FIGS. 15A, 15B, 
16 and 17. 
The first step in the spin echo scanning sequence, as in the other NMR 
imaging techniques, is to provide initial alignment of nuclear spins in 
object 100. To this end, an intense magnetic field, B.sub.0, is generated 
along, for example, the Z direction of a nominal Cartesian coordinate 
system centered in alignment with object 100 (FIG. 2A). As noted above, 
the nuclear spins of the atoms assume Larmor frequencies directly 
proportional to the magnetic field acting thereon, and tend to align with 
magnetic field B.sub.0. 
Next, a particular volume within object 100 is excited to nutate the spins 
of the atoms in the volume to a transverse orientation, preferably 
90.degree.. It should be recalled that nuclei with spins at a given Larmor 
frequency are responsive only to external electromagnetic signals 
substantially at the Larmor frequency. Thus, if object 100 is subjected to 
a position variant magnetic field, such as, for example, a magnetic 
gradient along the X direction (g.sub.x =.delta.B.sub.z /.delta.x) the 
nuclei located within the various Y-Z planes (planar volumes) located at 
different positions along the X axis will have different Larmor 
frequencies. A single Y-Z plane can thus be addressed by irradiating 
object 100 (while in the presence of the X position variant field) with an 
electromagnetic signal having a frequency spectrum corresponding to the 
plane. In practice, of course, the addressed volume of atoms have a finite 
X dimension, and accordingly, include spins of Larmor frequencies over a 
predetermined bandwidth. 
Thus, after initial alignment of the spins by field B.sub.0, an X-variant 
magnetic field, suitably a gradient g.sub.x (FIG. 3), is generated across 
object 100, to provide a Larmor frequency discriminant along the X axis. 
In the presence of gradient g.sub.x, a particular planar volume 104 (FIG. 
2B) within object 100 is addressed by irradiating object 100 with a 
90.degree. electromagnetic pulse X having a frequency spectrum consisting 
substantially of frequency components corresponding to the Larmor 
frequencies of the atoms disposed within plane (planar volume) 104. 
The relationship of 90.degree. pulse X and gradient g.sub.x is shown in 
FIG. 3. It should be noted that FIG. 3 (and FIG. 6) depicts the presence 
or absence of the gradient, rather than the shape or form of the gradient 
itself. 
To simplify the following discussion, the following conventions will be 
adopted. The process of irradiating the object with an electromagnetic 
signal of predetermined frequency spectrum in the presence of a position 
variant field to excite a predetermined volume of nuclei will hereinafter 
be referred to as a process of selective irradiation. Similarly, planar 
volumes wherein the nuclear spins are reoriented by 90.degree. or 
180.degree. will hereinafter be referred to as 90.degree. planes and 
180.degree. planes, respectively. 
As previously noted, the nuclear spins within volume 104 are, after 
90.degree. pulse X and Type I phase correction -g.sub.x, initially in 
alignment and at 90.degree. with respect to the original orientation and 
thus induce a relatively strong voltage in a coil disposed about the X 
axis. However, the induced voltage decays with time as the phases of the 
spins at different positions within volume 104 spread. Free induction 
decay (FID) signal 110 generated by the nuclei within volume 104 as a 
result of 90.degree. pulse X is shown in FIG. 3. 
Now a Type IV phase correction is applied by the gradient g.sub.z1. This 
gradient is applied to spread the phases along the z direction so that 
they may be refocused by g.sub.z2 during the first half of the spin echo 
read out. g.sub.z1 has the same area as the first half of g.sub.z2. FIG. 3 
shows g.sub.z1 not overlapping any other gradient for simplicity in 
presenting the FID. It is possible to have g.sub.z1 overlap -g.sub.x with 
no deleterious effect of the spin echo and a reduction in the time 
required for the sequence. 
The next step in the spin echo scanning sequence is to effect a 180.degree. 
rotation of nuclear spins within a volume transverse to and intersecting 
volume 104 such that unit volume 102 is common to both volumes. A 
Y-position variant magnetic field, suitably a gradient g.sub.y 
=.delta.B.sub.z /.delta.y is therefore generated across object 100, and a 
predetermined X-Z planar volume 106 (FIG. 2C) is excited by a 180.degree. 
pulse Y having a frequency spectrum corresponding to the band of Larmor 
frequencies of the nuclei within volume 106. Thus, the phases of the spins 
of the nuclei within volume 106 are reversed and volume 106 becomes a 
180.degree. plane. 
The effect of the phase reversal on those nuclei common to both planes 104 
and 106 (hereinafter referred to as intersection volume 108, FIG. 2D) is 
to recluster the spin phases. Thus, spin echo 112 (FIG. 3) is produced by 
intersection volume 108. 
Referring briefly to FIG. 4, it should be noted that 180.degree. pulse Y 
acts, in effect, as a mirror with respect to FID 110. As the phases begin 
to regroup, a reflection of the decay is provided and a peak signal is 
generated at a time 2.tau., where .tau. is equal to the time between the 
generation of 90.degree. pulse X and 180.degree. pulse Y, in accordance 
with the rule of equal times. The phases of the spins thereafter spread 
and the spin echo decays in a manner similar to the decay of the FID. As 
will hereinafter be explained, the rule of equal times can be utilized in 
a multiline scan system to provide non-interfering spin echoes from the 
multiple lines. Further, while the spin echo is of less magnitude than the 
FID due to transverse relaxation (T.sub.2) the mirroring effect provides 
twice the sampling period for collecting the data. Also the spin echo is a 
reflection only of that part of the FID arising from the nuclei in volume 
108. 
The spin echo is recorded in the presence of a position variant magnetic 
field wherein the intensity of the magnetic field varies as a function of 
position along the line of intersection. Referring again to FIGS. 2 and 3, 
the individual spin densities of unit volumes within intersection volume 
108 are determined by subjecting object 100 to a Z-position variant 
magnetic field, suitably gradient g.sub.z2 =.delta.B.sub.z /.delta.z (FIG. 
3) during the time period when spin echo 112 is generated. The spin echo 
signal is sampled and a Fourier transform of the sampled spin echo signal 
is performed to, in effect, measure the intensity of the various frequency 
components of the spin echo. The spin density of the particular unit 
volume 102 is thus represented by the intensity of the spin echo frequency 
component corresponding to the particular Larmor frequency of the unit 
volume. 
