MXPA00009820A - Recovery of signal void arising from field inhomogeneities in magnetic resonance echo planar imaging - Google Patents

Abstract

A prescan process is performed in which an EPI pulse sequence acquires a series of pilot images. Each pilot image is acquired with a different slice selection refocusing gradient and a rephasing map is produced which indicates the optimal refocusing gradient to be used for each pixel. The rephasing map is used to determine the optimal refocusing gradient amplitudes to use in a subsequent scan to reduce signal drop out caused by susceptibility gradients.

Description

RECOVERY OF THE NULL SIGN THAT ARISES FROM THE NON-HOMOGENEOUS FIELDS, IN THE FORMATION OF ECHO PLANAR IMAGES BY MAGNETIC RESONANCE This request is based on the provisional request for series No. 60 / 081,688 filed on April 14, 1998.

BACKGROUND OF THE INVENTION The field of the invention is in nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the recovery or momentary loss of signal from MR images, caused by local susceptibility gradients at the tissue boundaries. When a substance, such as human tissue is subjected to a uniform magnetic field (polarization of the Bo field), the individual magnetic moments of the spins in the tissue try to align it with this field of polarization, where the precessions around it are in their characteristic frequencies Larmor. If the substance, or tissue, is subjected to a magnetic field (excitation of the Bi field) that is in the x-plane and which is close to the Larmor frequency, the net aligned moment, M, can be rotated, or "tilted", within REF .: 12387 from the x-y plane, to produce a net transverse magnetic moment M. A signal is emitted by the excited spins, then the excitation signal B is terminated, this signal can be received and processed to form an image. When these signals are used to produce images, the gradients of the magnetic field (G », G% and GJ are used.) Typically, the region to form the image is scanned, by a sequence of measurement cycles, in which these gradients vary. According to the particular localization method used, the resulting series of NMR received signals are digitized and processed to reconstruct the image using one of the many known techniques of reconstruction.The imperfections in the gradients of the linear magnetic field are known ( G, -, G, and G) produce artifacts in reconstructed images.This is a well-known problem, for example, that eddy currents produced by impulse gradients will distort the magnetic field and produce image artifacts. methods for compensating such eddy current errors are also well known, for example, in US Patent Nos. 4, 698, 591; 4, 950, 994; 5,226,418 It is also well known that gradients can not be perfectly uniform in the volume of total image formation, which can lead to image distortion. Methods to compensate for this non-uniformity are well known, and for example, are described in U.S. Pat. No. 4,591,789. The methods of image formation are also dependent in the presence of a homogeneous polarization magnetic field B0. Many methods have been invented to obtain a homogenous and stable polarization magnetic field, which includes the use of adjustment coils, which can be adjusted periodically during calibration procedures. However, such measures can not be corrected for inhomogeneous magnetic fields that arise from the local susceptibility gradients produced when a patient is placed within the magnetic field. Such susceptibility gradients arise near the air-tissue limits, for example, in moderate cases these can lead to reducing signal intensities in pixels located near such limits, and in severe cases, these can lead to image artifacts, such as displacements. of pixels and complete loss of the signal. Methods have been proposed to recover the total and momentary loss of the signal in MR images, due to to non-homogeneous susceptibility gradients. These include methods that use gradient echo in 3D, which use a higher image resolution, using adjusted excitation pulses rf, and multigradiente echo acquisitions with inhomogeneous susceptibility compensation. J. Frahm et al., 'Direct FLASH MR Imaging of Magnetic Field Homogeneities by Gradient Compensation ", Magnetic Resonance in Medicine 6,474-480 (1988) recognizes that the total or momentary loss of the signal is due to the phase readjustment of the transverse magnetization in the local regions, which contain susceptibility gradients. Multiple images that are acquired with different gradient values of phase readjustment can be added to reduce the effects of susceptibility. R.T. Constable, "Functional MR Imaging Using Gradient-Echo Echo-Planar Imaging in the Presence of Large Static Field Inhomogeneities," JMRI 1995; 5: 746-752 proposes the acquisition of nine images with different gradient values of phase readjustment, which form a composite image, either by determining the average of the nine values for each pixel, or by choosing the maximum value for each pixel No effort has been made to analyze pilot images, acquired during a previous exploration with different gradients of phase readjustment, to determine the gradients Optimal phase readjustments used during a scan. BRIEF DESCRIPTION OF THE INVENTION The present invention is a method for acquiring an MR image of a subject, in which the susceptibility gradients produce total or momentary loss of the signal. More specifically, the method includes the use of a prior examination of the patient to acquire a plurality of pilot images, in which an image gradient is set at different values; a phase readjustment map is produced by calculating the optimal value of the image gradient, which produces the maximum brightness for each image pixel; to determine the adjusted image formation gradient, the optimum image gradient values are examined in the phase readjustment map; that runs a scan that uses the adjusted image gradient. A general object of the invention is to produce information that allows a preliminary exploration, which eliminates or reduces the total or momentary loss of the signal. The phase readout map, produced by the previous scan, indicates the optimum value of the image formation gradient to produce the optimum signal for each pixel in the reconstructed image. The map of Phase readjustment can be used to manually or automatically set one or more scans with a gradient of adjusted image formation image (s) that will produce the best image. Another object of the invention is to automatically determine the adjusted image formation gradient (s) to achieve better results. A histogram of the phase readjustment gradient is produced by counting the pixel numbers in the phase readjustment map, in each gradient value of image formation. The histogram of the phase readjustment gradient is scanned to determine the best gradient of image formation established. The foregoing and other objects and advantages of the invention will be shown from the following description. In the description, the reference is made to the accompanying drawings, which are part of the same, and in which is shown by means of an illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, reference is therefore made to the included claims, to interpret the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of an IRM system, which employs the present invention; Fig. 2 is an electrical block diagram of the transceiver, which forms part of the IRM system of Fig. 1; Fig. 3 is a graphical representation of a succession of EPI pulses; Fig. 4 is a flow chart, of a preliminary exploration process, which was performed by the IRM system of Fig. 1; Fig. 5 is a flow chart of an automatic phase readjustment process performed by the MRI system of Fig.l; Fig. 6 is a graphical representation of the waveform of the gradient of the selected cut, which is part of the succession of pulses of Fig. 3 and which is altered when the method of Fig. 4 is practiced; Fig. 7 is a graph of signal strength as a function of the gradient amplitude of a new approach; Fig. 8 is a graph of an illustrative histogram, showing the number of pixels in an image, which achieves a maximum value of signal strength at different gradient amplitudes of a new approach; and Fig. 9 is a graph of an illustrative segmentation map, showing the accumulated brightness of the pixel at specific gradient amplitudes of a new approach.

