Frequency calibration for MRI scanner

An NMR scanner performs a prescan before each NMR scan sequence in which the optimal RF excitation frequency is automatically determined and applied to the scanner's transceiver. The prescan sequence includes a pair of NMR measurements which provide data that allows the precise RF excitation frequency to be determined.

BACKGROUND OF THE INVENTION 
This invention relates to magnetic resonance (NMR) techniques. More 
specifically, this invention relates to the automatic adjustment of the RF 
transmitter and receiver to the optimal Larmor frequency. 
The magnetic resonance phenomenon has been utilized in the past in high 
resolution magnetic resonance spectroscopy to analyze the structure of 
chemical compositions. More recently, NMR has been developed as a medical 
diagnostic modality having applications in imaging the anatomy, as well as 
in performing in vivo, noninvasive spectroscopic analysis. As is now well 
known, the NMR phenomenon can be excited within a sample object, such as a 
human patient, positioned in a homogeneous polarizing magnetic field, 
B.sub.0, by irradiating the object with radio frequency (RF) energy at the 
Larmor frequency. In medical diagnostic applications, this is typically 
accomplished by positioning the patient to be examined in the field of an 
RF coil having a cylindrical geometry, and energizing the RF coil with an 
RF power amplifier. Upon cessation of the RF excitation, the same or a 
different RF coil is used to detect the NMR signals, frequently in the 
form of spin echoes, emanating from the patient lying within the field of 
the RF coil. In the course of a complete NMR scan, a plurality of NMR 
signals are typically observed. The NMR signals are used to derive NMR 
imaging or spectroscopic information about the patient being imaged or 
studied. 
Before the commencement of each NMR scan, it is common practice to adjust 
the frequency of the RF transmitter and receiver to insure that the 
excitation field is at the optimal Larmor frequency. This is necessary to 
produce the desired image contrast effects in certain NMR measurements and 
to insure the accuracy of slice selection location. In a human subject, 
for example, the NMR signal is produced primarily by the protons in water 
and fat molecules. The Larmor frequency of the protons in these two 
substances is slightly different and the Larmor frequency of both will 
vary slightly from patient to patient and at different locations within a 
patient due to inhomogeneities. In prior NMR scanners, it is common 
practice to perform a calibration sequence in which an NMR sequence is 
first executed and the NMR signal is processed to produce on a CRT screen 
a picture of signal amplitude versus RF frequency. The operator then 
examines this picture and manually adjusts the frequency of the RF 
receiver to desired value. For example, the displayed NMR signal may show 
two peaks, one at the Larmor frequency for fat protons and one at the 
Larmor frequency for water protons. The operator may choose either 
frequency, or a frequency therebetween, depending on the particular NMR 
measurement to be conducted. 
A method for automatically calibrating the RF frequency of an MRI scanner 
is described in U.S. Pat. No. 4,806,866. In this method an NMR signal is 
acquired from the region of interest and transformed to the frequency 
domain where the two peaks for fat and water are found by filtering the 
power spectrum signal and identifying peaks which are spaced apart the 
proper amount. All peaks are identified by taking the first derivative of 
the filtered power spectrum and searching for points where the derivative 
is zero. This method has worked well in most applications to automatically 
set the MRI scanner frequency to the precise Larmor frequency of fat or 
water, or to a frequency therebetween, such as the midpoint or certroid. 
However, it has been discovered that in some applications this method 
routinely finds the wrong peaks in the filtered power spectrum and does 
not accurately set the MRI scanner to the proper RF frequency. As a 
result, the images produced by pulse sequences that are particularly 
sensitive to RF excitation frequency are unsatisfactory, and the contrast 
characteristics of images produced with other pulse sequences are varied. 