If the gradient g.sub.z2 is such that its contribution to B.sub.z is zero 
at the center of intersection volume 108, unit volumes at equal Z 
distances on either side of the center will have Larmor frequencies at 
equal frequency increments above and below the center Larmor frequency. 
The demodulated signal components from those unit volumes will be at equal 
and opposite frequencies and can thus be discriminated by conventional 
quadrature detection techniques, as will be described. 
The spin echo scanning sequence just described is particularly advantageous 
in that it is readily adapted for rapid multi-line scanning. By 
selectively addressing a sequence of respective planes with 90.degree. 
pulses, then sequentially addressing selected transverse planes with 
180.degree. pulses, a timed sequence of spin echoes from plural lines of 
intersection can be effected. The aforementioned rule of equal times 
provides a time discrimination between the respective spin echoes. By way 
of example, a two line sequence will now be explained with reference to 
FIGS. 5 and 6. 
As in the single-line scanning sequence, a first plane 104a (FIG. 5A) is 
addressed by selective irradiation, suitably utilizating a 90.degree. 
pulse X.sub.1 in cooperation with an X-gradient g.sub.x (FIG. 6). 
Thereafter, successive selective irradiation processes are performed to 
cause phase reversals in transverse planes 106a and 106b (FIG. 5B). As 
shown in FIG. 6, a Y-position variant magnetic field, suitably a gradient 
g.sub.y1, is generated across object 100 and a 180.degree. pulse Y.sub.1 
of appropriate frequency spectrum, is applied to selectively excite volume 
106a to 180.degree.. A second X-Z plane 106b is excited by a selective 
irradiation process utilizing a second 180.degree. pulse Y.sub.2 and 
Y-gradient g.sub.y2 in predetermined time relation with pulses X.sub.1 and 
Y.sub.1. X-Z plane 106b is selected at a Y position different from that of 
plane 106a by suitably offsetting the frequency spectrum of 180.degree. 
pulse Y.sub.2 by a suitable frequency .DELTA.F, changing the main magnetic 
field B.sub.0 by a predetermined amount .DELTA.B (the shift .DELTA.B 
effecting a shift in relative Larmor frequency throughout the object) or 
by a combination of both. The .DELTA.B method is schematically illustrated 
in FIG. 6. 
The nuclei common to both the 90.degree. plane and 180.degree. plane, i.e., 
intersection volumes X.sub.1 Y.sub.1 and X.sub.1 Y.sub.2 (FIG. 5C) 
generate respective spin echoes (also designated X.sub.1 Y.sub.1 and 
X.sub.1 Y.sub.2 in FIG. 6) at respective times in accordance with the rule 
of equal times. The respective spin echoes are recorded in the presence of 
position variant magnetic fields, gradients g.sub.z1 and g.sub.z2 (FIG. 
6), respectively, to impart a Larmor frequency discriminant along the line 
of intersection. Fourier transforms are taken of the recorded spin echoes 
to develop indicia of the spin-densities, or T.sub.1 or T.sub.2 modified 
spin densities of unit volumes within the intersection volumes. 
It should be noted that the respective spin echoes X.sub.1 Y.sub.1 and 
X.sub.1 Y.sub.2 occur at time periods after pulse Y.sub.1 and after 
Y.sub.2, respectively, equal to the time periods between pulses Y.sub.1 
and X.sub.1 and pulses Y.sub.2 and X.sub.1. Thus, by proper relative 
timing of pulses Y.sub.1 and Y.sub.2 spin echoes X.sub.1 Y.sub.1 and 
X.sub.1 Y.sub.2 can be made to occur with a desired time interval 
therebetween to provide proper discrimination between lines. 
The spin phases of nuclei other than in the respective instantaneously 
selected planes are spread by exposure to the respective magnetic 
gradients. Spurious phase spreading due to the gradients and steps in 
magnetic field .DELTA.B is corrected by application of the various types 
of phase corrections. For example, with reference to FIG. 6, gradient 
-g.sub.x is the Type I phase correction required following a 90.degree. 
selective irradiation. The gradient g.sub.y1 causes phase spreading in 
180.degree. plane 106b prior to pulse Y.sub.2 and gradient g.sub.y2 causes 
phase spreading in 180.degree. plane 106a after pulse Y.sub.1. Correction 
should be made for such spurious phase spreading prior to the generation 
of a spin echo signal from nuclei within the affected plane. Accordingly, 
to avoid errors due to spurious phase spreading in spin echo X.sub.1 
Y.sub.1 a Type II negative gradient -g.sub.y2 is applied immediately upon 
termination of gradient g.sub.y2. However, negative gradient -g.sub.y2 
affects plane 106b after pulse Y.sub.2 and must itself be corrected. 
Correction for the effects of gradients g.sub.y1 and -g.sub.y2 on plane 
106b and thus echo X.sub.1 Y.sub.2 is provided by application of Type IV 
correction gradient g.sub.y3 and gradient correction Type III g.sub.y4, 
respectively. The first half of read out gradient g.sub.z1 must refocus 
spin phases to produce an echo. This requires that the spins be defocused 
at the start of the sequence. Gradient g.sub.z0 does this. The area of 
gradient g.sub.z0 is half that of g.sub.z1 such that the refocusing is 
complete at the center of the echo. The first half of g.sub.z1 is, in 
effect, a Type IV phase correction for g.sub.z0. g.sub.z0 may be applied 
at the same time as -g.sub.x. 
Phase spreading effects of the second half of gradient g.sub.z1 on plane 
106b are corrected by application of negative gradient -g.sub.z1 (a Type 
II phase correction). The spin echo is refocused during this correction 
and samples taken during this period can be used in signal averaging. 
FIG. 7 schematically illustrates a three line spin echo scanning sequence. 
A 90.degree. pulse X.sub.1 is applied at time t.sub.0 (in the presence of 
an X gradient) and a sequence of Y pulses (in the presence of Y gradients) 
irradiate the object at times 3.tau./2,2.tau. and 5.tau./2, respectively. 
Accordingly, a spin echo X.sub.1 Y.sub.1 is generated at a time 3.tau. by 
the nuclei disposed common to both the 90.degree. plane associated with 
pulse X.sub.1 and the 180.degree. plane associated with pulse Y.sub.1. 