Detailed description of the invention The loss of signal strength occurs due to the phase reset of the espm through a voxel (pixel volume, a three-dimensional pixel). This phase readjustment is the result of gradients and increases in susceptibility with respect to the thickness of the cut. Here, it is assumed that the phase distribution of the spins varies only in the direction of the cut (z), the gradients in the particular direction are neglected (x and y). The intensity of the signal S of a three-dimensional pixel can then be written as a function of the gradient deviation of a new focus of the selected Gref cut (relative to the precise roll of the gradient of the selected cut) as: Where M ^ is the magnetization,? the gyromagnetic relation, TE the return signal time, GSU3 the susceptibility gradient (assumed to be linear), t the duration of the gradient impulse of a new approach, and z the thickness of the cut. The evaluation of the integral and calculation of the absolute value of S leads to I S (GL ") | = M? Z I smc [?? z (Gs STE + G-» «t) / 2] | (2) The observed signal has its maximum in Gr "£ = - G,", TE / t. By performing a series of scans with different Gr "* a corresponding series of images can be reconstructed, the signal magnitudes in each pixel x, and in these images they are subsequently fitted to a smc function indicated by equation (2). From this best fit, the signal level of maximum value (x, y) of each pixel and the gradient of a corresponding new focus Gma (x, y) that will produce this maximum value signal will be determined. These values are stored in a phase readjustment map.

