Abstract:
NMR images indicative of thermal changes in tissues undergoing therapy are produced using a gradient-recalled echo pulse sequence. Prior to therapy a contrast agent which shortens spin T 1  relaxation time is injected into the patient and a reference phase image indicative of proton chemical shift is acquired. Temperature maps are produced in real-time as the therapy is subsequently performed by repeatedly acquiring NMR data, reconstructing measurement phase images and subtracting the reference phase image. The temporal rate at which the temperature maps are produced is increased by segmenting k-space and acquiring less than all the segments during each repetition.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. provisional application No. 60/134,916, filed May 19, 1999, which is incorporated herein by reference as if fully set forth herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with United states Government support awarded by the following agencies: NIH Grant No. HL57501. The United States has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the in vivo measurement of temperature changes using NMR imaging techniques. 
     When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited spins after the excitation signal B 1  is terminated and this signal may be received and processed to form an image. 
     When utilizing these signals to produce images, magnetic field gradients (G x , G y  and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
     MR-guided interventional procedures employ the MRI system to produce real-time images which enable the procedure to be monitored. Such procedures, include MR-guided biopsies, hyperthermia, cryoablation, and ablation using laser, radiofrequency, and focused ultrasound. A critical part of such MR-guided ablation procedures is the ability to monitor spatially localized changes in temperature using heat-sensitive MR pulse sequences. Four different NMR properties of tissues have shown potential as parameters sensitive to temperature changes. These are the spin-lattice relaxation time T 1 , the molecular diffusion coefficient D, and the water proton chemical shift (PCS). The PCS method is based on the dependence of the water proton resonance frequency on temperature. Using this phenomenon, the phase of a gradient-echo image can be used to measure temperature as described, for example, in U.S. Pat. Nos. 5,307,812; 5,323,779; 5,327,884 and 5,711,300. 
     It has been shown that to achieve the optimal signal-to-noise ratio (SNR) in a temperature-sensitive phase image, the echo time (TE) of the pulse sequence used to acquire the NMR data should be equal to the spin-spin relaxation time constant T 2 * of the imaged subject matter. The T 2 * constant of tissues is typically on the order of 40 ms. As a result, a conventional scan can be quite long in duration if the TR period of the pulse sequence is set long enough to accommodate a 40 ms TE. In a 256×256 voyel 2D acquisition using pulse sequence with a TR=50 ms, for example, 13 seconds is required to acquire a complete 2D image data set. When a 3D temperature map is to be produced, the acquisition time becomes much longer and is not very useful in real-time imaging applications. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for producing a temperature map which indicates the temperature of in vivo tissues or in vitro temperature calibration phantoms. More particularly, the present invention is a method in which an NMR contrast agent which alters the spin lattice relaxation time (T 1 ) of spins in the subject to be measured is applied to the subject, an NMR pulse sequence is performed with an MRI system to acquire an NMR data set from which an image may be reconstructed, and a phase image which indicates temperature is reconstructed from the NMR data. The NMR pulse sequence is continuously repeated to update the acquired NMR data such that temperature maps may be produced in real time to indicate temperature changes occurring during a medical procedure or the like. The temporal rate at which updated temperature maps are produced can be further increased by updating the NMR data set with central k-space samples at a higher rate than peripheral k-space samples are acquired. 
     A general object of the invention is to increase the temporal rate at which NMR temperature maps can be produced without reducing their SNR. It has been discovered that NMR contrast agents, such as Gd DPTA, enable the TE/TR period of a PCS sensitive NMR pulse sequence to be substantially reduced without diminishing the SNR of the resulting temperature map. This results in a shorter scan time and a consequent higher temperature map temporal rate. Importantly, it has been discovered that the temperature sensitivity of the NMR pulse sequence is not significantly altered by the contrast agent and the temperature measurement is not significantly affected by variations in contrast agent concentration. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an MRI system which employs the present invention; 
     FIG. 2 is an electrical block diagram of the transceiver which forms part of the MRI system of FIG. 1; 
     FIG. 3 is a graphic representation of a preferred pulse sequence used to acquire the phase image data according to the present invention; and 
     FIG. 4 is a flow chart of the steps used in the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is shown the major components of a preferred MRI system which incorporates the present invention. The operation of the system is controlled from an operator console  100  which includes a keyboard and control panel  102  and a display  104 . The console  100  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on the screen  104 . The computer system  107  includes a number of modules which communicate with each other through a backplane  118 . These include an image processor module  106 , a CPU module  108  and a memory module  113 , known in the art as a frame buffer for storing image data arrays. The computer system  107  is linked to a disk storage  111  and a tape drive  112  for storage of image data and programs, and it communicates with a separate system control  112  through a high speed serial link  115 . 