Similarly, spin echoes X.sub.1 Y.sub.2 and X.sub.1 Y.sub.3 are generated 
by the nuclei common to the planes associated with pulses X.sub.1 and 
Y.sub.2, and pulses X.sub.1 and Y.sub.3, at times 4.tau. and 5.tau., 
respectively. Phase correction (not shown) can be effected in a manner 
similar to that described in conjunction with FIG. 6. The spin echoes are 
recorded in the presence of a Z gradient and Fourier transforms are 
performed to develop indicia of the relative normal, or T.sub.1 or T.sub.2 
modified spin densities. 
As schematically illustrated in FIG. 8, a plurality of Y-Z planes can be 
sequentially excited in cooperation with one or more X-Z planes to produce 
a multi-line scan. For example, assume Y-Z planes 104a, 104b and 104c 
(FIG. 5A) are excited to 90.degree. orientations by a selective 
irradiation process including irradiation of the object by successive 
90.degree. pulses X.sub.1, X.sub.2 and X.sub.3 in the presence of X 
gradients. As noted above, the planes along the X axis (and Y axis) are 
uniquely selected by choosing appropriate frequency spectrums for the 
respective 90.degree. pulses, by offsetting the magnetic field B.sub.0, or 
by a combination of both techniques. After Y-Z planes 104a, 104b and 104c 
are excited to 90.degree., selected X-Z planes 106a and 106b are excited 
to 180.degree. by a similar process of selective irradiation utilizing 
180.degree. pulses Y.sub.1 and Y.sub.2. The intersections of the 
respective Y-Z planes and X-Z planes result in a plurality of lines of 
intersection as shown in FIG. 5C, X.sub.1 Y.sub.1, X.sub.2 Y.sub.1, 
X.sub.3 Y.sub.1, X.sub.1 Y.sub.2, X.sub. 2 Y.sub.2 and X.sub.3 Y.sub.2. 
Each such line of intersection generates a spin echo signal denoted in 
FIG. 8 by the respective associated line of intersection in accordance 
with the aforementioned rule of equal times. 
By proper choice of the time intervals between respective pulses, the 
respective spin echoes are generated in a non-interfering sequence. For 
example, assuming 90.degree. pulses X.sub.1, X.sub.2 and X.sub.3 to be 
generated at times 0, .tau. and 2.tau., respectively, and 180.degree. 
pulses Y.sub.1 and Y.sub.2 generated at 11.tau./4 and 3.tau., 
respectively, the spin echo from the line of intersection X.sub.3 Y.sub.1 
will be generated at time 7.tau./2; X.sub.3 Y.sub.2 at time 4.tau.; 
X.sub.2 Y.sub.1 at 9.tau./2; X.sub.2 Y.sub.2 at 5.tau.; X.sub.1 Y.sub.1 at 
11.tau./2; and X.sub.1 Y.sub.2 at time 6.tau.. Correction for phase 
spreading due to the effects of the respective gradients would again be 
made in a manner similar to that described above with respect to FIG. 6. 
The respective spin echoes are then recorded in the presence of a Z 
gradient and Fourier transforms performed to develop indicia of the 
normal, T.sub.1 modified or T.sub.2 modified spin densities of the 
individual unit volumes within the lines of intersection. Thus, utilizing 
a spin echo sequence in accordance with the present invention, a 
multiplicity of lines within volume 100 can be scanned in rapid sequence, 
at rates not limited by the spin-lattice relaxation time T.sub.1 of the 
object as in scanning techniques using saturation. 
In some instances, it is desirable to take plural readings of the spin echo 
from a given line of intersection for signal averaging purposes. As noted 
above, Mansfield and Maudsley have indicated that after the FID has 
decayed, various signal refocusing arrangements can be employed to recall 
the signal for signal averaging purposes. A similar process may be 
employed to recall the spin echo signal. Referring now to FIG. 9, a spin 
echo X.sub.1 Y.sub.1 is generated by first selectively irradiating a first 
plane (planar volume) with a 90.degree. pulse X.sub.1, and thereafter 
selectively irradiating a transverse plane with a 180.degree. pulse 
Y.sub.1. Assuming the time interval between pulse X.sub.1 and Y.sub.1 to 
be equal to .tau..sub.1, spin echo X.sub.1 Y.sub.1 is generated with a 
peak at a time .tau..sub.1 after the irradiation of 180.degree. pulse 
Y.sub.1, in accordance with the rule of equal times. If a further 
180.degree. pulse Y.sub.1 ' having a frequency spectrum including that of 
pulse Y.sub.1 is generated at a time .tau..sub.2 after the peak of spin 
echo X.sub.1 Y.sub.1, the phases of the nuclear spins generating spin echo 
X.sub.1 Y.sub.1 will be reversed, causing the phases to converge and 
thereby create a copy or replica X.sub.1 Y.sub.1 ' of spin echo X.sub.1 
Y.sub.1. 
The rule of equal time applies to the phenomenon of copying. Accordingly, 
replica X.sub.1 Y.sub.1 ' occurs at a time .tau..sub.2 after the 
generation of 180.degree. pulse Y.sub.1 '. Further replicas can be 
generated by applications of additional 180.degree. pulses. However, the 
overall amplitudes of the signals decrease in accordance with the 
transverse relaxation time T.sub.2. Such decay represents a practical 
limit on the number of replicas that can be produced. 
It should be appreciated that if 180.degree. pulse Y.sub.1 ' includes 
spectral components not in pulse Y.sub.1, a phase reversal will be 
effected of portions of the original 90.degree. plane not affected by 
pulse Y.sub.1. Thus, such portions of the original 90.degree. plane will 
produce a parasitic spin echo P at a time 2.tau..sub.1 +.tau..sub.2 after 
the generation of pulse Y.sub.1 '. The parasitic echo P can be avoided by 
making the frequency spectrum of 180.degree. pulse Y.sub.1 ' identical to 
that of pulse Y.sub.1. 
Replicas of a plurality of spin echoes can be generated by application of a 
single 180.degree. pulse having a frequency spectrum encompassing the 
individual frequency spectra of the original 180.degree. pulses, i.e., 
covering the respective 180.degree. planes. Such a procedure is 
schematically illustrated in FIGS. 10 and 11. In FIG. 10, a first plane is 
selectively irradiated by 90.degree. pulse X.sub.1, then respective 
transverse planes are selectively irradiated in sequence by 180.degree. 
pulses Y.sub.1, Y.sub.2 and Y.sub.3, respectively, applied at times 
3.tau./2, 2.tau. and 5.tau./2. Accordingly, spin echoes from the 
respective lines of intersection X.sub.1 Y.sub.1, X.sub.1 Y.sub.2 and 
X.sub.1 Y.sub.3 are generated at times 3.tau., 4.tau. and 5.tau., 
respectively. 
