Abstract:
Image artifacts produced by NMR signals emanating from outside the designed spherical volume in an MRI system are suppressed by interleaving preparatory pulse sequences with image pulse sequences performed during a scan. Each preparatory pulse sequence includes a pair of RF pulses having equal, but opposite flip angles. Spins in the image field of view are tipped and then restored by the pair of RF pulses and spins outside the designed spherical volume are saturated.

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reduction of image artifacts caused by signals produced outside the field of view. 
     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, 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. 
     To accurately excite spins and resolve the locations of the resulting NMR signals the polarizing magnetic field B 0  must be highly homogeneous and the imaging gradient fields G x , G y  and G z  must be highly linear. Numerous structures and methods are known in the art to accomplish this in commercial MRI system, and the region where these fields meet the requirements is referred to as the designed spherical volume (“DSV”). The DSV may range for example, from a diameter of 40 to 48 cm. Outside the DSV, the polarizing magnetic field B 0  can become very inhomogeneous and the imaging gradients G x , G y  and G z  can become highly nonlinear. They are also very poorly controlled in these outer regions. 
     Referring particularly to FIG. 2, the DSV of a typical MRI system is indicated by dashed line  10  and a subject to be scanned  12  is placed in the DSV  10 . A field of view (FOV) from which accurate NMR data is acquired to reconstruct an image is indicated by dotted lines  14 . Portions of the subject  12  are outside the DSV  10 , and the spins therein are subject to the RF excitation fields and magnetic fields produced by the MRI system while imaging the FOV  14 . The NMR signals produced by spins located outside the DSV  10  can produce image artifacts. These image artifacts from outside the DSV  10  can be aliased into the reconstructed image because of the limited imaging FOV  14 , and they can be ghosted into the image because of the data inconsistency. 
     Methods and apparatus are known to reduce these artifacts. One solution is to increase the imaging FOV  14  to reduce aliasing. Hardware solutions include design of gradient coils with a larger linear region or RF transmit coils which significantly reduce RF excitation of spins outside the DSV  10 . These are costly solutions which require major system changes. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for reducing image artifacts caused by signals emanating from outside the system DSV. More particularly, the imaging pulse sequence performed by the MRI system is preceeded by a preparatory pulse sequence which excites spins in the regions outside the DSV and spoils the resulting NMR signals. The imaging pulse sequence follows the preparatory pulse sequence before the longitudinal magnetization of spins outside the DSV have time to recover. 
     Since the RF and magnetic fields outside the DSV produce unpredictable results, it has been discovered that the use of conventional spatial presaturation pulse sequences do not suppress the image artifacts. The present invention solves this problem by applying a first RF pulse that is selective to the FOV along one gradient axis, applying a second RF pulse having an equal, but opposite flip angle and that is selective to the FOV along another gradient axis, and spoiling the NMR signals produced by the two RF pulses. Spin longitudinal magnetization in the FOV is restored by the application of the two equal but opposite flip angle RF pulses and spin longitudinal magnetization outside the FOV and along the two gradient axes is suppressed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an MRI system which employs the present invention; 
     FIG. 2 is a schematic representation of the DSV, FOV and saturation regions produced by the MRI system when practicing the preferred embodiment of the invention; and 
     FIG. 3 is a graphic representation of a preparatory pulse sequence which employs 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. 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  122  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 . 
     To acquire an image, the MRI system performs a series of pulse sequences under the direction of the pulse generator  121 . There are many imaging pulse sequences known in the art and the present invention may be used with any of them. The prescribed imaging pulse sequence directs the operation of the transceiver  150  to produce RF pulses and receive resulting NMR signals, and it directs the gradient system  127  to produce the required gradient fields G x , G y  and G z . As shown in FIG. 2, the prescribed imaging pulse sequence directs the acquisition of NMR data that will enable an image of the FOV  14  to be reconstructed. The size, shape and orientation of the FOV  14  is determined by the scan parameters of the particular prescribed imaging pulse sequence used during the scan. 
     Referring particularly to FIG. 3, the present invention employs a preparatory pulse sequence that is performed by the MRI system prior to the prescribed imaging pulse sequence. This preparatory pulse sequence may be repeated prior to each imaging pulse sequence in the series of imaging pulse sequences performed during the scan, or it may be interleaved between groups of imaging pulse sequences. The repetition rate of the preparatory pulse sequence will depend on such factors as the particular imaging pulse sequence used and the severity of the image artifacts to be suppressed. 
     Referring particularly to FIGS. 2 and 3, the preparatory pulse sequence begins with application of a selective RF pulse  200  having a flip angle α in the presence of a slab select G x  gradient  202 . The slab select G x  gradient  202  has a strength sufficient to excite a slab of spins in the Y-Z plane having a “region one” thickness indicated in FIG. 2 of substantially the same extent of the FOV  14  along the x-axis. As a result, spins in the subject which are located in this slab along the Y and Z axes are saturated. 
     The next step is the application of a second RF pulse  204  in the presence of a slab select G z  gradient  206 . The second RF pulse  204  has the same flip angle α, but the phase of the RF is reversed by 180° such that spins subjected to both RF pulses  200  and  204  are substantially unaffected. The slab select G z  gradient  206  has a strength sufficient to excite a slab of spins in the X-Y plane having a “region two” thickness indicated in FIG. 2 of substantially the same extent as the FOV  14  along the z-axis. Spins in the subject disposed in this slab along the X and Y axes are excited. The first and second regions intersect, and the region of intersection is substantially coincident with the FOV  14 . Those spins outside the FOV  14  along the x-axis are saturated, and those spins inside the region of overlap (i.e. the FOV  14 ) are restored to their original longitudinal magnetization. 
     Before performing an imaging pulse sequence the transverse magnetization produced by the RF pulses  200  and  204  is dephased, or “spoiled”. This can be accomplished simply by a short delay before beginning the imaging pulse sequence (if the B 0  field inhomogeneities are significant outside the DSV  10 ), but in the preferred embodiment dephasing is accelerated by the application of spoiler gradients  208 ,  210  and  212  along respective gradient axes x, y and z. As a result, spins outside the FOV  14  are substantially saturated and their transverse magnetization is dephased such that no net NMR signal is detected. 
     It can be appreciated that a number of variations are possible without departing from the spirit of the invention. The preferred preparatory pulse sequence suppresses spin longitudinal magnetization located outside the FOV  14  along both the x and z-axes. If the subject  12  does not extend outside the FOV  14  along the x-axis, the first RF pulse  200  need not be selective and the slab select G x  gradient pulse  202  need not be produced. In this alternative all spins in the bore of the MRI system are saturated by the first RF pulse  200  and a slab of spins in region two are restored by the second RF pulse  204 . In the preferred embodiment the overlapping part of region one and region two slabs are coextensive with the prescribed boundaries of the FOV  14 , but this is not a requirement. The regions one and two may define an intersecting volume which is larger than the FOV  14 , but this volume should not extend beyond the boundary of the DSV  10 , at least along those axes in which the subject extends beyond the DSV  10 . 
     It is also possible to achieve fat suppression in the FOV  14  using the preparatory pulse sequence. This is achieved by controlling the time delay between the first and second RF pulses  200  and  204 . If this time period is such that fat spins are allowed to dephase 180° with respect to water spins, the longitudinal magnetization of fat spins will not be restored by the second RF pulse  204 . Rather, they will be tipped  2 α and spoiled by the subsequent gradients. 
     It is also possible to use other combinations of Rf pulse excitations during the preparatory pulse sequence. For example, other “binomial” excitations such as 1, −2, 2, −1 RF pulses may be used, where the numbers designate the relative flip angles of the RF pulses.