SUMMARY OF THE INVENTION 
The present invention is an improvement to an NMR scanner in which the 
adjustment of RF frequency is made automatically during a sequence 
performed just prior to each NMR scan. More specifically, NMR scanner 
performs a first, wideband NMR measurement in which the frequency of the 
highest peak in the NMR signal is determined at the region of interest in 
the subject, the RF transmit and receive frequencies are set to this 
determined frequency, a second, narrowband NMR measurement is made to 
obtain a second NMR signal from the region of interest, the NMR signal is 
analyzed to determine the frequencies of the fat and water peaks therein 
by transforming the NMR signal to the frequency domain, correlating the 
transformed NMR signal with a correlation model of the fat and water peaks 
to produce a correlation frequency, locating the water peak frequency to 
one side of the correlation frequency and locating the fat peak frequency 
to the other side of the correlation frequency, and the frequency of the 
RF transmitter and RF receiver is set with respect to these determined 
frequencies. 
A general object of the invention is to improve the accuracy of the RF 
frequency calibration of an NMR system. The correlation model is an 
idealized representation of the fat and water peaks that should be present 
in the transformed NMR signal. The correlation process "slides" this model 
over the NMR signal spectrum and locates the frequency where it fits best. 
The water and fat peaks should be located on opposite sides of this 
correlation frequency at one half the chemical shift expected between the 
fat and water peaks. This technique improves performance particularly well 
when the NMR signal spectrum has many candidate peaks, since the 
correlation process picks out the pair of peak candidates which matches 
best to the expected spectrum. 
The foregoing and other objects and advantages of the invention will appear 
from the following description. In the description, reference is made to 
the accompanying drawings which form a part hereof, and in which there is 
shown by way of illustration a preferred embodiment of the invention. Such 
embodiment does not necessarily represent the full scope of the invention, 
however, and reference is made therefore to the claims herein for 
interpreting the scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, there is shown the major components of a 
preferred NMR system which incorporates the present invention and which is 
sold by the General Electric Company under the trademark "SIGNA". The 
operation of the system is controlled from an operator console 100 which 
includes a console processor 101 that scans a keyboard 102 and receives 
inputs from a human operator through a control panel 103 and a plasma 
display/touch screen 104. The console processor 101 communicates through a 
communications link 116 with an applications interface module 117 in a 
separate computer system 107. Through the keyboard 102 and controls 103, 
an operator controls the production and display of images by an image 
processor 106 in the computer system 107, which connects directly to a 
video display 118 on the console 100 through a video cable 105. 
The computer system 107 is formed about a backplane bus which conforms with 
the VME standards, and it includes a number of modules which communicate 
with each other through this backplane. In addition to the application 
interface 117 and the image processor 106, these include a CPU module 108 
that controls the VME backplane, and an SCSI interface module 109 that 
connects the computer system 107 through a bus 110 to a set of peripheral 
devices, including disk storage 111 and tape drive 112. The computer 
system 107 also includes a memory module 113, known in the art as a frame 
buffer for storing image data arrays, and a serial interface module 114 
that links the computer system 107 through a high speed serial link 115 to 
a system interface module 120 located in a separate system control cabinet 
122. 
The system control 122 includes a series of modules which are connected 
together by a common backplane 118. The backplane 118 is comprised of a 
number of bus structures, including a bus structure which is controlled by 
a CPU module 119. The serial interface module 120 connects this backplane 
118 to the high speed serial link 115, and pulse generator module 121 
connects the backplane 118 to the operator console 100 through a serial 
link 125. It is through this link 125 that the system control 122 receives 
commands from the operator which indicate the scan sequence that is to be 
performed. 
The pulse generator module 121 operates the system components to carry out 
the desired scan sequence. It produces data which indicates the timing, 
strength and shape of the RF pulses which are to be produced, and the 
timing of and length of the data acquisition window. The pulse generator 
module 121 also connects through serial link 126 to a set of gradient 
amplifiers 127, and it conveys data thereto which indicates the timing and 
shape of the gradient pulses that are to be produced during the scan. The 
pulse generator module 121 also receives patient data through a serial 
link 128 from a physiological acquisition controller 129. The 
physiological acquisition control 129 can receive a signal from a number 
of different sensors connected to the patient. For example, it may receive 
ECG signals from electrodes or respiratory signals from a bellows and 
produce pulses for the pulse generator module 121 that synchronizes the 
scan with the patient's cardiac cycle or respiratory cycle. And finally, 
the pulse generator module 121 connects through a serial link 132 to scan 
room interface circuit 133 which receives signals at inputs 135 from 
various sensors associated with the position and condition of the patient 
and the magnet system. It is also through the scan room interface circuit 
133 that a patient positioning system 134 receives commands which move the 
patient cradle and transport the patient to the desired position for the 
scan. 
