News Column

Patent Issued for Magnetic Resonance Imaging Device

August 8, 2014



By a News Reporter-Staff News Editor at Health & Medicine Week -- Hitachi Medical Corporation (Tokyo, JP) has been issued patent number 8787639, according to news reporting originating out of Alexandria, Virginia, by NewsRx editors (see also Hitachi Medical Corporation).

The patent's inventors are Shirai, Toru (Hachioji, JP); Bito, Yoshitaka (Kokubunji, JP).

This patent was filed on January 28, 2011 and was published online on July 22, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "The currently most widely used nuclide for imaging in clinical MRI is hydrogen nucleus (proton), which is the major constituent of subjects.

"MRI realizes two-dimensional or three-dimensional imaging of forms or functions of human head, abdominal part, limbs, and so forth by imaging spatial distribution of proton density or relaxation information of magnetic resonance signals.

"When a human body is an object of the measurement, as the major sources of protons that can be detected by MRI, there are water and fats. Fats existing in human bodies reduce contrast of images of the abdominal part, spine, limbs, etc. of a subject.

"Therefore, there have been proposed methods for suppressing fat signals in clinical measurements. As one of them, there is known the Dixon method (Non-patent document 1), in which fat signals are suppressed by using phase difference between water and fat signals.

"Hereafter, the Dixon method will be explained.

"Difference of 'chemical shift', which indicates resonant frequency, between water and fats is 3.5 ppm, due to the difference in the molecular structures. The difference of the resonant frequency between water and fats is proportional to magnetic field strength, and when the magnetic field strength is 1.5 teslas, it corresponds to about 224 Hz.

"The Dixon method utilizes the phase difference produced by the difference in the frequency of water and fats signals.

"Data are obtained for a plurality of images with different times from excitation of nuclear spins to acquisition of signals (henceforth referred to as echo time). Specifically, images are obtained with such an echo time that water and fat signals become in-phase, and such an echo time that water and fat signals become out-of-phase.

"When the frequency difference of water and fat signals is represented by df, such a time t.sub.in that the water and fat signals become in-phase is represented as n/df, and such a time t.sub.out that the water and fat signals become out-of-phase is represented as (n+1/2)/df, wherein n is an integer.

"In the Dixon method, two of images, an image I.sub.1 and an image I.sub.2, are obtained with the echo times t.sub.in and t.sub.out, respectively.

"If a water signal is represented by W, and a fat signal is represented by F, I.sub.1 and I.sub.2 can be represented by the following equations (1) and (2), respectively. I.sub.1=W+F (1) I.sub.2=W-F (2)

"By addition and subtraction of these equations for two images, the water signal and the fat signal are separated as represented by the following equations (3) and (4). W=(I.sub.1+I.sub.2)/2 (3) F=(I.sub.1-I.sub.2)/2 (4)

"On the basis of the above, each of the water signal W and the fat signal F can be calculated, and separate images of water and fats can be obtained.

"However, in the Dixon method, the phase rotation induced by inhomogeneity of the static magnetic field produced when a subject is inserted into the static magnetic field space is not taken into consideration. If spatial inhomogeneity of the static magnetic field exists, phase rotation is induced depending on the position, which is different from the chemical shift, and therefore there arises a problem that water and fats cannot be completely separated by the simple addition and subtraction as shown by the equations (3) and (4).

"Therefore, as water/fat separation methods that take inhomogeneity of the static magnetic field into consideration, the methods described in Non-patent documents 2 and 3 are known.

"Non-patent document 2 describes a method in which a least square estimation processing is repeatedly performed with three images obtained with different echo times to determine three variables, water signal W, fat signal F, and frequency difference f induced by inhomogeneity of the static magnetic field, to separate water and fats images.

"In the method of Non-patent document 3, two of images, images I.sub.1 and I.sub.2, are obtained with such an echo time that water and fat signals become in-phase, and such an echo time that water and fat signals become out-of-phase, respectively, as in the Dixon method. However, in the method of Non-patent document 3, phase rotation induced by inhomogeneity of the static magnetic field is estimated by using the region growing method, and thereby water and fats signals are separated. In addition, in the method of Non-patent document 3, after the image I.sub.1 is obtained with such an echo time t.sub.in that water and fat signals become in-phase, the gradient magnetic field pulse is reversed, and the image I.sub.2 is obtained with such an echo time t.sub.out that water and fat signals become in-phase. That is, the in-phase image and out-of-phase image are obtained by one measurement."

Supplementing the background information on this patent, NewsRx reporters also obtained the inventors' summary information for this patent: "Object to be Achieved by the Invention

"In the method of Non-patent document 2 mentioned above, it is necessary to carry out the measurement three times, and therefore it has a problem that the measurement time is prolonged.

"Further, in the method of Non-patent document 3, water and fat in-phase image and out-of-phase image are obtained by one measurement by reversing the gradient magnetic field pulse. However, the time interval dt of t.sub.in and t.sub.out is in inverse proportion to the magnetic field strength. For example, when the magnetic field strength is 3.0 teslas, dt is (1.12+2.24.times.n) ms, when the magnetic field strength is 1.5 teslas, dt is (2.24+4.48.times.n) ms, and when the magnetic field strength is 0.5 tesla, dt is (6.70+13.39.times.n) ms, wherein n is an integer.

"Because dt is narrow at a magnetic field strength of 3.0 teslas or 1.5 teslas, the method suffers from restrictions concerning measurement, for example, the measurement band must be widened, and number of sampling points must be decreased. If the measurement band is widened, the SN ratio (henceforth abbreviated as SNR) is degraded, and thus image quality is degraded. If the number of sampling points is decreased, the spatial resolution of the image is degraded, and thus it becomes difficult to diagnose minute pathological lesions.

