US 4,720,679 A · Magnetic resonance imaging with phase encoded chemical shift correction

Abstract

A shimming magnetic field control (22) causes shimming magnetic fields for improving uniformity of a main magnetic field generated by main magnets (10). A resonance excitation control (32) selectively applies a resonance excitation pulse (34) and inversion pulse (36) for inverting the spin magnetization of water and lipid dipoles. A phase sensitive detector (30) selectively receives resonance signal components which are transformed by a transform algorithm (70) into a real image (72) and an imaginary image (74). The inversion pulse (36) is shifted by a time such that the water and lipid spin magnetizations are out of phase by a predetermined amount. With a 90.degree. phase difference, the real image represents water and the imaginary represents lipid. A phase image (80) is derived from a reference image pair (76, 78). A phase unwrap circuit (82) removes ambiguities attributable to the spin magnetizations becoming dephased by multiples of 2.pi. to create a phase map (84). A shim adjustment circuit (94) measures uniformity of the phase map and causes the shim field control circuit (22) to adjust the shimming magnetic fields to optimize the measured uniformity. The phase map is inversely transformed (100) to generate a field map (102) indicative of geometric distortion. A geometric distortion correction circuit (106) selectively interpolates values in the real and imaginary images in accordance with the field map to correct the image for geometric distortion.

Claims49

Dependent claims

A method of generating both a water image and a lipid image with a single magnetic resonance scan, the method comprising:

exciting magnetic resonance in water and lipid dipoles of an image region such that magnetic resonance signal components are generated thereby;
inverting spin magnetization of the resonating dipoles to cause a magnetic resonance echo, the inversion being timed such that water and lipid spin magnetizations are n.pi..+-.90.degree. out of phase during the echo, when n is an integer;
collecting magnetic resonance signal components adjacent the echo, whereby the n.pi..+-.90.degree. out of phase spin magnetizations corresponding to water and lipid are independent;
transforming the collected echo magnetic resonance signal components into a set of pixel values;
dividing the pixel values between a real image and an imaginary image, one of which is the water image and the other of which is the lipid image, whereby collecting data with the n.pi..+-.90.degree. out of phase spin magnetizations enables independent water and lipid images to be collected concurrently during a single scan.

2 more elements

A method of magnetic resonance imaging comprising:

(a) exciting magnetic resonance in water and lipid dipoles in an image region such that magnetic resonance signal components are generated:
(b) spatially encoding the magnetic resonance signal components;
(c) transforming the spatially encoded resonance signal components into a first image corresponding to water dipole generated resonance signal components and a second image corresponding to the lipid dipole generated resonance signal components;
(d) generating a phase map from transformed resonance signals;
(e) inverse transforming the phase map to create a field map;
(f) interpolating at least one of the images in accordance with the field map, whereby the image is corrected for geometric distortion attributable to magnetic field inhomogeneities caused by static, gradient, and local field errors.

3 more elements

A method of magnetic resonance imaging comprising:

(a) generating a generally uniform, main magnetic field longitudinally through an image region;
(b) applying magnetic field gradients in the image region for spatially encoding resonance signal components;
(c) applying an excitation radio frequency pulse for exciting magnetic resonance of at least water and lipid dipoles in the image region, the water and lipid dipoles resonate at different frequencies such that the respective spin magnetizations thereof progressively dephase, i.e. become progressively more out of phase;
(d) applying an inversion pulse such that the dephasing spin magnetizations commence rephasing;
(e) phase sensitively detecting the resonance signal components when the rephasing spin magnetizations are about a n.pi..+-..pi./2 degrees out of phase, where n is an integer;
(f) transforming the detected resonance signal components into real and imaginary images, one image corresponding to resonance signals generated by the water dipoles and the other image corresponding to resonance signal components generated by the lipid dipoles.

3 more elements

A method of magnetic resonance imaging comprising:

(a) applying a magnetic resonance excitation pulse;
(b) applying an inversion pulse a time t.+-..epsilon. after the excitation pulse;
(c) collecting magnetic resonance signals around a first echo about a time 2t after the excitation pulse;
(d) applying a second inversion pulse about a time 3t after the excitating pulse;
(e) collecting magnetic resonance signals around a second echo about a time 4t after the excitation pulse;
(f) performings steps (a) through (e) with .epsilon.=0 and transforming the magnetic resonance signals collected during the first echo into a first image pair and during the second echo into a second image pair;
(g) performing steps (a) through (e) with .epsilon..noteq.2n.pi. such that the water and lipid dipoles are placed in a preselected phase relationship, where n is an integer including zero and transforming the magnetic resonance signals collected during the first echo into a third image pair and during the second echo into a fourth image pair;
(h) deriving first, second, third, and fourth magnitude images from the first, second, third, and fourth image pairs, respectively;
(i) adding the first and third magnitude images to create a first echo dominant image and adding the second and fourth magnitude images to create a second dominant image;
and,
(j) subtracting the first and third magnitude images to create a first echo subdominant image and subtracting the second and fourth magnitude images to create a second echo subdominant image.