The gradient waveforms produced by the pulse generator module 121 are 
applied to a gradient amplifier system 127 comprised of G.sub.x, G.sub.y 
and G.sub.z amplifiers 136, 137 and 138, respectively. Each amplifier 136, 
137 and 138 is utilized to excite a corresponding gradient coil in an 
assembly generally designated 139. The gradient coil assembly 139 forms 
part of a magnet assembly 141 which includes a polarizing magnet 140 that 
produces either a 0.5 or a 1.5 Tesla polarizing field that extends 
horizontally through a bore 142. The gradient coils 139 encircle the bore 
142, and when energized, they generate magnetic fields in the same 
direction as the main polarizing magnetic field, but with gradients 
G.sub.x, G.sub.y and G.sub.z directed in the orthogonal x-, y- and z-axis 
directions of a Cartesian coordinate system. That is, if the magnetic 
field generated by the main magnet 140 is directed in the z direction and 
is termed B.sub.0, and the total magnetic field in the z direction is 
referred to as B.sub.z, then G.sub.x =.differential.B.sub.z 
/.differential.x, G.sub.y =.differential.B.sub.z /.differential.y and 
G.sub.z =.differential.B.sub.z /.differential.z, the magnetic field at any 
point (x,y,z) in the bore of the magnet assembly 141 is given by 
B(x,y,z)=B.sub.0 +G.sub.x X+G.sub.y YG.sub.z Z. The gradient magnetic 
fields are utilized to encode spatial information into the NMR signals 
emanating from the patient being scanned. 
Located within the bore 142 is a circular cylindrical whole-body RF coil 
152. This coil 152 produces a circularly polarized RF field in response to 
RF pulses provided by a transceiver module 150 in the system control 
cabinet 122. These pulses are amplified by an RF amplifier 151 and coupled 
to the RF coil 152 by a transmit/receive switch 154 which forms an 
integral part of the RF coil assembly. Waveforms and control signals are 
provided by the pulse generator module 121 and utilized by the transceiver 
module 150 for RF carrier modulation and mode control. The resulting NMR 
signals radiated by the excited nuclei in the patient may be sensed by the 
same RF coil 152 and coupled through the transmit/receive switch 154 to a 
preamplifier 153. The amplified NMR signals are demodulated, filtered, and 
digitized in the receiver section of the transceiver 150. The 
transmit/receive switch 154 is controlled by a signal from the pulse 
generator module 121 to electrically connect the RF amplifier 151 to the 
coil 152 during the transmit mode and to connect the preamplifier 153 
during the receive mode. The transmit/receive switch 154 also enables a 
separate RF coil (for example, a head coil or surface coil) to be used in 
either the transmit or receive mode. 
In addition to supporting the polarizing magnet 140 and the gradient coils 
139 and RF coil 152, the main magnet assembly 141 also supports a set of 
shim coils 156 associated with the main magnet 140 and used to correct 
inhomogeneities in the polarizing magnet field. The main power supply 157 
is utilized to bring the polarizing field produced by the superconductive 
main magnet 140 to the proper operating strength and is then removed. 
The NMR signals picked up by the RF coil 152 are digitized by the 
transceiver module 150 and transferred to a memory module 160 which is 
also part of the system control 122. When the scan is completed and an 
entire array of data has been acquired in the memory modules 160, an array 
processor 161 operates to Fourier transform the data into an array of 
image data. This image data is conveyed through the serial link 115 to the 
computer system 107 where it is stored in the disk memory 111. In response 
to commands received from the operator console 100, this image data may be 
archived on the tape drive 112, or it may be further processed by the 
image processor 106 and conveyed to the operator console 100 and presented 
on the video display 118. 