The result of the phase readjustment map can be used in several ways, to reconstruct an image or plan a subsequent examination of the patient. Description of the preferred modality Referring first to FIG. 1, there is shown the main components of a preferred IRM system, which incorporates the present invention. The operation of the system is controlled from a console operator 100, which includes a keyboard and control panel 102 and a screen 104. The console 100 communicates through a link 116, with a computer system 107 that allows a operator control the production and display of images on the screen 104. The computer system 107 includes a number of modules that communicate with each other through an interconnection mechanism. These include an image processor module 106, a CPU module (central processing unit) 108 and a memory module 113, known in the art as a regulator of recurring pulse cycles, for storing the image data. The computer system 107 is linked to a disk storage 111, and to a tape drive 112, for the storage of image data and programs, this communicates with a system control 122 through a high-speed serial link 115. The control system 122 includes a set of modules connected by an interconnection mechanism. These include a module of the CPU 119, and a pulse generator module 121, which connects the console operator 100 through a serial link 125. By means of the link 125 the control system 122 receives commands from the operator, which indicates the scan sequence to be performed. The impulse generator module 121 operates the components of the systems to perform the desired scanning sequence. This produces information that indicates the timing, amplitude and shape of the RF impulses that are produced, the timing and the length of the window of the information obtained. The pulse generator module 121 is connected to a gradient amplifier instrument 127, to indicate the timing and shape of the impulse gradients to be produced during the scan. The pulse generator module 121 also receives information from the patient, from a physiological acquisition controller 129, which receives the signals, from a number of different sensors connected to the patient, such as the ECG signals from electrodes or respiration signals from of inhalations. Y finally, the impulse generator module 121 connects to an interface circuit of the scanning room 133, which receives the signals from sensor vanes, associated with the condition of the patient and the magnetic system. This is also done through the scanning circuit of the scanning room, for a system that positions the patient 134, which receives the commands to move the patient to the desired position for scanning. The waveform gradients produced by the pulse generator module 121 are applied to a gradient amplifier system 127, which comprises the amplifiers G <;, G ^ and G. Each gradient amplifier excites a corresponding gradient coil in an assembly 139, generally designated to produce the gradients of the magnetic field, which are used to spatially code the acquired signals. The gradient of the assembly coil 139 forms part of an assembly of the magnet 141, which includes a polarization magnet 140, and a whole-body coil RF 152. A transceiver module 150, within the control system 122 produces pulses that are amplified by an RF amplifier 151, and connects to the RF coil 152 by a transmitter / receiver switch 154. The radiated signals resulting from the core inside the patient can be perceived, by the same RF coil 152 and connected through the transmitter / receiver switch 154 to a preamplifier 153. The amplified NMR signals are demodulated, filtered and digitized in the receiving section of the transceiver 150. The transmitter / receiver switch 154 is controlled by a signal from the pulse generator module 121 to an RF amplifier 151 electrically connected to the coil 152, during the transmission mode and connects the pre-amplifier 153 during the reception mode . Transmitter / receiver switch 154 also enables a different RF coil. { for example, a coil head or coil surface) to be used either in the transmission or in the reception mode. The NMR signals collected by the RF coil 152 are digitized by the transceiver module 150, and transferred to the memory module 160 within the control system 122. When the scanning is complete and a complete series of data has been acquired in the module of memory 160, a serial processor (matrix) 161 operates to transform the data into Fourier within an established image data. This adjusted image data is transmitted via the serial link 115 to a computer system 107, where it is stored in the memory of the disk 111. In response to the commands received from the operator console 100, these adjusted image data can be filed in the tape drive 112, or this can be further processed by the image processor 5 106 and transmitted to the operator console 100, and presented in the screen 104. Referring particularly to Figures 1 and 2, transceiver 150, produces RF excitation in field Bi through power amplifier 151 to a coil 152A and receives the resultant signal induced in a coil 152B. As indicated above, coils 152A and B can be separated, as shown in Fig. 2, or can be a single full-body coil, as shown in Fig. 1. The base, or carrier, frequency 5 of the RF excitation field is produced under the control of a frequency synthesizer 200, which receives a series of digital signals from the CPU module 119, and the pulse generator module 121. These digital signals indicate the frequency and phase of the carrier signal produced at an output 201. The RF command bearer is applied to a modulator and converter 202, where its amplitude is modulated in response to a signal R (t), also received from the pulse generating module 121. The signal R (t) defines the development of the RF excitation pulse to occur and occurs within module 121, sequentially reading a series of stored digital values. These stored digital values can, in turn, be changed from the console operator 100, to allow any development to occur from the desired RF pulse. The magnitude of the RF excitation pulse produced at the output 205 is attenuated by an excitation attenuator circuit 206, which receives a digital command, from an interconnection mechanism 118, the attenuated RF excitation pulses are applied in the power amplifier. 151, which handles RF coil 152A. For a more detailed description of this part of the transceiver 122, reference is made to U.S. Pat. No. 4,952,877.

