NMR probe for imaging small samples

A small sample imaging apparatus replaces the dewar which normally passes along the axis of an NMR imaging probe. The small sample imaging apparatus incorporates an integral RF coil and capacitor resonant circuit. The coil and capacitor are positioned adjacent to each other and remotely at the end of a projecting stalk. Stray inductance effects are thereby avoided and the stalk physically positions the resonant circuit at substantially the properly centered location. The stalk which positions the coil and capacitor may be formed from a length of rigid coaxial cable which carried RF energy to the resonant circuit. When large samples are imaged, the conventional probe coil insert can be utilized in the normal manner. However, when smaller samples are images, the adapter can be attached to the probe and the adaptor resonant circuit can be utilized.

FIELD OF THE INVENTION 
This invention relates to imaging of solids and liquids produced by nuclear 
magnetic resonance (NMR) imaging of NMR-active nuclei and, in particular, 
to an improved .mu.-imaging probe for imaging small solid and liquid 
samples. 
BACKGROUND OF THE INVENTION 
Nuclear magnetic resonance is a phenomenon exhibited by a select group of 
atomic nuclei and is based upon the existence of nuclear magnetic moments 
in these nuclei (termed "NMR active" nuclei). When an NMR active nucleus 
is placed in a strong, uniform and steady magnetic field, the spin 
magnetization of the nucleus precesses at a natural resonance frequency 
known as the Larmor frequency, which is characteristic of each nuclear 
type and is proportional to the applied field strength at the location of 
the nucleus. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 
C, .sup.19 F and .sup.31 P. The resonant frequencies of the nuclei can be 
observed by monitoring with an RF receiver the transverse magnetization 
which results after a strong RF pulse is applied at, or near, the Larmor 
frequency. 
In order to use the NMR phenomenon to obtain an image of a sample, a 
magnetic field is applied to the sample, along with a magnetic field 
gradient which depends on physical position so that the field strength at 
different sample locations differs. When a field gradient is introduced, 
as previously mentioned, since the Larmor frequency for a particular 
nuclear type is proportional to the applied field strength, the Larmor 
frequencies of the same nuclear type will vary across the sample and the 
frequency variance will depend on physical position. By suitably shaping 
the applied magnetic field and processing the resulting NMR signals for a 
single nuclear type, a nuclear spin density image of the sample can be 
measured. Because the measured NMR signal is a function of the total 
number of nuclei of a given type, it is common to use a nucleus which is 
found in abundance in the sample to be imaged. For example, .sup.1 H 
(protons) are commonly used because they are abundant in many materials 
and therefore, generate a large NMR signal. 
FIG. 1A illustrates a portion of a prior art NMR imaging apparatus. As 
discussed above, the sample must be placed in a uniform magnetic field, a 
field gradient must be applied and RF pulses must also be applied in order 
to obtain the image. Accordingly, in conventional NMR spectrometers, the 
sample is usually mounted in a "probe" device for performing the actual 
imaging experiment. In FIG. 1A, probe 1 is utilized to hold and lower the 
sample into a magnet chamber (not shown) which provides the constant 
magnetic field. The probe comprises a hollow body 2. Passing through body 
2 is a hollow copper tube 4. Several insulating platform plates 6, 8 and 
10 are transversely mounted on the end of copper tube 4 which support and 
physically space the probe components. The sample is actually positioned 
within an RF coil 3 that serves a number of functions as described below. 
The RF coil is, in turn, surrounded by a gradient coil set 12 mounted on a 
hollow insulting form 14 which slides over coil 3 and electrically 
connects to the NMR probe by means of plug 16 and socket 20. In 
conventional systems, the coil set 12 may, for example, contain a Golay 
coil which generates a magnetic field gradient in a known manner. The coil 
set is positioned over coil 3 by means of platform plates 6, 8 and 10 
which closely fit to the inner diameter 22 of coil form 14. A dewar 19 
generally extends through tube 4 to allow cool or hot air to be blown over 
coil 3 to provide the system with the capability of varying the 
temperature of the coil during operation. 
In order to properly accomplish NMR imaging, the probe device utilized 
should satisfy several design parameters. First, it is important to use an 
RF coil with a size that approximates that of the sample being imaged in 
order to make the system efficient and to improve the signal-to-noise 
ratio. More particularly, RF coil 3 serves two purposes. First, it 
transmits the strong RF pulse to the sample that is required to nutate the 
spin magnetization into a plane transverse to the static magnetic field 
direction and, second, it is used to receive the NMR signals generated by 
the nuclei. With regard to RF pulse transmission, it is well-known that 
physically larger coils require more energy than smaller coils to generate 
an RF field of a given strength at points within the coil. Since a coil 
sized slightly larger than the sample will deliver sufficient RF energy at 
all points within the sample to perform the experiment, a coil that is 
much larger than the sample will generate a significant field outside of 
the sample, thereby wasting much of the energy utilized to generate the RF 
pulse. Consequently, such a system is inefficient. 