Application of a wide spectrum 180.degree. pulse Y' at, for example, time 
11.tau./2, will generate replicas of the spin echoes in reverse order. In 
accordance with the rule of equal times, replica X.sub.1 Y.sub.3 ' is 
generated at a time 6.tau.(.tau./2 after the application of pulse Y', an 
interval equal to the time between the occurrence of spin echo X.sub.1 
Y.sub.3 at 5.tau. and pulse Y'). Similarly, replica X.sub.1 Y.sub.2 ' 
occurs at time 7.tau. and replica X.sub.1 Y.sub.1 ' occurs at time 8.tau.. 
Application of a second wide spectrum 180.degree. pulse Y" at, for example, 
time 17.tau./2 will create further replicas of the spin echoes, this time 
in the same order as the original spin echoes. Replicas X.sub.1 Y.sub.1 ", 
X.sub.1 Y.sub.2 " and X.sub.1 Y.sub.3 ", respectively, occur at times 
9.tau., 10.tau. and 11.tau.. 
It should be noted, however, that wide spectrum 180.degree. pulse Y' causes 
phase reversals in portions of the 90.degree. plane not phase reversed by 
pulse Y.sub.1, and accordingly, a parasitic spin echo P is generated. In 
accordance with the rule of equal times, such parasitic echo is generated 
at a time 11.tau.. Thus, parasitic echo P renders replica Y.sub.3 X.sub.1 
' substantially useless. 
The parasitic echo can be reduced by reducing the portions of the 
90.degree. plane not phase reversed by pulse Y.sub.1. The effects of the 
parasitic echoes can also be lessened or spread between various of the 
replicas by changing the relative timings of wide spectrum 180.degree. 
pulses Y' and Y". 
FIG. 11 shows an alternate spin echo scanning sequence utilizing copying. 
In this instance, a plurality of 90.degree. planes are generated by 
selective irradiation with respective 90.degree. pulses X.sub.1, X.sub.2 
and X.sub.3, followed by a 180.degree. pulse Y.sub.1 of predetermined 
frequency spectrum. The resultant lines of intersection generate 
respective spin echoes X.sub.3 Y.sub.1, X.sub.2 Y.sub.1 and X.sub.1 
Y.sub.1 in accordance with the rule of equal times. Thereafter copying is 
effected by selective irradiation employing respective 180.degree. pulses 
Y.sub.1 ' and Y.sub.1 " of identical frequency spectrum to pulse Y.sub.1. 
It should be appreciated that since the frequency spectrum of the 
respective 180.degree. pulses are identical, no parasitic echoes are 
generated. 
It is possible that FID 110 following a selective 90.degree. RF pulse will 
interfere with the following spin echo 112 emanating from an intersection 
volume 108. This type of interference can be eliminated during signal 
averaging. Consider a single line scan as illustrated in FIG. 3. If 
g.sub.y is zero and no 180.degree. RF pulse is applied, FID 110 may last 
through the time when spin echo 112 occurs. Furthermore, the g.sub.z2 
gradient reclusters the FID to increase the interference. Under these 
conditions, FID 110, produced by spins in the entire X-plane selected by 
the 90.degree. RF pulse, produces a signal during the spin echo time that 
is quite strong. 
During the actual imaging process the strength of the FID is substantially 
reduced by the phase spreading produced by the gradient pulses g.sub.z1 
and g.sub.y. In portions of the X selected plane it is possible for phase 
spreading produced by g.sub.z1 to be compensated for by phase spreading 
due to g.sub.y. As usual the phase spreading is a function of the area of 
the gradient pulse. If g.sub.z1 is on for t.sub.1 seconds and g.sub.y is 
on for t.sub.2 seconds, then at the end of the g.sub.y pulse the phase 
spread for each point in the X selected plane, except for where it 
intersects the Y selected plane is .theta.=g.sub.z1 z t.sub.1 +g.sub.y Y 
t.sub.2. 
The portions of the plane which experience no net phase spreading at the 
start of the echo can be found by setting .theta.=0. 
This gives z=-(g.sub.y t.sub.2 /g.sub.z1 t.sub.1)y, a diagonal line through 
the X selected plane. Thus spins around this diagonal line will produce an 
interference signal during the spin echo. This interference signal is 
dependent only on the values of the gradient pulses and the distribution 
of the object in the X selected plane. 
To remove this interference signal, the phases of the 90.degree. and 
180.degree. RF pulses can be changed so that FID 110 can be inverted while 
spin echo 112 is not. Phase in this case is defined with respect to the 
output of a master oscillator 166 (described below) employed in both the 
generation of the RF pulses and detection of the resulting FID and spin 
echo signals. When the phases of the 90.degree. pulses and the 180.degree. 
pulses are 0.degree. with respect to the output of frequency synthesizer 
168, the apparatus to be described below produces an FID having a positive 
polarity and a spin echo having a negative polarity. The effect of FID 110 
can be eliminated by scanning intersection volume 108 twice, once 
producing a positive FID and spin echo and once producing a negative FID 
and a positive spin echo, and adding the corresponding portions of the two 
signals together. The positive and negative interference from the FIDs 
will cancel when added, leaving the spin echo. 
For example, if in a first read out, the phase of the 90.degree. RF pulse 
is 0.degree. and the phase of the 180.degree. RF pulse is 90.degree., the 
polarity of both FID 110 and spin echo 112 will be positive. If, in a 
second read out, the phase of the 90.degree. RF pulse is 180.degree. and 
the phase of the 180.degree. RF pulse is 0.degree. the polarity of the FID 
signal 110 will be negative while the polarity of spin echo 112 will be 
positive. When corresponding portions of the signals from the two read 
outs are added, the effects of FID 110 cancel leaving only the effects of 
spin echo 112. 
Other combinations of RF signal phase can produce the same results. For 
example, corresponding portions of a positive FID and a negative spin echo 
signal can be subtracted from a positive FID and a positive spin echo 
signal. Note that the repetition of line scans would usually be necessary 
to improve image contrast (signal-to-noise ratio) so that this repeated 
line scanning and summation does not detract from the utility of the 
technique. 