Referring particularly to FIGS. 1 and 2, the transceiver 150 includes 
components which produce the RF excitation field B.sub.1 through power 
amplifier 151 at a coil 152A and components which receive the resulting 
NMR signal induced in a coil 152B. As indicated above, the coils 152A and 
B may be separate as shown in FIG. 2, or they may be a single whole-body 
coil as shown in FIG. 1. The base, or carrier, frequency of the RF 
excitation field is produced under control of a frequency synthesizer 200 
which receives a set of digital signals (CF) through the backplane 118 
from the CPU module 119 and pulse generator module 121. These digital 
signals indicate the frequency and phase of the RF carrier signal which is 
produced at an output 201. The commanded RF carrier is applied to a 
modulator and up converter 202 where its amplitude is modulated in 
response to a signal R(t) also received through the backplane 118 from the 
pulse generator module 121. The signal R(t) defines the envelope, and 
therefore the bandwidth, of the RF excitation pulse to be produced. It is 
produced in the module 121 by sequentially reading out a series of stored 
digital values that represent the desired envelope. These stored digital 
values may, in turn, be changed from the operator console 100 to enable 
any desired RF pulse envelope to be produced. The modulator and up 
converter 202 produces an RF pulse at the desired Larmor frequency at an 
output 205. 
The magnitude of the RF excitation pulse output through line 205 is 
attenuated by an exciter attenuator circuit 206 which receives a digital 
command, TA, from the backplane 118. The attenuated RF excitation pulses 
are applied to the power amplifier 151 that drives the RF coil 152A. For a 
more detailed description of this portion of the transceiver 122, 
reference is made to U.S. Pat. No. 4,952,877 which is incorporated herein 
by reference. 
Referring still to FIGS. 1 and 2, the NMR signal produced by the subject is 
picked up by the receiver coil 152B and applied through the preamplifier 
153 to the input of a receiver attenuator 207. The receiver attenuator 207 
further amplifies the NMR signal and this is attenuated by an amount 
determined by a digital attenuation signal (RA) received from the 
backplane 118. The receive attenuator 207 is also turned on and off by a 
signal from the pulse generator module 121 such that it is not overloaded 
during RF excitation. 
The received NMR signal is at or around the Larmor frequency, which in the 
preferred embodiment is around 63.86 MHz for 1.5 Tesla and 21.28 MHz for 
0.5 Tesla. This high frequency signal is down converted in a two step 
process by a down converter 208 which first mixes the NMR signal with the 
carrier signal on line 201 and then mixes the resulting difference signal 
with the 2.5 MHz reference signal on line 204. The resulting down 
converted NMR signal on line 212 has a maximum bandwidth of 125 kHz and it 
is centered at a frequency of 187.5 kHz. The down converted NMR signal is 
applied to the input of an analog-to-digital (A/D) converter 209 which 
samples and digitizes the analog signal at a rate of 250 kHz. The output 
of the A/D converter 209 is applied to a digital detector and signal 
processor 210 which produce 16-bit in-phase (I) values and 16-bit 
quadrature (Q) values corresponding to the received digital signal. The 
resulting stream of digitized I and Q values of the received NMR signal is 
output through backplane 118 to the memory module 160 where they are 
employed to reconstruct an image. 
To preserve the phase information contained in the received NMR signal, 
both the modulator and up converter 202 in the exciter section and the 
down converter 208 in the receiver section are operated with common 
signals. More particularly, the carrier signal at the output 201 of the 
frequency synthesizer 200 and the 2.5 MHz reference signal at the output 
204 of the reference frequency generator 203 are employed in both 
frequency conversion processes. Phase consistency is thus maintained and 
phase changes in the detected NMR signal accurately indicate phase changes 
produced by the excited spins. The 2.5 MHz reference signal as well as 5, 
10 and 60 MHz reference signals are produced by the reference frequency 
generator 203 from a common 20 MHz master clock signal. The latter three 
reference signals are employed by the frequency synthesizer 200 to produce 
the carrier signal on output 201. For a more detailed description of the 
receiver, reference is made to U.S. Pat. No. 4,992,736 which is 
incorporated herein by reference. 