8 more elements

A method of magnetic resonance imaging comprising:

(a) inverting spin magnetization;
(b) exciting magnetic resonance in water and lipid dipoles in an image region such that magnetic resonance signal components are generated;
(c) inverting the spin magnetization of the resonating water and lipid dipoles;
(d) collecting the magnetic resonance signal components around an echo;
(e) performing steps (a) and (b) another time;
(f) performing step (c) another time with the inversion time shifted relative to step (c) such that the resonating water and lpid dipoles are placed in a preselected phase relationship;
(g) performing step (d) another time following the time shifted inversion;
(h) performing steps (b), (c), and (d) another time;
(i) performing steps (b), (f), and (g) another time;
(j) transforming the magnetic resonance signal components collected during steps (d), (g), (h), and (i) into first, second, third and fourth image representations each consisting of magnitude and phase components respectively;
(k) adding the first and second image representation magnitude components to create a first dominant image;
(l) subtracting the first and second image representation magnitude components to create a first subdominant image;
(m) adding the third and fourth image representation magnitude components to create a second dominant image;
(n) substracting the third and fourth image representation magnitude components to create a second subdominant image;
(o) correcting the signs of the first and second dominant and subdominant images;
(p) forming a first T1 relaxation parameter image from the sign corrected first and second dominant images;
and,
(q) forming a second T1 relaxation parameter image from the first and second subdominant images.

15 more elements

An apparatus for magnetic resonance imaging comprising:

(a) magnetic resonance excitation means for exciting magnetic resonance in water and lipid dipoles in an image region such that magnetic resonance signal components are generated;
(b) gradient field means for spatially encoding the magnetic resonance signals;
(c) transforming means for transforming the spatially encoded resonance signal components into a first image corresponding to water dipole generated resonance signal components and a second image corresponding to lipid dipole generated resonance signal components;
(d) a means for generating a phase image;
(e) a phase unwrapping means for removing ambiguities in the phase image attributable to water and lipid spin magnetizations that are dephased by at least 2.pi. radians to create an ambiguity corrected phase map;
(f) inverse transform means for inversely transforming the phase map to create the field map;
and,
(g) geometric distortion correction means for interpolating at least one of the images in accordance with the field map.

5 more elements

An apparatus for magnetic resonance imaging comprising:

a main magnetic field means for generating a generally uniform, main magnetic field longitudinally through an image region;
a gradient field means for applying gradient magnetic fields in the image region for spatially encoding the resonance signal components;
a resonance excitation means for selectively exciting magnetic resonance of at least water and lipid dipoles in the image regions, the resonance excitation means including means for applying an excitation radio frequency pulse which induces the water and lipid dipoles to resonate at different resonance frequencies such that the respective magnetizations thereof progressively dephase and an inversion pulse applying means for applying an inversin pulse which inverts the spin magnetizations such that the dephasing spin magnetizations commence rephasing;
a phase sensitive detection means for detecting the magnetic resonance signal components when the rephasing magnetizations are about a preselected angle out of phase;
and,
transforming means for transforming the detected resonance signal components into real and imaginary images for storage in real and imaginary image memories, one image corresponding to resonance signals generated by the water dipoles and the other image corresponding to dipoles generated by the lipid dipoles.

3 more elements

An apparatus for magnetic resonance imaging comprising:

a magnetic resonance exciation means for exciting magnetic resonance of at least water and lipid dipoles in an image region by applying a magnetic resonance excitation pulse, a first inversion pulse about a time t.+-..epsilon. after the excitation pulse and a second inversion pulse about a time 3t.+-..epsilon. after the excitation pulse;
a magnetic resonance signal collecting means for receiving magnetic resonance signals around a first echo about a time 2t after the excitation pulse and around a second echo about a time 4t after the excitation pulse;
a transform means for transforming resonance signals collected around the first echo when .epsilon.=0 into a first memory means, for transforming resonance signals collected adjacent the second echo when .epsilon.=0 into a second memory means, for transforming resonance signals collected adjacent the first echo when .epsilon.=n.pi. into a third memory means and for transforming resonance signals collected around the second echo with .epsilon.=n.pi. into a fourth memory means, where n is an integer;
a magnitude image deriving means for deriving first, second, third, and fourth magnitude images from the first, second, third, and fourth memory means respectively;
a phase image means for deriving phase images from at least some of the first, second, third, and fourth memory means;
a first echo dominant image generating means for generating a first echo dominant image by additively combining the magnitude images corresponding to resonance signals collected during the first echo with .epsilon.=0 and .epsilon.=n.pi.;
a subdominant image generating means for subtractively combining the magnitude images corresponding to resonance signals collected during the first echo when .epsilon.=0 and .epsilon.=n.pi.;
a second echo dominant image generating means for generating a second echo dominant image by additively combining the magnitude images corresponding to resonance signals collected during the second echo with .epsilon.=0 and .epsilon.=n.pi.;
a second subdominant image generating means for subtractively combining the magnitude images corresponding to resonance signals collected during the second echo when .epsilon.=0 and .epsilon.=n.pi.;
a sign adjusting means for adjusting the sign of pixel values in each of the first and second dominant and subdominant images.

7 more elements

Dependencies are inferred from the claim text.

Magnetic resonance imaging with phase encoded chemical shift correction

Life of this patent

Magnetic resonance imaging with phase encoded chemical shift correction

Life of this patent

Abstract

Claims49