A slight extension of this phase shifting will also correct for any FID 
from the Y selected plane where the value of the 180.degree. pulse is not 
exactly correct. If, for example, the RF pulse was only 170.degree. due to 
misalignment or RF absorption in object 100, there will be an FID from the 
Y plane. This FID may be either in phase or quadrature. The following 
Table I lists all the possible phase combinations for the 90.degree. and 
180.degree. RF pulses which give inphase spin echoes: 
TABLE I 
______________________________________ 
Phase of Phase of Sign of Sign of 
Sign of 
90.degree. 180.degree. 
spin x plane 
y plane 
RF Pulse RF Pulse echo FID FID 
______________________________________ 
1. 0.degree. 
0.degree. 
- + + 
2. 0.degree. 
90.degree. 
+ + + Quad 
3. 0.degree. 
180.degree. 
- + - 
4. 0.degree. 
270.degree. 
+ + - Quad 
5. 180.degree. 
0.degree. 
+ - + 
6. 180.degree. 
90.degree. 
- - + Quad 
7. 180.degree. 
180.degree. 
+ - - 
8. 180.degree. 
270.degree. 
- - - Quad 
______________________________________ 
Many combinations of phases with additions and/or subtractions of line 
scans can be derived from this table to eliminate the two different types 
of interference FIDs, but at least four line scans are required and the 
number of line scans must be a multiple of four. One possible combination 
would be to add the results of line scans 2, 4, 5 and 7 from Table I. 
Another possibility is to add the results from line scans 2 and 5, and add 
the results from line scans 1 and 6, and then subtract the intermediate 
sums. Table I can also be employed for selecting the RF phases for 
multi-line sequences. 
If an image (particularly an image of a volume) were constructed employing 
the sequence of applying the 90.degree. RF pulse, applying the 180.degree. 
RF pulse, reading out the spin echo, and waiting for the excited planes to 
fully relax, an extraordinarily long time would be consumed. As described 
above, to speed the development of a volume image, a number of 90.degree. 
planes and 180.degree. planes may be excited, after which the spin echoes 
from all of the intersection volumes are read out. However, the 
180.degree. RF pulses must be applied within the spin-spin relaxation time 
(related to T.sub.2) in order to produce spin echo 112. In many 
substances, T.sub.2 is too short to make practical the scanning of more 
than one line at a time. 
In the case where a short T.sub.2 makes it impractical to scan many lines 
at a time, and a long T.sub.1 precludes rapid repeated irradiations of a 
particular plane, the following process can be used to rapidly produce 
images, particularly multi-planar images. 
In FIG. 12, line 11 is defined by two selective irradiations. The planes 
excited by these irradiations must be allowed to relax (in the 
spin-lattice, T.sub.1 sense) before they can be irradiated again. If 
reirradiation is delayed one T.sub.1 unit of time, relaxation will have 
occurred to the extent of 63.2%. If reirradiation is delayed five T.sub.1 
periods, relaxation will have occurred to the extent of 99.3%. During this 
waiting period, it is possible to read other lines as long as these other 
lines do not intersect any of the previously irradiated planes. Line 22 in 
FIG. 12 is an example of such a line. Note that intersection volume 22 is 
in neither of the planes excited to produce a read out from intersection 
volume 11. After line 22 is read out, lines 33-88 can be sequentially 
scanned while the preceding planes recover. If enough intersecting volumes 
11-88 are included, the planes defining intersection volume 11 have 
relaxed by the time intersection volume 88 has been read out. Thus, lines 
11-88 can be read out again for averaging purposes, or lines 12-81 can be 
read out to develop an image of the volume. In the latter case, the final 
set of lines to be read out would be 18, 21, 32, 43 . . . 87, and would 
complete the image of eight lines in each of eight planes. 
Where the relaxation time is approximately equal to eight irradiation and 
sample periods, it is possible to develop images for eight planes in the 
time it takes to develop an image for one plane if the time between lines 
were spent waiting for T.sub.1 type recovery. In practice, the number of 
planes which can be scanned depends on T.sub.1, T.sub.2 and the time 
required to select and sample a line. For example, at roughly room 
temperature and a magnetic field strength of 3.52 K gauss (0.352 Tesla), 
T.sub.1 may typically be 0.5 to 1.0 seconds, while T.sub.2 may be about 
100 ms. In the preferred embodiment the time between the first turn on of 
g.sub.x and the turn off of g.sub.z2 is 50 ms. 
In practice, the borders around excited planes are not precise. To ensure 
that border areas are not excited in two consecutive read outs, it may 
also be advantageous to skip some planes. Thus, every other plane can be 
skipped so that the first sequence would be 11, 33, 55, 77. 
If only one plane is of interest and an unusual resolution element is 
acceptable, a variation of this rapid multi-plane scanning can be applied 
to a single plane through the object, as illustrated in FIG. 13. The 
selective irradiation planes are aligned at non-orthogonal angles such 
that they have an intersection volume with a diamond or rhombus 
cross-section rather than a rectangular cross-section. The slant angle of 
the selected planes is achieved by applying two gradients at the same 
time. It is possible to stack these diamond shapes along a line so that 
the result is a single planar image. In this manner each line being read 
out is not disposed in planar volumes previously excited. The volume of 
the planar image composed of diamonds is only half that of a planar image 
composed of squares having contiguous surfaces with the same resolution, 
so that for the same image contrast (signal-to-noise ratio) four times the 
number of read outs is necessary. This increased averaging requirement 
will reduce the advantage of avoiding T.sub.1 recovery delays. For 
example, if T.sub.1 is ten times longer than the line acquisition time, 
the requirement of four times more averaging increases the imaging speed 
by a factor of 2.5, instead of the full 10 fold increase. 
An advantage can be gained in terms of the volume of the line being excited 
if the cross-sections are rhomboid as shown in FIG. 14. The volume of each 
line is now much closer to the volume of a rectangular element so that 
less signal averaging is required. Care must now be exercised as to the 
sequence in which the lines are scanned because some lines are in 
previously irradiated areas. For example, the irradiation which defines 
line 1 in FIG. 14 also irradiates parts of lines 2-8. Thus, the next line 
which can be scanned is line 9. To scan the entire plane, one would first 
do a group out of the series 1, 9, 18, 27 . . . and next do a group out of 
the series 2, 10, 19, 28 . . . . The maximum speed gained would occur when 
the number of lines in the plane is large enough so that there are 
sufficient lines in each series to fully occupy the T.sub.1 relaxation 
time with scanning. 