Still referring to FIGS. 1 and 2, the signal produced by the subject is picked up by the receiver coil 152B and applied through the preamplifier 153 to the output of another receiver amplifier 207. The receiver amplifier 207 subsequently amplifies the signal for a given amount of a digital attenuation signal received from the interconnection mechanism 118. The received signal is at or around the Larmor frequency, this high frequency signal is reduced in a two-stage process with a descending converter 208 , which first mixes the NMR signal with the carrier signal on line 201 and subsequently mix the resulting difference signal, with the reference signal at 2.5 MHz on line 204. The signal of the down converter is applied to the output of an analog-to-digital converter (A / D) 209, which samples and digitizes the analog signal, and applies to This is a digital detector and signal processor 210, which produces values (I) in phase of 16 bits and values (Q) of quadrature of 16 bits corresponding to the receiving signal. The resulting flow of the digitized values I and Q of the receiver signal are produced through the interconnection mechanism 118 to the memory module 160, where they are used to reconstruct an image. The pulse sequence EPI employed in the preferred embodiment of the invention is illustrated in Fig. 4. An RF excitation pulse 250 at 90 ° is applied in the presence of a pulse gradient in the selected cut 251, to produce the magnetization transversal in a cut. Excited spins are re-enriched, or refocused, by a negative division 252 on the gradient of the selected cut. A total of Ny (e.g., Ny = 64) different echo NMR signals, indicated in 253 are then acquired during the succession of EPI pulses. The echo signals NMR 253, are called echoes gradients produced by the application of an oscillating output reading gradient 255. The output reading sequence is started with a pre-stepped 256 output reading gradient gradient, and echo signals 253 occur as the oscillating output reading gradients between positive and negative values. A total of N (eg, N = 64) samples are taken from each echo signal NMR 253 during each output output reading of pulse gradient 255. The successive echo NMR signals of, are phase encoded separately, by a series of coding pulse gradients per phase 258. A phase coding division is pre-phased 259, before the echo signals are acquired in a central view position (kJ = 0) at the desired time point (TE) . The coding pulses of the subsequent phase 258, happen as the pulse gradients of the output readings of the polarity switch 255, these move to the phase coding in a monotone and upward fashion through the space Ky. In the realization of the pulse sequence EPI, different frequencies N have been acquired, which encoded samples N? in a separate way, coding in phase the echo NMR 253 signals. After remixing the time of each echo, this spatial series of Nx x Ny of the element k of complex numbers are transformed to Fourier, along its dimensions (y and kx) to produce a set image data, which indicates the magnitude of the NMR signal to along each of its two dimensions (x and y). One aspect of the present invention is a pre-scan process, in which a phase readjustment map is produced to assist the operator in pre-establishing the optimum scan. This preliminary scanning process is carried out by the control system 122, under the direction of a program, which is executed after the patient is placed on the magnet 141, and prior to the scan. This pre-scan phase uses the succession of EPI pulses described above, as it will become apparent, the size of the division of a new approach 252 is varied during the process. This should be apparent to those skilled in the art, however, variations in the selection of the cut gradient is not limited to this particular division gradient. The application of a phase readjustment gradient along the selection of the cutting axes at any time after the RF excitation and at the previous reading of the first NMR signal that is sufficient.