     The system control  122  includes a set of modules connected together by a backplane. These include a CPU module  119  and a pulse generator module  121  which connects 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  connects to a set of gradient amplifiers  127 , to indicate the timing and shape of the gradient pulses to be produced during the scan. The pulse generator module  121  also receives patient data from a physiological acquisition controller  129  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. And finally, the pulse generator module  121  connects to a scan room interface circuit  133  which receives signals from various sensors associated with the 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 to move 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 x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding gradient coil in an assembly generally designated  139  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly  139  forms part of a magnet assembly  141  which includes a polarizing magnet  140  and a whole-body RF coil  152 . A transceiver module  150  in the system control  122  produces pulses which are amplified by an RF amplifier  151  and coupled to the RF coil  152  by a transmit/receive switch  154 . The resulting 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. 
     The NMR signals picked up by the RF coil  152  are digitized by the transceiver module  150  and transferred to a memory module  160  in the system control  122 . When the scan is completed and an entire array of data has been acquired in the memory module  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 display  104 . 
     Referring particularly to FIGS. 1 and 2, the transceiver  150  produces the RF excitation field B 1  through power amplifier  151  at a coil  152 A and receives the resulting signal induced in a coil  152 B. As indicated above, the coils  152 A and B may be separate as shown in FIG. 2, or they may be a single wholebody 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) from the CPU module  119  and pulse generator module  121 . These digital signals indicate the frequency and phase of the RF carrier signal 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 from the pulse generator module  121 . The signal R(t) defines the envelope of the RF excitation pulse to be produced and is produced in the module  121  by sequentially reading out a series of stored digital values. 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 magnitude of the RF excitation pulse produced at output  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  152 A. 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 signal produced by the subject is picked up by the receiver coil  152 B and applied through the preamplifier  153  to the input of a receiver attenuator  207 . The receiver attenuator  207  further amplifies the signal by an amount determined by a digital attenuation signal (RA) received from the backplane  118 . 
     The received signal is at or around the Larmor frequency, and 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 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 and applies it to a digital detector and signal processor  210  which produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received signal. The resulting stream of digitized I and Q values of the received signal are output through backplane  118  to the memory module  160  where they are employed to reconstruct an image. 
     The 2.5 MHz reference signal as well as the 250 kHz sampling signal and the 5, 10 and 60 MHz reference signals are produced by a reference frequency generator  203  from a common 20 MHz master clock signal. These provide a reference phase for the received NMR signals such that the phase is accurately reflected in the I and Q values. 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. 
     To practice the present invention a scan is performed using an imaging pulse sequence, and an image is reconstructed in which the phase information at each image pixel is preserved. A two-dimensional or a three-dimensional image pulse sequence may be employed, and a Fourier transformation is performed along each axis of the acquired array of complex signal samples. The phase at each image pixel may be calculated as the argument of the complex value at the pixel: φ=tan −1 Q/I. As will be described below, this phase measurement may be used to calculate a phase difference (Δφ) at each image pixel which indicates tissue temperature change. 
     In the preferred embodiment a gradient recalled echo pulse sequence is employed to acquire this phase image data. Referring particularly to FIG. 3, an RF excitation pulse  220  having a flip angle of 30° is produced in the presence of a slab select gradient pulse  222  to produce transverse magnetization in the 3D volume of interest as taught in U.S. Pat. No. 4,431,968. This is followed by a phase encoding gradient pulse  224  directed along the z axis and a phase encoding gradient pulse  226  directed along the y axis. A readout gradient pulse  228  directed along the x axis follows and a partial echo (60%) NMR signal  230  is acquired and digitized as described above. After the acquisition, rewinder gradient pulses  232  and  234  rephase the magnetization before the pulse sequence is repeated as taught in U.S. Pat. No. 4,665,365. 
     As is well known in the art, the pulse sequence is repeated and the phase encoding pulses  224  and  226  are stepped through a series of values to sample 3D k-space. In the preferred embodiment sixteen phase encodings are employed along the z axis. For each particular y phase encoding, therefore, sixteen acquisitions with sixteen different z phase encodings are performed to sample completely along the k z  axis. This is repeated 80 times with 80 different y phase encodings to sample completely along the k y  axis. 
     Sampling along the k x  axis is performed by sampling the echo signal  230  in the presence of the readout gradient pulse  228  during each pulse sequence. Only a partial sampling of the echo signal  230  along the k x  axis is performed and the missing data is computed using a homodyne reconstruction or by zero filling. The echo peak is located near the end of the acquisition window, which is contrary to the normal practice of locating the peak near the beginning of the acquisition window. This partial, asymmetric sampling of the echo signal  230  enables the echo time (TE) of the pulse sequence to be as long as possible within the pulse repetition rate (TR). This is contrary to prior asymmetric partial sampling methods which sample the echo peak early in the acquisition window to shorten the TE time. The echo time (TE) is further increased by decreasing the readout bandwidth (e.g. BW=3 kHz). 
     Tissue magnetic susceptibility changes as a function of temperature. This susceptibility change in turn causes spin resonance frequency shifts which vary linearly with temperature. For water the spin resonance frequency changes at a rate of approximately 0.01 ppm/° C. The resulting frequency shifts at three polarizing field strengths are set forth in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 B 0  = 0.5 T 
                 B 0  = 1.0 T 
                 B 0  = 1.5 T  
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Water Rate = 0.01 
                 Δv = 0.21 
                 Δv = 0.42 
                 Δv = 0.63 
               