Referring now to FIGS. 15A, 15B and 16, suitable apparatus for effecting 
spin echo scanning sequences in accordance with the present invention will 
be described. FIGS. 15A and 15B show the overall apparatus in block 
schematic form, while apparatus utilized in generating the various 
magnetic fields (control circuitry excepted) is shown in exploded 
perspective in FIG. 16. 
Object 100 is disposed within an RF coil 114 disposed about the nominal X 
axis of a Cartesian coordinate system. RF coil 114 is utilized in 
irradiation of object 100, to pick up the spin echo signals generated by 
object 100 and, further, suitably provides a support holder for object 
100. 
Coil 114 is, in turn, disposed between respective pole tips 116 and 118 of 
main magnet 120. Magnet 120 is utilized to generate the main magnetic 
field B.sub.0 along the Z-direction of the nominal coordinate system 
(transverse to the axis of coil 114). Magnet 120 suitably comprises a 
conventional iron core having wound thereabout, conductors cooperating 
with a power supply (not shown) and are suitably water cooled to maintain 
a constant temperature. 
Z-position variant fields are selectively provided by Z gradient coils 
(g.sub.z) 122 and 124. Coils 122 and 124 are disposed on the respective 
faces of pole tips 116 and 118 and are suitably in a Maxwell coil 
geometry. That is, the coils are circular and concentric with the pole tip 
with the radius of the coils determined in accordance with the separation 
of the coils. Z gradient coils 122 and 124 are electrically connected in 
series such that the magnetic fields generated thereby are in opposition 
and cancel at the origin of the coordinate system. 
X-position gradient magnetic fields are selectively provided by X gradient 
(g.sub.x) coils 126 and 128. Coils 126 and 128 are suitably of a 
rectangular configuration disposed on pole tips 116 and 118, respectively, 
so as to simulate infinite conductors running in an X direction. Coils 126 
and 128 are electrically connected to generate opposing magnetic fields 
which also cancel at the origin of the coordinate system. 
Y-position variant magnetic fields are selectively provided by Y gradient 
(g.sub.y) coils 130 and 132. Coils 130 and 132 are suitably of the same 
shape and area as X gradient coils 126 and 128 but are disposed on pole 
tips 116 and 118 in a manner to simulate infinite conductors running in 
the Y direction. Coils 130 and 132 are also electrically connected to 
generate opposing fields which cancel at the origin of the coordinate 
system. 
Position-independent changes in main field B.sub.0 can be effected, if 
desired, by .DELTA.B coils 134 and 136, disposed about the outside of pole 
tips 116 and 118 and electrically connected in series such that they 
provide an additive magnetic field (with respect to each other). For 
further description of suitable coils for producing magnetic gradients 
reference is made to the aforementioned U.S. Pat. No. 4,015,196 issued 
Mar. 29, 1977 to Moore et al. 
RF coil 114 is electrically connected through an appropriate matching 
impedance network 138 to one terminal of a directional network or circular 
(magic-T network) 140. Impedance matching network 138 suitably comprises a 
pair of variable capacitors for tuning RF coil 114. The variable 
capacitors are suitably of a non-magnetic material such as copper or brass 
and preferably are situated as closely as possible to RF coil 114. 
Magic-T network 140 selectively couples RF coil 114 (in a mutually 
exclusive manner) to transmitter 142 and to a preamplifier 144. As is well 
known in the art, magic-T network 140 operates to connect transmitter 142 
to the coil during such times that transmitter 142 is transmitting, and 
connects coil 114 to preamplifier 144 during such periods when transmitter 
142 is not transmitting. 
Preamplifier 144 is suitably of conventional type, providing a high gain 
and wide bandwidth. It is desirable to switch the preamplifier off during 
transmission or interim periods to provide greater isolation from 
transmitter 142. Accordingly, a gated preamplifier can be utilized. The 
output signals from preamplifier 144 are applied through diode detector 
146 and lowpass filter 148, if desired, for monitoring during tuning of 
the system, and are applied through a buffer 150 to a demodulator 152. 
Demodulator 152 is suitably a quadrature demodulator, so that not only 
frequency offset (from the center frequency, as will be explained) and 
amplitude information are provided, but the sign of the frequency offset 
can be determined as well, for relating the respective frequency 
components to positions in object 100 on opposite sides of the origin. 
Further, use of quadrature demodulation avoids phase errors due to circuit 
delays. Demodulator 152 is receptive of signals indicative of the 
transmitted signal (as well as the received signal), and generates inphase 
(I) and quadrature (Q) output signals (the Q output signal being at 
90.degree. phase relative to the I output signal). The I and Q output 
signals include components indicative of the sum and difference of the 
received signal frequency and the transmitted signal frequency. The spin 
echo signal (minus the carrier frequency) is the vector sum of the I and Q 
signals. A more detaled description of a suitable demodulator 152 will be 
made in conjunction with FIG. 17. 
The I and Q demodulator output signals are applied through respective 
lowpass filters 154, to a two channel analog-to-digital (A/D) converter 
156. A/D converter 156 is, in turn, connected through a direct memory 
access (DMA) interface 158 to a suitable computer 160. 
Transmitter 142 is suitably a class A amplifier having bandwidth sufficient 
to cover a desired bandwidth of Larmor frequencies, and is driven by 
signals from a modulator 162. Modulator 162 suitably comprises a balanced 
mixer (filtered) and is receptive of a pulse shape control signal from 
suitable microcomputer control 164 (as will be hereinafter described) and 
a signal at a desired Larmor carrier (center) frequency. 
The Larmor carrier frequency signal is suitably developed from the output 
signal of a crystal oscillator 166 by a frequency synthesizer/phase 
generator 168 in a phase locked relationship with crystal 166 and suitable 
bandpass filter and level setting circuitry 170. Bandpass filter 170 
operates to develop a sine wave with a predetermined constant envelope 
from the output of frequency synthesizer. A more detailed description of a 
suitable frequency synthesizer/phase generator 168 will be hereinafter 
provided in conjunction with FIG. 17. The generation of the Larmor center 
frequency signal is performed responsive to control signals from 
microcomputer controller 164. 