With regard to signal reception, only portions of the RF coil that are 
close to the sample are capable of gathering the weak NMR signals. Those 
portions of the coil remote from the sample receive only noise. Therefore, 
the RF coil should be approximately equal in size to the sample being 
imaged to ensure that it will gather all the NMR signals emitted from the 
sample without unduly increasing the amount of noise received. 
Consequently, in order to facilitate the imaging of various size samples, 
the prior art system is capable of accommodating various RF inserts 15, 
each having a differently-sized RF coil so that the prior art probe can 
easily accommodate different-sized samples. Such an insert is shown in 
FIG. 1B. Each RF insert 15' includes a disk insulator 18' which 
incorporates a pair of connectors 13' which slide over the extension posts 
11 in order to physically and electrically connect insert 15' to probe 1. 
Each RF insert 15' also includes a pair of leads 17' that extend from 
connectors 13' to electrically connect to RF coil 3'. 
In the probe, RF coil 3 is connected to adjustable capacitors 5 to form a 
resonant circuit. Each adjustable capacitor 5 is connected to a tuning 
handle 7 via a tuning rod 9 so that the capacitors can be manually 
adjusted, thereby enabling the RF resonance circuit to be tuned. 
For reasons known to those skilled in the art, it is important to center 
the RF coil in the middle of the constant magnetic field and the gradient 
field. Consequently, when in position, the RF insert is slid over 
extension posts 11 until the coil is centered. In order to accommodate 
coils of relatively large diameters, the RF inserts 15' are designed so 
that the coil centers are positioned several inches away from the 
insulator disk 18'. When large diameter RF coils are used, the length of 
leads 17 which connect the coil to the system is relatively short and does 
not significantly interfere with system performance. However, when smaller 
RF coils are utilized, in order to physically center coil 3 properly, 
relatively long leads 17 must be used to connect coil 3 to the connectors 
13. At the resonance frequencies used in a typical NMR system, the stray 
inductance introduced by the leads 17 into the resonant circuit becomes 
significant in relation to the inductance of the smaller RF coil 3. This 
stray inductance does not contribute to either RF field generation or NMR 
signal gathering because it is positioned at a significant distance away 
from the sample. Therefore, the large stray inductance significantly 
reduces the efficiency of the probe circuit. 
Accordingly, it is an object of the present invention to provide an 
efficient NMR probe for small samples. 
It is another object of the present invention to provide an NMR probe which 
can accommodate samples of various sizes with efficient imaging 
capability. 
It is still another object of the present invention to provide apparatus 
which can be used with existing imaging probes to more efficiently image 
small samples. 
It is yet another object of the present invention to provide small sample 
imaging apparatus which can be used with existing .mu.-imaging probes. 
It is a further object of the present invention to provide apparatus which 
can be used with existing .mu.-imaging probes to more efficiently image 
small samples without permanently modifying the existing probe. 
It is still a further object of the present invention to provide small 
sample imaging apparatus which replaces the prior art RF coil insert with 
apparatus that efficiently utilizes a small diameter coil. 
It is yet a further object of the present invention to provide small sample 
imaging apparatus which replaces the prior art RF coil insert and reduces 
stray capacitance to improve efficiency. 
SUMMARY OF THE INVENTION 
The foregoing problems are solved and the foregoing objects are achieved in 
one illustrative embodiment of the invention in which small sample imaging 
apparatus replaces the dewar which normally passes along the axis of an 
imaging probe. The small sample imaging apparatus incorporates an integral 
RF coil and capacitor resonant circuit. The coil and capacitor are 
positioned adjacent to each other and remotely at the end of a projecting 
stalk. Stray inductance effects are thereby avoided and the stalk 
physically positions the resonant circuit at substantially the properly 
centered location. The entire apparatus can be adjusted longitudinally 
within the dewar channel to properly position the coil at the center of 
the magnetic field. 
More particularly, the stalk which positions the coil and capacitor is 
formed from a length of rigid coaxial cable which carries the RF energy to 
the resonant circuit. When large samples are imaged, the conventional 
probe coil insert can be utilized in the normal manner. However, when 
smaller samples are imaged, the adapter can be attached to the probe and 
the adapter resonant circuit can be utilized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 illustrates the small sample imaging apparatus 21 of the present 
invention into the prior art probe body that was shown in FIG. 1. The 
small sample imaging apparatus is inserted in the direction of arrow 23 
through the copper tube 4 which normally houses the dewar 19 of the prior 
art probe. When the small sample imaging apparatus is used, the dewar is 
removed and is not used. 