The present invention relates to the automatic adjustment of the RF carrier 
frequency produced by the synthesizer 200 in the transceiver 150. This RF 
frequency must be precisely set in order to provide optimal results from 
the NMR scanner. The optimal RF frequency usually changes from scan to 
scan, and the present invention is implemented routinely at the beginning 
of each scan as part of a "prescan" sequence in which other system 
parameters are also adjusted, or calibrated. One such adjustment, for 
example, is described in U.S. Pat. No. 5,107,215 which is entitled "RF 
Power Calibration For An NMR Scanner". The prescan sequence is executed by 
the main computer 107 in response to a set of stored program instructions 
and it produces the digital signals CF, TA and RA which are employed to 
operate the transceiver 150 as described above. 
Referring to FIG. 3, the prescan sequence is entered and the various data 
structures which it requires are initialized as indicated at process block 
250. A loop is then entered at process block 251 in which the prescan 
waits for a signal from the main scan program. The main scan program 
provides data to the prescan, such as the operator's criteria for center 
frequency selection. A test is then made at decision block 252 to 
determine if the operator has chosen the automatic frequency adjust mode 
of operation. If not, the prescan continues to perform its other functions 
and it is presumed that the operator is satisfied with the current 
frequency setting or intends to manually adjust the RF frequency after the 
automatic prescan and prior to the scan. Otherwise, a first, broadband, 
NMR measurement is performed to coarsely determine the proper RF frequency 
as indicated by process block 253. As will be explained in more detail 
below, this measurement employs the NMR signal to detect the frequency of 
its peak amplitude and this frequency is output (CF.sub.1) to the 
transceiver 150. The optimal transmitter attenuation (TA) is then 
calculated at process block 254 in accordance with the teachings in the 
above-cited U.S. patent. If either the carrier frequency (CF.sub.1) or the 
transmitter attenuation (TA) cannot be determined automatically, this is 
detected at decision block 255 and the process branches to log the error 
at 256 for display to the operator. 
Referring still to FIG. 3, if the automatic frequency adjustment mode has 
been selected, the system branches at decision block 251 and a second, 
narrowband, NMR measurement is performed at process block 258 to determine 
the exact RF frequency setting (CF.sub.2). As will be explained in more 
detail below, this second measurement and the subsequent analysis of the 
NMR signal employs data which has been input by the operator through the 
console (FIG. 1). More specifically, the RF frequency may be set to any 
one of the following frequencies: 
WATER--the Larmor frequency of the nuclei associated with water molecules; 
FAT--the Larmor frequency of the nuclei associated with fat molecules; 
MIDPOINT--a frequency midway between the WATER and FAT Larmor frequencies; 
PEAK--the frequency which produces the largest peak in the transformed NMR 
signal; and 
CENTROID--weighted center frequency of the transformed NMR signal. 
After determining one of these selected frequencies the prescan process 
determines at process block 259 the receiver attenuation setting (RA). If 
either CF.sub.2 or RA cannot be determined, the system branches at 
decision block 260 to indicate an error. Otherwise, the main computer 
outputs the calculated values CF.sub.2, TA and RA to the transceiver 150 
as indicated at 261. The prescan process then loops back to block 251 and 
waits for another call from the main scan program. Of course, the main 
scan program now proceeds to perform the programmed scan with the 
transceiver 150 finely tuned to the selected RF frequency. 
The determination of the optimal RF frequency for the particular scan to be 
performed requires the execution of two NMR measurements. The first of 
these is illustrated by the pulse sequence in FIG. 4 which is executed as 
part of the procedure for finding the coarse frequency CF.sub.1. The 
second NMR measurement is illustrated by the pulse sequence in FIG. 5 
which is executed as part of the procedure for finding the exact RF 
frequency CF.sub.2. These pulse sequences are orchestrated in the standard 
manner by the pulse generator 121 (FIG. 1) under the direction of the 
prescan program as described above. 
Referring to FIG. 4, the coarse pulse sequence begins by exciting the spins 
in a selected slice through the center of the region of interest. This is 
accomplished in standard fashion with a 90.degree. excitation pulse 
produced while a gradient G.sub.z is applied. The G.sub.z gradient is then 
reversed to rephase the spins and the A/D converter is enabled to acquire 
the NMR signal. This signal is demodulated and its quadrature phases I and 
Q are acquired. In the preferred embodiment the signals I and Q are 
digitized at a 4 kHz sampling rate and 256 samples are acquired. These 
samples represent the magnitude of the components of the NMR signal as a 
function of time and they are stored in the computer system as a file 
S(t)=S.sub.I (t)+jS.sub.q (t). Together these signals also indicate the 
phase of the NMR signal. The file S(t) is then transformed to the 
frequency domain using a fast-Fourier transformation of the complex data. 