Microcomputer controller 164, in essence, controls the sequence of events 
within the NMR system: the interfacing of a computer 160 with the system, 
display of data such as by a CRT terminal 172, the generation of field 
gradients, and the timing, amplitude, frequency and phase of transmitted 
electromagnetic signals. Microcomputer controller 164 is suitably based on 
a microcomputer such as an LSI-11. A microcomputer such as the LSI-11 can 
be modified for more rapid operation (for example, with respect to turning 
on or off the carrier signal, selection of gradient direction and 
selection of phase) by addition of special purpose hardwired interface 
circuits. Reference is made to "An NMR Sequencer for Imaging," by J. 
Hoenninger and L. Crooks in press. 
In effecting generation of the magnetic field gradients, microcomputer 
controller 164 generates respective control signals indicative of desired 
gradient values and particular gradient directions. A similar control 
signal indicative of particular values of .DELTA.B and/or .DELTA.f are 
generated, if desired. The gradient value control signals, and/or the 
.DELTA.B signal, are applied to conventional voltage-to-current converters 
and amplifiers 174 which, in turn, apply the gradient signals to the 
appropriate (g.sub.z) coils 122, 124; (g.sub.x) coils 126, 128; and 
(g.sub.y) coils 130, 132 and applies the .DELTA.B signals to coils 134 and 
136. 
Microcomputer controller 164 provides control signals representative of the 
desired RF pulse shape to modulator 162 to thus set the amplitude and 
duration of the electromagnetic signals and, accordingly, the spin 
reorientation (nutation) angle, e.g., 90.degree., 190.degree., effected by 
the signal. The desired electromagnetic pulse shape and amplitude scale 
factor are maintained in memory and selectively utilized to develop the 
control signals. 
In the selective irradiation process, it is desirable that the 
electromagnetic pulses have a narrow band frequency spectrum. Accordingly, 
a (sin t)/t pulse shape is utilized (which provides an almost square 
frequency spectrum). A Gaussian pulse shape (which provides a Gaussian 
frequency spectrum) has also been considered. 
Microcomputer controller 164 also suitably includes provisions for 
controlling sampling by A/D converter 156 and for controlling transmission 
of data from DMA interface 158 to computer 160. Responsive to signals from 
microcomputer controller 164, A/D converter 156 takes a predetermined 
number of samples of the demodulated signals, and transmits the data to 
main computer 160 (memory 176) through DMA 158. When a group of samples is 
received by main computer 160, they are stored in the appropriate 
locations in memory 176. The programming of microcomputer controller 164 
and main computer 160 are coordinated such that the data received from DMA 
158 can be properly interpreted with respect to sequence. Thereafter a 
Fourier transform of the data is performed and the Fourier transforms of 
respective line scans are displayed on CRT display 178. If desired, 
provisions can be made for controlling servo-mechanisms for positioning 
the specimen (object 100) with respect to the various coils. 
With reference now to FIG. 17, a more detailed description of a suitable 
quadrature demodulator 152 and a suitable frequency synthesizer/phase 
generator 168 will be provided. To minimize leakage, frequency 
synthesizer/phase generator 168 and demodulator 152 suitably operate 
primarily with intermediate frequencies other than the frequency actually 
transmitted by transmitter 142 and received by preamplifier 144. In this 
regard, reference is made to U.S. Pat. No. 3,651,396 issued Mar. 21, 1972 
to Hewitt et al. Accordingly, crystal oscillator 166 provides a 
square-wave output signal of predetermined frequency (10 MHz) to frequency 
synthesizer/phase generator 168. Assuming the desired Larmor frequency to 
be on the order of 15 MHz, the 10 MHz oscillator signal is then suitably 
applied to a frequency divider 180 (.div.5), to develop a 2 MHz signal. 
The 2 MHz signal is suitably applied to a programmable frequency 
synthesizer 182 and to a phase locked loop 184. Programmable frequency 
synthesizer 182 is receptive of control signals indicative of the desired 
frequency output from microcomputer controller 164. The output signal of 
frequency synthesizer 182 is applied through a gate 186, under the control 
of microcomputer controller 164 and lowpass filter 188, to a conventional 
balanced mixer 190. 
Phase locked loop 184 suitably includes a voltage controlled oscillator 
operating about a predetermined center frequency such as 52 MHz, phase 
locked to the 2 MHz signal. The 52 MHz output signal of phase locked loop 
184 is suitably applied to a conventional four phase generator 192 
providing signals at the 13 MHz intermediate frequency with relative 
phases of 0.degree., 90.degree., 180.degree., and 270.degree.. The output 
signals of phase generator 192 are applied to suitable selector gating 
circuitry 194, responsive to control signals from microcomputer controller 
164. The selected intermediate frequency signal is passed through a 
lowpass filter 196 to mixer 190 to develop a difference signal component 
at the desired Larmor carrier frequency. 
Assuming the desired frequency to be 15 MHz, programmable frequency 
synthesizer 182 is set by microcomputer controller 164 to provide a 28 MHz 
signal, whereby the difference component produced by modulator 190 (28 
MHz-13 MHz) is at the desired 15 MHz frequency. The difference component 
is extracted by bandpass filter and level setting circuitry 170 to produce 
a sinewave of constant envelope at the desired Larmor carrier frequency. 
The output of programmable frequency synthesizer 182 is also applied to a 
buffer amplifier 198 in demodulator 152. The buffered signal is applied 
through a gate 200 (under the control of microcomputer controller 164) to 
a balanced mixer 202. Mixer 202 is also receptive of the received signal 
as amplified by preamp 144 and buffer 150. In the example wherein the 
transmitted carrier frequency is 15 MHz, and the synthesizer output 28 
MHz, the difference component of the output signal of mixer 202 will be at 
the intermediate frequency 13 MHz. The difference component is extracted 
from the mixer output signal by a lowpass filter 204 and is applied to a 
suitable IF amplifier 206 tuned to 13 MHz. The output signals from IF 
amplifier 206 are applied to respective balanced mixers 208 and 210. 
Mixers 208 and 210 are respectively receptive of the 0.degree. and 
90.degree. phase intermediate signals from phase generator 192 in 
frequency synthesizer/phase generator 168. Thus, mixers 208 and 210 
respectively provide inphase and quadrature audio frequency signals 
indicative of the respective magnetization components of the spin echo 
signal. 