"Since the time interval dt can be widened by adjusting the value of the integer n, the restrictions concerning measurement are eased to some extent. However, it is difficult to obtain an image with arbitrary contrast for both of the two images to be obtained.

"When the magnetic field strength is 0.5 tesla, dt is large, and therefore the measurement time per one slice is prolonged. That is, in the two-dimensional multi-slice measurement, the number of slices per desired repetition time TR must be reduced, and therefore the number of slices desired to be obtained must also be decreased. Moreover, in the three-dimensional measurement, the measurement time is simply prolonged. If the measurement time is prolonged, motion artifacts of the subject increase. If the artifacts increase, errors in processing increase, and separation of water and fats signals becomes difficult.

"Moreover, in human tissues in which water and fats are intermingled, water signals and fat signals are compensated with each other, and hence SNR of the out-of-phase image is lowered. Therefore, it becomes impossible to correctly calculate the phase rotation induced by inhomogeneity of the static magnetic field due to the noises. As a result, the errors in processing increase, and separation of water and fats signals becomes difficult.

"Furthermore, it also suffers from the problem that it is difficult to determine a water image or a fat image from each of the obtained two images.

"An object of the present invention is to realize favorable water/fat separation in water/fat separation imaging, even if the measurement is not performed with such an echo time that the phases of water and fat signals become in-phase or out-of-phase. A further object of the present invention is to realize a device and a method enabling distinction of obtained two separation images, i.e., determining which is each of them, a water image or a fat image.

"Means for Achieving the Object

"According to the present invention, two original images are measured with such two of echo times that phase differences of water and fat signals do not become positive and negative values, and do not become integral multiples of .pi.. For example, when phase difference of water and fat signals obtained at a certain echo time t.sub.1 is represented by P.sub.1, and phase difference of water and fat signals obtained at an echo time t.sub.2, which is dt after the echo time t.sub.1, i.e., t.sub.1+dt=t.sub.2, is represented by P.sub.2, the measurement is performed with such echo times t.sub.1 and t.sub.2 that the following equations are satisfied. P.sub.1.noteq.n.sub.1.pi. (n.sub.1 is an integer) (5) P.sub.2.noteq.m.sub.1.pi. (m.sub.1 is an integer) (6) 2n.sub.2.pi.P.sub.1.noteq.-2m.sub.2.pi.P.sub.2 (n.sub.2 and m.sub.2 are integers) (7)

"From two of the original images obtained under the aforementioned conditions, two of water/fat ratio maps are calculated algebraically or by numerical analysis. From two of the calculated water/fat ratio maps, two of phase maps are calculated. By combining two of the calculated phase maps, two of minimum phase difference maps, each of which gives the minimum spatial phase difference, are calculated. On the basis of dispersion in differential maps obtained by spatially differentiating the minimum phase difference maps, it is determined whether they are correct minimum phase difference maps. By using one of the phase difference maps determined to be correct, phase correction of one of the original images is performed. By using the corrected original image, water and fat separation processing is performed.

"Specifically, the present invention provides a magnetic resonance imaging device comprising a static magnetic field generation part for generating a static magnetic field in a space in which a subject is placed, an irradiation part for irradiating a radio frequency magnetic field pulse on the subject, a reception part for receiving magnetic resonance signals generated from the subject by irradiation of the radio frequency magnetic field pulse, a gradient magnetic field application part for applying a gradient magnetic field for imparting spatial information to the magnetic resonance signals, a control part for operating the irradiation part, the gradient magnetic field application part, and the reception part according to a predetermined pulse sequence to perform imaging with two different echo times, and an image reconstruction part for reconstructing original images from the magnetic resonance signals obtained with the two different echo times, which further comprises a phase difference map calculation part for calculating maps of phase difference induced by inhomogeneity of the static magnetic field from the original images, a phase-corrected image calculation part for calculating a phase-corrected image by carrying out phase correction of one of the original images using the phase difference map, and a water/fat separation image calculation part for calculating a water/fat separation image from two images, the phase corrected image and the original image not subjected to the phase correction.

"The aforementioned magnetic resonance imaging device measures original images, for example, at such two of echo times that the aforementioned equations (5), (6) and (7) are satisfied. The aforementioned phase difference map calculation part comprises, for example, a water/fat ratio calculation part for calculating two of water/fat ratio maps algebraically or by numerical analysis from two of the original images, a phase map calculation part for calculating two of phase maps from two of the water/fat ratio maps, a minimum phase difference map calculation part for calculating two of minimum phase difference maps from combinations of points on two of the phase maps so as to give a spatial minimum phase difference for each point, and a phase difference map determination part for determining whether two of the minimum phase difference maps are correct minimum phase difference maps on the basis of dispersion in differential maps obtained by spatially differentiating the minimum phase difference maps.

"Effect of the Invention

"According to the present invention, in a method of obtaining images with different echo times for separation of water and fats images, the restrictions concerning the measurement are eased, and favorable water/fat separation can be realized. Furthermore, it becomes possible to determine which is each of the obtained two images, a water image or a fat image."

For the URL and additional information on this patent, see: Shirai, Toru; Bito, Yoshitaka. Magnetic Resonance Imaging Device. U.S. Patent Number 8787639, filed January 28, 2011, and published online on July 22, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8787639.PN.&OS=PN/8787639RS=PN/8787639

Keywords for this news article include: Mathematics, Magnetic Resonance, Numerical Analysis, Hitachi Medical Corporation.

Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC


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Source: Health & Medicine Week


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