The transformed data is stored as a file F(f)=F.sub.i (f)+jF.sub.q (f). 
The magnitude of the transformed signal F.sub.f is then calculated: 
##EQU1## 
and the resulting transformed signal .vertline.F(f).vertline. is smoothed 
by digitally filtering out higher frequency components as disclosed in 
"Numerical Recipes" by William H. Press et al. and published in 1986 by 
Cambridge University Press, pp. 495-497. This transformed signal is 
graphically represented in FIG. 6, although it can be appreciated that the 
precise shape of this waveform will differ with each NMR measurement. 
Referring particularly to FIG. 6, the transformed NMR signal is now 
analyzed to determine the frequency of the highest peak. This is 
accomplished by taking the derivative of the transformed NMR 
.vertline.F(f).vertline. signal and identifying those frequencies at which 
the derivative changes from a positive to negative sign. The magnitude of 
the signal is then measured at each of these frequencies to determine the 
frequency of the highest peak. The frequency is returned as the first 
pass, or coarse, RF frequency CF.sub.1 which is output to the transceiver 
150. The RF frequency has now been set so that a higher resolution NMR 
measurement can be performed during the second pass. 
The pulse sequence for the second NMR measurement which is performed during 
the second pass adjustment of the RF frequency is shown in FIG. 5. In 
addition to a slight change in the RF frequency of the excitation field Bi 
as a result of the first pass adjustment, a number of other differences 
exist in this second NMR measurement. After the selected slice is excited 
and rephased by the ninety degree B.sub.1 pulse and the G.sub.z pulses, 
the excited spins are tipped 180.degree. by a second excitation pulse 300 
which is produced midway between two G.sub.z gradient pulses 301 and 302. 
As a result, when the A/D converter is subsequently turned on the NMR 
signal which is acquired is an echo pulse. In addition, by applying 
gradient pulses in either the Y or X direction as indicated at 303 and 
304, this NMR signal can be further position encoded to a specific region 
within the selected slice. This is particularly useful where there is 
considerable tissue in the selected slice along one of these axes, but 
outside the region of interest. If not eliminated by position encoding, 
such tissue will effect the NMR signal and may result in a less than 
optimal RF frequency for the NMR scan of the region of interest. Because 
these position encoding gradient pulses are optional, they are indicated 
by dashed lines in FIG. 5, however, the system does automatically apply 
position encoding gradient pulses along the dimension thought to have the 
most tissue. 
During the second NMR measurement the I and Q signals are sampled and 
digitized at a 1 kHz sample rate by the analog to digital converter 209 
(FIG. 2). Two hundred and fifty-six such digitized samples are obtained 
and are processed as described above to produce a file 
.vertline.F(f).vertline. containing the magnitude of the transformed and 
filtered NMR signal. The graphic representation of this transformed NMR 
signal is shown in FIG. 7 and it can be seen that because of the lower 
sampling rate a much narrower frequency range is covered. 
Referring particularly to FIG. 8, the transformed NMR signal is now 
analyzed to determine the optimal selected RF frequency. The first step in 
this process is to correlate the transformed NMR signal with a correlation 
model as indicated at process block 270. The correlation model is shown in 
FIG. 9 and is comprised of two pulses 271 and 272 which are separated in 
frequency by an amount which corresponds to the chemical shift between 
water and fat in the particular NMR system being used. In the 1.5 Tesla 
NMR system of the preferred embodiment the spacing between pulses 271 and 
272 is 210 Hertz and each pulse 271 and 272 is 30 Hertz wide. The 
correlation model is a simplified and idealized signal representing the 
two peaks produced by fat and water. 
The correlation is performed by convolving the correlation model of FIG. 9 
with the NMR signal spectrum .vertline.F(f).vertline. of FIG. 7. The 
correlation model is positioned with the pulse 271 at the left end (i.e. 