One key feature of the small sample imaging apparatus is that it utilizes 
its own capacitor 31 and RF coil 25 which can be matched to the size of a 
given small sample. Consequently, when the small sample imaging apparatus 
is utilized, capacitors 5 and RF coil insert 15 of the prior art probe are 
not utilized to form the resonant circuit. Because the small sample 
imaging apparatus does not utilize an RF coil insert 15, the resonant 
circuit does not include the long leads 17 that are associated therewith. 
Consequently, the small sample imaging apparatus does not suffer from the 
problem experienced by the prior art resulting from large stray inductance 
introduced into the circuit as a result of long leads. 
The remote resonant circuit of the small sample imaging apparatus is formed 
on a stalk which supports the entire circuit at substantially the correct 
physical position. The supporting stalk is formed from coaxial cable 27 
which also passes through the apparatus and is utilized to connect the 
circuit, through a conventional BNC bayonet connector 28, to a source 
capable of providing an appropriate RF pulse. The conventional coaxial 
cable is comprised of three components that are most clearly shown in FIG. 
3. The signal carrying wire 27a is enclosed within an insulating layer 27b 
which is in turn enclosed within a solid metal shield 27c. In the 
preferred embodiment of the invention, the signal carrying wire 27a is 
connected, via a lead 29, to the RF sample coil 25. The small RF coil is 
further connected, via a second lead 29, to a variable capacitor 31. The 
variable capacitor 31 can be adjusted through the use of the tuning wand 
33. A metal supporting bracket 35 has a hole through which cable 27 
passes. Bracket 35 is soldered to the outer shield 27c of cable 27 to form 
a rigid connection. Bracket 35 is also provided with a threaded opening 34 
into which a threaded nipple on capacitor 31 is screwed. Bracket 35 
thereby establishes an electrical connection between one side of variable 
capacitor 31 and the metal shield 27c of coaxial cable 27. A short wire, 
37, acts as a shunt inductance between the cable conductor 27a and the 
cable shield 27c to complete the resonant circuit. An electrical schematic 
circuit diagram of the equivalent resonance circuit for the preferred 
embodiment is shown in FIG. 4A. Other resonant circuits could also be 
utilized to practice the present invention. FIGS. 4B and 4C each 
demonstrate an electrical schematic circuit diagram for resonant circuits 
that could be utilized in alternate embodiments of the invention. In the 
circuit shown in FIG. 4B, the RF coil 25B is connected with three variable 
capacitors 31B as shown. In the circuit shown in FIG. 4C, the RF coil 25C 
is connected in series with two variable capacitors 31C. 
The remainder of the small sample imaging apparatus is comprised of a 
tubular body 41 that extends through the length of the existing probe. In 
the preferred embodiment of the invention, body 41 is made from aluminum 
and the outer surface is copper-plated and electro-flashed with a rare 
earth metal such as, for example, rhodium. This is a typical manufacturing 
process which is used because the finish allows solder to adhere to the 
body 41, but does not tarnish. The tubular body 41 is open at each end. On 
the body end closest to the RF coil 25, 41 is closed by a top cover 43 
which is illustratively formed from the same material as the body 41 and 
press-fitted into the end of body 41. Cover 43 is provided with holes 45 
and 47 for respectively allowing the passage of the tuning wand 33 and 
coaxial cable 27; the holes 45 and 47 are best shown in FIG. 5. At this 
top end of the apparatus, there is no interconnection between the small 
sample imaging apparatus and the existing probe structure. Therefore, body 
41 can be adjustably slid relative to the existing probe in order to 
perform small alignment adjustments. The opposite end of body 41 (as shown 
in FIG. 6) is provided with an annular flange 75 that extends around its 
circumference. The annular flange 75 is, in turn, provided with a key 77 
extending out therefrom. This end of body 41 is also internally tapped at 
79. An internally threaded collar 85 fits over the end of body 41 and a 
shoulder 89 of the collar 85 bears against flange 75. Body 41 is completed 
by a threaded cap 83 having an externally threaded nipple 81 which screws 
into threads 79 of body 41. Cap 83 holds collar 85 captive, but rotatable. 
Body 41 is also not connected to the interior surface of the existing probe 
tube 4.. However, a sliding contact is made between body 41 and tube 4 as 
body 41 is friction fitted within the aluminum tube. Additionally, a 
frictional and electrical connection is formed between the body 41 and the 
aluminum tube 4 by means of a set of retractable springs. 