               
                   
                 ppm/° C. 
                 Hz/° C. 
                 Hz/° C. 
                 Hz/° C.  
               
               
                   
                   
               
             
          
         
       
     
     Typical temperature changes in tissues due to various therapies and procedures are set forth in Table 2. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Temperature 
                 Frequency Shift at 
               
               
                   
                 Therapy 
                 Change 
                 1.5 T  
               
               
                   
                   
               
             
             
               
                   
                 Localization 
                  0-5° C. 
                 0-3.2 Hz 
               
               
                   
                 Focused Ultrasound 
                 23-63° C. 
                 14.5-40 Hz  
               
               
                   
                 Classic 
                  8-13° C. 
                 5-8.2 Hz 
               
               
                   
                 Hyperthermia  
               
               
                   
                   
               
             
          
         
       
     
     Thermally induced chemical shift changes can be sensitively monitored using phase images because the MR signal phase (proportional to frequency) shifts linearly as a function of temperature. For water spins (0.01 ppm/° C.), a relative 9.2 degree/° C. phase shift occurs at 1.5T for a 40 msec echo time (TE) in a gradient-recalled echo pulse sequence. 
     To produce a temperature map the pulse sequence of FIG. 3 is first used to acquire a reference phase image before the medical procedure is begun. The resulting 3D k-space data set (I R , Q R ) provides a reference phase image that reflects not only the normal temperature of spins in the region of interest, but also phase shifts caused by other well-known factors. As will be described in more detail below, after the medical procedure to be monitored is started, additional 3D k-space data sets (I 1 , Q 1 ) are acquired using the pulse sequence of FIG.  3 . The information necessary to produce a temperature map is contained in the phase difference between the reference and measurement images. This information can be extracted in a number of ways. First, the phase difference (Δφ) may be calculated at each image pixel 
     
       
         Δφ=tan −1   Q   R   /I   R −tan −1   Q   1   /I   1 . 
       