Also, if the weighted moment gradient is used to compensate for the phase dispersion of the blood flow, more than one new division approach can be used and varied. Referring particularly to FIG. 4, the first step in the scanning process indicated in process block 300 is to download the succession of EPI pulses to pulse generator 121, with appropriate parameters for the image of the pilot cut to be acquired. . In the preferred embodiment, a sharp angle of 90 °, TE of 50 ms, a TR of 2000 ms, FOV of 24 x 24 cm, a thickness of the cut of 7 mm, a space of 2 mm, 18 cuts, a ( series) matrix of 64x64 image formation and a bandwidth of the receiver at 125 kHz. As will be apparent from the following description, this EPI pulse sequence is repeated a number of times, during the previous scan to acquire the necessary NMR data, to produce a phase readjust map. As best shown in Fig. 6, the amplitude of the phase reset division 252, over the gradient of the selected cut is started through a series of values during the previous scan. These steps are in a range of from 0.6 times the nominal amplitude indicated in 254 to 1.4 times of the nominal amplitude indicated in 261.

The nominal amplitude, which in theory mentions that this amplitude transverse magnetization was out of phase, after the application of the gradient of the selected cut 251, ba or ideal conditions. Referring again to Fig. 4, after a succession of EPI pulses is discharged and the phase readjustment division 252 is set to 0.6 of nominal amplitude, a circuit is introduced in which the succession of EPI pulses is performed, to acquire the image data, as indicated in process block 302. The NMR image data of the acquired space k are transformed to Fourier, along the two dimensions and at an image magnitude is reconstructed as indicated in process block 304. This data acquisition and image reconstruction are repeated after an increase in the amplitude of the phase readjustment division, as indicated in process block 306. When the amplitude of the division of Phase readjustment reaches a nominal value of 1.4, the circuit is energized as indicated in process block 308. In the preferred embodiment, twenty amplitude stages of the phase readjustment division are acquired and reconstructed. Eight corresponding images of pilot magnitude |? 0. For each image I is a 64x64 matrix of pixel intensity values p (x, y).

The corresponding values of intensity of the pixel p (x, y) in the twenty images are examined later to determine the phase readjustment, which occurs in different tissues through the cut image. Referring to Fig. 7, if the images are taken through the human brain, for example, the intensity values p (x, y) of the pixels corresponding to the upper prefrontal cortex outside the susceptibility gradients will have a maximum value amplitude in the amplitude of the nominal phase readjustment division 261. In other amplitudes of the division of a new approach, the signal intensity produced by the prefrontral cortex will be smaller and they would be on the sync curve indicated by the solid line 310. On the other hand, the intensity values of the pixel p (x, y) correspond to the visual cortex near the cerebellum, which will have a maximum value of amplitude at a lower focus division amplitude, as indicated by the line stitch 312. In other words, the maximum value of the signal intensity can be obtained from each pixel, in which the image is determined in part by the accuracy with which the gradient of the division of a new e nfoque offset the transverse magnetization at that location of the pixel (x, y). The following stages in the process of Pre-scan examines the amplitude of the intensity values of the corresponding pixel p (x, y) in the twenty pilot images and calculates the amplitude of the division of a new optimal focus, for each of the 64x64 pixel locations. Referring again to Fig. 4, a circuit is introduced into the twenty pixel intensity values, for a pixel location p (x, y) are examined to find an optimum amplitude of the gradient of a new focus for that location of the pixel. pixel As indicated in process block 316, this is carried out by adjusting the values to a quadratic curve, which approximates the central part of the smc function. As indicated in process block 318, the maximum value of this quadratic curve is located later, and the amplitude of the gradient of the division of a new optimum approach Gra ^ (x, y) indicated in this maximum value is stored in a Phase readjustment map of element 64x64, in a location that corresponds to the location of the pixel x, and in the pilot images. In addition, the maximum intensity value of the pixel S, ax (x, y) at this maximum value, is also stored in the phase readout map in the same location, as indicated in process block 320.