More particularly, body 41 is provided with a series of longitudinal 
grooves 51 that are best shown in FIG. 6. In the preferred embodiment of 
the invention, three grooves are utilized and each is spaced equally 
around the circumference of body 41. Each groove 51 is provided with a 
pair of slanted holes 53, with one hole being positioned at each end of 
the groove; the slanted holes are illustrated in FIGS. 6 and 7. A spring 
55 is provided within each groove and is retained by its elongate ends 
which extend through the holes 53 at opposite ends of the groove. In the 
preferred embodiment of the invention, the springs are formed from 
non-magnetic spring material such as, for example, phosphor-bronze wire. 
Each spring 55 extends slightly out of its corresponding groove beyond the 
outer surface of the body 41 and contacts the inner surface of tube 4 when 
body 41 is in position. 
A rigid, but adjustable connection is provided between the opposite end of 
the small sample imaging apparatus and the end of the existing probe body 
and is shown in FIGS. 2, 6 and 8. FIG. 8 illustrates the additional 
components utilized to support the apparatus within the existing probe. A 
hollow cylindrical insert support 57 is attached to the body of the 
existing probe 59 via right-angle brackets 61 as shown in FIG. 2. Insert 
support 57 slides over apparatus body 41 and is provided with opposing 
tapped holes 63 which accept set screws 65. The insert support further has 
a male threaded end 67 and a counterbored section 68 which accommodate a 
sliding stop as discussed below. In the preferred embodiment of the 
invention, the insert support is formed from the same copper-plated and 
electro-flashed aluminum that is utilized to form the body 41. 
A hollow cylindrical sliding stop 69 slides into counterbore 68 and 
illustratively is also formed from the same material as the body 41. Stop 
69 is utilized to longitudinally adjust the position of the small sample 
apparatus relative to the existing probe body as is more fully described 
below. Additionally, stop 69 is also utilized to adjust the rotational 
position of the small sample apparatus relative to the existing probe body 
so that the RF coil can be aligned with an appropriate gradient produced 
by the magnet chamber in order to accomplish NMR imaging. Normally, 
rotational and longitudinal alignment is performed with the the probe in 
place while a simple one-dimensional imaging experiment is being 
conducted. 
Once the proper rotational position of the small sample apparatus is 
determined, set screws 65 slide in opposing tapped holes 63 and are 
tightened until they contact the stop 69, thereby preventing rotation of 
the stop 69 relative to the support 57. Stop 69 is provided with a stop 
face 71 in which a keyway 73 is located. Keyway 73 accommodates key 77 
(FIG. 6) located at the end of the apparatus body 41. Therefore, once the 
key 77 of the apparatus body 41 is inserted into the keyway 73, the 
rotational position of the small sample apparatus is fixed relative to the 
existing probe body. 
Stop 69 can also be slid longitudinally to position the RF coil at the 
proper location in the magnetic field as described above. After this 
positioning has been determined, the stop 69 is locked into position 
relative to the support 57, and consequently the existing probe body, by 
tightening the set screws 65. Thereafter, collar 85 is turned to engage 
the threaded end 67 of support 57. Flange 75 is thereby drawn up against 
stop face 71 as shown in FIG. 2 to rigidly lock the body 41 into the probe 
body 4. 
As shown in FIGS. 6 and 9, cover 83 is provided with a hole 91 that enables 
the tuning wand 33 to pass therethrough and is also provided with a tapped 
hole 92. Hole 92 accommodates a split sleeve 95 for securing the coaxial 
cable 27 to body 41. In particular, coaxial cable 27 is passed through a 
hole 97 in the sleeve 95 as well as the passageway 93. The threaded nipple 
96 of sleeve 95 is slightly tapered and slotted so that when sleeve 95 is 
screwed into tapped hole 92, nipple 96 tightens around the outer sheath of 
cable 27 locking it in place. 
Although the small sample imaging apparatus of the present invention 
enables improved system performance for imaging small samples, it does not 
require the use of a separate and discrete imaging probe. Consequently, 
when large samples are imaged, the small sample imaging apparatus of the 
present invention need not be utilized. Rather, the imaging probe can 
simply be utilized, along with an RF insert 15, in the-manner described 
with regard to the prior art probe illustrated in FIG. 1. 
It should be understood the various changes and modifications of the 
embodiment shown in the drawings may be made within the scope of this 
invention. Thus, it is intended that all matter contained in the above 
description and shown in the accompanying drawings shall be interpreted in 
an illustrative, and not limiting, sense.