As noted above, the I and Q output signals are filtered and sampled, then 
stored in memory 176 of computer 160. Computer 160, in effect, computes 
the vector sum of the I and Q components and performs a Fourier transform 
on the vector sum to develop indicia of the indicative amplitudes of the 
respective frequency components of the spin echo signals. 
It should be appreciated that since demodulator 152 and frequency 
synthesizer/phase generator 168 operate primarily at intermediate 
frequencies other than the Larmor carrier frequency, leakage from the 
transmitter into demodulator 152 is substantially reduced. 
By way of example, the operation of the apparatus of FIGS. 15A, 15B, 16 and 
17 during a single line scan will be described. The particular desired 
sequences of selective irradiations is entered (or recalled from memory) 
and the sequence initiated upon a ready signal from main computer 160 to 
microcomputer controller 164. Main magnet 120 is activated to bring the 
nuclear spins into initial alignment. 
Microcomputer controller 164 computes or recalls from memory or a lookup 
table, the pulse shapes, amplitudes, phases and durations necessary to 
effect 90.degree. and 180.degree. spin nutations, the desired respective 
time intervals between pulses and the desired values of the gradients. 
Appropriate control signals are generated to selectivity irradiate a 
particular volume (Y-Z plane 104, FIG. 2) with a 90.degree. pulse. An 
appropriate signal is applied to g.sub.x coils 126 and 128 to develop an X 
gradient across object 100. An RF pulse shape signal of appropriate 
amplitude and duration to effect the desired rotational angle (i.e., 
90.degree.) is generated and applied to modulator 162. 
Simultaneously, appropriate control signals are applied to frequency 
synthesizer/phase generator 168 to effect generation of a carrier signal 
at the Larmor frequency associated with plane 104. Programmable frequency 
synthesizer 182 (FIG. 17) is loaded with an appropriate frequency code 
from microcomputer controller 164, and a signal generation mode initiated. 
As indicated above it is desirable to change the phase of the 90.degree. 
and 180.degree. RF pulses to selectively cause the resulting FIDs and spin 
echoes to assume different polarities. Accordingly, microcomputer 
controller 164 also generates an appropriate control signal to selector 
gate 194 of frequency synthesizer/phase generator 168 (FIG. 17) to pass 
the appropriately phased IF signal to mixer 190. 
Modulator 162 thus applies a shaped RF pulse, at the desired Larmor 
frequency (and phase) to transmitter 142. Transmitter 142, accordingly, 
generates a signal through T network 140, RF switch 212 and 50 ohm 
matching network 138 to RF coil 114 and thereby irradiates object 100. 
Thus, Y-Z plane 104 in object 100 is excited to 90.degree. by a process of 
selective irradiation. 
Microcomputer controller 164 then generates appropriate gradient control 
signals of opposite polarity for a predetermined period, to generate a 
negative gradient for phase correction and then removes the gradient 
control signal from the g.sub.x coils 126, 128 and thereby turns off the X 
gradient. 
An appropriate signal is applied to g.sub.z coils 122 and 124 to develop a 
Z gradient across object 100. After a predetermined time these signals are 
removed thereby turning off the Z gradient. The timing of these events may 
be modified so that the Z gradient is applied for a time which overlaps 
the time during which the X gradient used for phase correction is applied. 
Appropriate control signals are then generated to effect selective 
irradiation of X-Z plane 106 (FIG. 2C) by a 180.degree. pulse. An RF pulse 
shape signal of amplitude and duration corresponding to a 180.degree. 
rotational angle is applied to modulator 162 and appropriate control 
signals are applied to frequency synthesizer 182 and phase selector 192 to 
set the frequency and phase of the carrier signal respectively. Gate 186 
is enabled and the modulated signal (180.degree. pulse) transmitted into 
the coil to effect the selective irradiation of plane 106. Simultaneously 
an appropriate signal is applied to g.sub.y coils 130 and 132 to develop a 
Y gradient across object 100. After the appropriate time duration has 
elapsed the transmitter and gradient are turned off via gate 186 and the 
gradient value output of microcomputer controller 164. 
After a time period in accordance with the rule of equal times, the line of 
intersection 108 (FIG. 2D) of the transverse planes (104 and 106) 
generates a spin echo. At a time just prior to the expected generation of 
the spin echo, appropriate control signals are used to apply a gradient 
value signal to g.sub.z coils 122 and 124, and thus develop a Z gradient 
across object 100. Gate 200 is enabled to, in effect, turn on the 
demodulator. 
The spin echo signals are induced in coil 114 and are applied through T 
network 140 to preamplifier 144 and thereafter through buffer 150 to 
quadrature demodulator 152. Quadrature demodulator 152 in cooperation with 
lowpass filter 154 produces an audio signal which is selectively sampled 
at a predetermined rate by A/D converter 156 in response to control 
signals from microcomputer controller 164. The digitized samples are then 
loaded into memory 176 through interface 158 and computer 160. 
The sequence is repeated a predetermined number of times and appropriate 
signal averaging performed by computer 160. Fourier transforms are then 
performed by computer 160 and the results displayed on CRT display 178. 
Where a multi-line scan is to be effected, subsequent selective 
irradiations of parallel planes are performed by either changing the 
carrier frequency by appropriate loading of programmable frequency 
synthesizer 182 with the Larmor frequency .+-..DELTA.f and/or appropriate 
changes made in the basic magnetic field by application of appropriate 
.DELTA.B signals to .DELTA.B coils 134 and 136. 
It should be appreciated that the above described apparatus is illustrative 
of the various types of apparatus that can be used to practice spin echo 
scanning sequences in accordance with the present invention. It should be 
appreciated that other apparatus can be utilized as well. For example, it 
is presently thought that the apparatus described in the Garroway et al 
U.S. Pat. No. 4,021,726 can be adapted for a spin echo scanning sequence. 
Further, it should be noted that while the various conductors shown 
interconnecting the elements of FIGS. 15A, 15B, 16 and 17 are shown as 
single lines, they are not so shown in a limiting sense and may comprise 
plural connections as is understood in the art. 
It will be understood that the above description is of illustrative 
embodiments of the present invention, and that the invention is not 
limited to the specific forms shown. Modifications may be made in the 
sequence or arrangements of pulses or in the design or arrangement of 
elements without departing from the spirit of the invention as expressed 
in the appended claims.