The process is repeated for each pixel location (x, y) in the pilot images I i-c • The next pixel is selects in process block 322, and processes 316, 318 and 320 are repeated. When all the locations of the pixels (x, y) have been processed, as determined in decision block 324, the previous scan is energized at 326. There are several ways in which the phase readjustment map can be used to establish previously the subsequent exploration. In a modality, a histogram is produced, in which the number of pixels in each amplitude of the gradient of a new approach is calculated. More specifically, a number of gradient amplitudes of new focus "boxes" are defined and the phase readout map is scanned to count the number of pixels in each box. The resulting histogram is subsequently displayed to the operator, as shown in Fig. 8. This histogram will always have a wide maximum value 328 in the gradient amplitude of a new nominal approach 261, since many pixels in an MR image are will properly offset this nominal amplitude. However, other smaller maximum values may occur in the histogram, such as 330 and 332, which indicate a significant number of pixels that can be brightened with a gradient amplitude of a new focus division. These can be identified in a manual way and used to establish previously the Subsequent exploration of the patient. Another preferred embodiment of the invention employs the phase readjust map, produced by the scanning process to previously establish an automatic form the subsequent scan. This is done by the control system 122, under the direction of an automatic phase reset process. This process is illustrated in Fig. 5. Referring in a particular way to Fig. 5, the automatic phase readjustment process filters the phase readjustment map, as indicated in process block 340, by the minimum intensity of the corresponding magnitude images I. If the sum of the twenty intensity values, at a particular location of the pixel (x, y) does not exceed a minimum value, then the corresponding location (x, y) in the phase readjustment map is set to zero. This is eliminated from the additional processing in the back plane of the pixels. The next step, as indicated by process block 342, is to calculate a brightness accumulation histogram. As described above, this involves counting the number of pixels in the phase readjustment map in each amplitude of the phase readjustment gradient. However, contrary to the histogram described above, each gradient of the phase readjustment box is not according to the number of pixels, but is instead the sum of the corresponding maximum intensity values stored in the phase readjustment map. This histogram, in this way indicates the accumulated brightness in each value of the gradient of phase readjustment, or of the box of the histogram.

Subsequently, a circuit is introduced, in which the best amplitudes of the adjusted new focus gradient are determined from the gradient histogram of phase readjustment. This is done by scanning the histogram of the phase readjustment gradient, to determine the adjusted gradient, which will produce the highest accumulated brightness. Instead of reaching only the highest accumulated brightness value. However, a band, or window, of the highest levels of brightness accumulation is selected by sliding the window through the histogram box. The extension of this window is established in the preferred modality of the extension of the central division in the smc function at half of its maximum value. This window advances through the successive boxes of the histogram of the phase readjustment gradient, and the accumulated brightness of the pixels within the window is calculated. When the accumulated maximum brightness are found, as indicated in process block 344, the pixels within the window are indicated with a segment number. The segment number is stored in the phase readout map at the corresponding locations of the pixel (x, y) and the observed pixels are removed from the phase readjustment gradient histogram so that these are not included in the subsequent segments. And finally, as indicated in the process block 346, the phase readjustment gradient adjusted for the segment is stored completely with an accumulated brightness value of all the pixels observed in this segment. The image pixel segmentation process, which selects the highest brightness accumulation window is repeated in decision block 348, until all the pixels have been selected and removed from the histogram of the phase readjustment gradient. For cutting edge studies this is carried out in a typical way in five or fewer steps. As a result, the 64x64 matrix of the pilot image pixels are segmented and a gradient of a new adjusted focus and an accumulated brightness value are stored for each segment. Fig. 9 is a graphic illustration of those stored segment values, where the axis horizontal is the gradient amplitude of a new approach and the vertical axis is the accumulated brightness of the pixels in the segment. Five segments labeled A, B, C, D and E are shown in this segmentation map example. The scan now proceeds automatically to acquire an image data at each of the stored new gradient amplitudes of a new focus, as indicated in process block 350. In the example of Fig. 9, this requires five acquisitions of different images in five different amplitudes of the gradient of a new approach. The number of image acquisitions can be reduced either automatically or manually if necessary. For example, the two or three segments with the highest accumulated brightness can be selected and their gradient amplitude from a new approach used in two or three acquisitions. The resulting reconstructed images are then combined into a single composite image, as indicated in process block 352. This was done by selecting the pixel values of each acquired image, which uses the segment numbers stored in the readjustment map. of phase. That is, for the first image, the pixels corresponding to the pixels of segment 1 in the readjustment map of phase are selected and stored in the corresponding locations (x, y) within the composite image. The same procedure is used for each of the acquired images and their corresponding segment numbers. When fewer images of the identified segments are acquired, the pixels of the non-acquired segments are selected from one of the acquired images. The choice is determined for those gradient amplitudes of a new focus of the acquired images, which are closer to the optimal stored value Ca »(x, y) for a particular pixel. This should be evident to those skilled in the art, that the phase readjustment map produced by the process of a previous scan, can be used in various ways, to improve the quality of the MR images, in which the structures are obscured or they are lost momentarily due to susceptibility gradients. The invention is created to have a particular application for IRM operation, where the activity of various parts of the brain forms an image. It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (13)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A method that produces an image of a subject with an MRI system, characterized in that it contains the following steps: performing a preliminary scan to produce a phase readout map, the previous scan includes: a) acquisition of pilot images of the subject , using a succession of NMR pulses, which employs a magnetic field gradient of image formation, wherein the gradient of the magnetic field of image formation is set or adjusted to different values of a plurality of pilot image acquisitions; b) the calculation of the optimum value of the gradient of the magnetic field of image formation, for each pixel in the pilot images, by examining the magnitudes of the corresponding pixels of a plurality of pilot images acquired; c) the storage of the optimal values of the gradient of the magnetic field of image formation at the corresponding locations of the pixel in the phase readjust map; d) the determination of a gradient of the established or adjusted image formation magnetic field, which uses the values stored in the phase readjustment map; and e) performing a patient scan, using the succession of NMR pulses, with the gradient of the established or adjusted magnetic field of image formation, which was determined in step d)