+500 Hz) of the NMR signal spectrum, and the corresponding frequency bins 
of each waveform are multiplied and then summed. Be setting the magnitudes 
of the pulses 271 and 272 to "1" this step is simplified to merely adding 
up the magnitudes of the NMR signal spectrum "under" each pulse 271 and 
272. The correlation model is then moved to the right one frequency bin 
and the convolution step is repeated. This continues until the righthand 
pulse 272 of the correlation model reaches the last frequency bin on the 
righthand end of the NMR signal spectrum (i.e. -500 Hz). The largest value 
produced by this series of convolutions is the best correlation and the 
corresponding frequency (f.sub.cor) of this maximum value is produced at 
process block 270. This result is shown in FIG. 10 by the dashed line 274. 
Referring again to FIG. 8, the peaks in the NMR signal spectrum 
.vertline.F(f).vertline. are now located as indicated at process block 
275. This is accomplished by taking the derivative of the stored signal 
.vertline.F(f).vertline. and identifying the frequencies at which the 
derivative changes from plus to minus. As indicated at decision block 276, 
the peaks located to the left of the correlation frequency f.sub.cor are 
candidates for the water peak, and if there is only one of them, then the 
WATER frequency is set to the frequency of this single peak as indicated 
at process block 277. If there are multiple peaks to the left of the 
correlation frequency f.COPYRGT.or as determined at decision block 278, 
then the WATER frequency must be calculated. As indicated by process block 
179 and shown by area 280 in FIG. 10, the center of mass of the 
.vertline.F(f).vertline. values in a region of 30 Hz centered at a 
frequency of 105 Hz to the left of the correlation frequency f.sub.cor is 
calculated. This is determined by numerically integrating across the 
region 280 until the accumulated area is equal to one-half the total area 
of the region 280. The WATER frequency is set to this value as indicated 
by process block 280. 
On the otherhand, if no peaks are found to the left of the correlation 
frequency f.sub.cor, then the automatic analysis is considered a failure 
and both the FAT and WATER frequencies are set to the frequency value 
f.sub.max at which the NMR signal spectrum .vertline.F(f).vertline. is 
maximum, as indicated at process block 281. A failure flag is set at 282 
to alert the operator of the failure and the process exits at 283. 
Referring still to FIG. 8, if a water peak is found, the system searches 
for a fat peak to the right of the correlation frequency f.sub.cor as 
indicated at decision blocks 285 and 286. If a Single peak is present, the 
WATER frequency is set to this peak's frequency as indicated at process 
block 287. If multiple peaks are found, the center of mass calculation is 
made in the region 288 of FIG. 10 to produce a frequency CMR as indicated 
at process block 289. This calculation is the same as that described above 
for CML, but a different 30 Hz portion of the spectrum centered 105 Hz to 
the right of the correlation frequency is used. The FAT frequency is set 
to this center of mass frequency CMR as indicated at process block 290. As 
with water, if no peaks are found to the right of the correlation 
frequency f.sub.cor, then the values of both FAT and WATER are set to 
f.sub.max at process block 281, and the failure flag is set at process 
block 282. 
Referring still to FIG. 8, if peaks have been found to each side of 
f.sub.cor for both fat and water, a check is made at decision block 297 to 
determine if they are reasonable. For example, if they are not separated 
in frequency by the expected chemical shift (i.e. 210 Hz in the preferred 
embodiment) then the failure flag is set at process block 298 to alert the 
operator. Also, a failure is indicated if either of the identified peaks 
has a magnitude less than 10% of the peak value of the NMR spectrum signal 
.vertline.F(f).vertline. . If no failures occur, the process exits at 299. 
The process of FIG. 8 returns three values, a WATER frequency, a FAT 
frequency and a failure flag state. The system uses one of these values 
directly to produce CF.sub.2 if either FAT or WATER has been selected by 
the operator, or they may be used to calculate CF.sub.2 if the operator 
selects the MIDPOINT. The MIDPOINT is simply the frequency midway between 
the two identified peaks. If the failure flag has been set, the operator 
is notified and he has the option of manually changing the frequency that 
has been returned.