     
     These phase difference values (Δφ) are multiplied by a constant to produce numbers indicative of relative temperature. This is the preferred method when a quantitative temperature map is produced. 
     While the present invention may be employed in many different situations, it has particular applicability to MR guided interventional procedures which involve heating of tissues in a patient. Referring to FIGS. 1 and 4, the patient is positioned in the bore of the MRI system magnet such that the anatomy of interest can be imaged. As indicated at block  250 , the patient is then injected with an NMR contrast agent which travels through the patient&#39;s vasculature and permeates the anatomy of interest. In the preferred embodiment the contrast agent is Gd-DPTA manufactured by Nycomed of New York, N.Y. and sold under the trademark “OMNISCAN”. Other contrast agents which function to shorten the T 1  spin-lattice relaxation time of the subject spins may also be used and the contrast agent may also be injected directly into the tissue subcutaneously. 
     As indicated at process block  252 , after the contrast agent has permeated the subject tissues the reference phase image is acquired. As described above, the pulse sequence of FIG. 3 is used to acquire a complete k-space data set (I R , Q R ) which provides the reference phase at each pixel in the reconstructed image. The medical procedure which is to be monitored can then be started, as indicated at process block  254 . The system enters a loop indicated at  256  in which temperature maps of the subject anatomy are produced as rapidly as possible to provide the attending physician with real-time time feedback regarding the tissue heating which the procedure is producing. For example, if a thermal ablation device is used to ablate tissue at a particular location, the real-time three-dimensional temperature map indicates the temperature rise of the tissue at that location and in the surrounding region. The physician can thus observe that the proper tissue is being treated and that the temperature increase is sufficient to accomplish the desired result. In addition, the three-dimensional temperature map will indicate when the surrounding tissues are approaching an overheated condition and the ablation procedure should be terminated. To accomplish these goals it is important that the temperature map be updated at as high a temporal rate as possible. This is particularly true if 3D anatomic images are also being produced from the same NMR data to help guide the ablation device into the proper position. 
     Referring particularly to FIG. 4, after the medical procedure is started, the pulse sequence of FIG. 3 is employed as indicated at process block  260  to acquire k-space data (I 1 , Q 1 ). During the first pass through the loop  256  a complete k-space NMR data set is acquired. However, k-space is divided into segments as described in U.S. Pat. No. 5,713,358 issued Feb. 3, 1998 and entitled “ Method For Producing A Time - Resolved Series Of  3 D Magnetic Resonance Angiograms During The First Passage Of Contrast Agent ”. In the preferred embodiment 3D k-space is divided into 8 segments (4 in k y  direction and 2 in k z  direction) and the segments closer to the origin of k-space (i.e. k=0) are sampled more frequently during subsequent passes through the loop  256 . As indicated by process block  262 , a temperature map is then reconstructed using the acquired 3D k-space data set. The updated k-space data is Fourier transformed along all three axes and the phase at each resulting pixel is calculated from the real (I) and imaginary (Q) components. The corresponding phase at each pixel in the reference phase image is subtracted to produce a temperature map that may be output directly to a display as indicated at process block  264 . In this embodiment the brightness of each displayed image pixel is indicative of temperature difference at its corresponding voxel in the patient. 
     The temperature map produced in the preferred embodiment indicates the change in temperature between the time the reference phase image is acquired and the time the measurement phase image is acquired. An alternative embodiment is to measure the rate at which the temperature is increasing during the treatment procedure. In this case the reference phase image is updated at regular intervals during the treatment by using acquired measurement data. The temperature map in this embodiment indicates the temperature change since the last reference phase image update, or in other words, the rate at which the temperature is increasing during treatment. 
     In an alternative embodiment, a magnitude image is also produced from the image data set reconstructed from the acquired 3D k-space data set. The brightness of each pixel in this magnitude image is as follows: 
     
       
         
           M={square root over (I 2   +Q   2 +L )}. 
         
       
     
     The temperature values indicated by the temperature map are combined with this magnitude image by modulating the color of each corresponding pixel therein. Cooler temperatures are indicated by blue and the hottest temperatures are indicated by yellow. This color modulation of the magnitude image provides the physician with an anatomic image that clearly shows the anatomy being treated and the location and orientation of the medical device used to perform the treatment. The color of the anatomic structures indicates their temperature. The temperature desired for treatment may also be input and used as a threshold that produces a specific color (e.g. red) on the temperature map. That is, tissues which reach this threshold temperature are displayed red on the temperature map. 
     As indicated at by the decision block  266 , the system remains in the loop  256  until the medical procedure is completed. As indicated above, during subsequent passes through the loop  256  the 3D k-space data set is only partially updated by acquiring two of the eight defined k-space segments. During each pass the central-most k-space segment is sampled along with two of the other seven “peripheral” segments. The rate at which the peripheral segments are updated is a function of their distance from the center of k-space as described, for example, in the above-cited U.S. Pat. No. 5,713,358. It has been discovered that temperature variations throughout the image have low spatial frequency components and that only the central most segment of k-space need be sampled to accurately depict these variations. Thus, a substantial increase in temporal resolution may be achieved by acquiring only three of the eight segments without sacrificing temperature image resolution. By sampling the remaining peripheral segments during the procedure, however, the details of the anatomic image are also updated at a reasonable rate. 
     A rapid, high spatial and high temperature resolution in vivo temperature mapping technique has been presented. While a gradient-recalled echo pulse sequence is used to produce the phase images in the preferred embodiment, other well-known imaging pulse sequences can be used. Spin echo pulse sequences can also be used, and either 2D or 3D pulse sequences will work.