2. The method as mentioned in claim 1, characterized in that step b) includes adjusting the magnitudes of the corresponding pixels in the pilot images to a curve, and determining the maximum value of the curve.

3. The method mentioned in claim 2, characterized in that, the maximum value of the curve indicates the optimum value of the gradient of the magnetic field of image formation, for a pixel and an optimum value of brightness for the pixel, the optimum value of the brightness it is also stored in the readjustment map of phase in the corresponding pixel location.

4. The method mentioned in claim 1, characterized in that it determines a magnetic field gradient of image formation established or adjusted step d) that includes the production of a histogram of the phase readjustment map, which is indicative of the numbers of pixels having an optimum brightness in each of a plurality of values of the magnetic field gradient in the image formation.

5. The method mentioned in claim 4, characterized in that the gradient of the established or adjusted image formation magnetic field is determined by displaying the histogram, so that the adjustment can be manually selected.

6. The method mentioned in claim 1, characterized in that it includes the storage of each location of a pixel in the phase readout map at a pixel value, which indicates the brightness of the pixel at a corresponding optimum value of the magnetic field gradient of the pixel. stored image formation, and in which a gradient of the magnetic field of image formation established or adjusted step d) is determined which includes the production of a histogram of the accumulated brightness of the phase readjustment map, which is indicative of the sum of the brightness values of the pixel stored at the pixel locations in the phase readjustment map, which has an optimum brightness in each plurality of magnetic field gradient values in the image formation.

7. The method mentioned in claim 6, characterized in that, the magnetic field gradient of fixed image formation is determined, by scanning the histogram of the accumulated brightness.

8. The method mentioned in claim 7, characterized in that the scan is carried out by moving a window through the accumulated brightness histogram and locating the established or adjusted magnetic field gradient, which provides the accumulated maximum brightness within the window.

9. The method mentioned in claim 1, characterized in that the succession of pulses NMR is a succession of planar echo image formation.

10. The method mentioned in claim 9, characterized in that, the gradient of the magnetic field of image formation is a gradient of the selected cut.

11. The method mentioned in claim 10, characterized in that the gradient of the magnetic field of image formation is a gradient of a new focus.

12. The method mentioned in claim 1, characterized in that step d) includes a determination of a plurality of gradients of set or adjusted magnetic fields of imaging, which are stored in the segment numbers of the "phase readjustment map". , which indicate the settings for the locations of the pixels, in which step e) includes carrying out an exploration and reconstruction of an image, for each of the gradients of the established or adjusted magnetic fields, and f) combining each one of the images within a single output image, selecting the pixel values of each one, which uses the stored segment numbers.

13. The method mentioned in claim 12, characterized in that, the succession of pulses NMR, is a succession of pulses of echo-planar imaging and the magnetic field gradient of image formation is a gradient of the cut.