Unipolar fast spin echo for permanent magnet MRI

Patent metadata

Field	Value
Publication number	US-10261146-B2
Application number	US-201514700207-A
Country	US
Kind code	B2
Filing date	Apr 30, 2015
Priority date	May 21, 2014
Publication date	Apr 16, 2019
Grant date	Apr 16, 2019

Abstract

A method of reducing artifacts produced during Fast Spin Echo measurements made using permanent magnet NMR instruments. The method includes applying encoding gradients that do not switch signs throughout the experiment. Prior to the 90° RF pulse, a strong RM gradient pulse is given to produce a dominant and constant residual magnetization. The encoding is done through the combination of encoding gradients with the aid of the 180° RF pulses of the echo train. A first constant encoding gradient is given before the first 180 pulse. Then two variable encoding gradients are provided after each 180 pulse; one applied prior to and one applied subsequent to each acquisition in the echo train.

First claim

What is claimed is:

1. A method for reducing artifacts produced during a Fast Spin Echo (FSE) sequence in a nuclear magnetic resonance instrument comprising a gradient power amplifier and a permanent magnet comprising a plurality of pole pieces, said permanent magnet characterized by a gradient-dependent residual magnetization B g , said FSE pulse sequence comprising, in order:
a 90° radio frequency (RF) pulse characterized by a duration of the 90° RF pulse (T 90 );
an echo train comprising a first 180° RF pulse, a first echo, X−1 additional 180° RF pulses and X−1 additional echoes, where X≥1; and,
a navigator pulse;

wherein said method comprises:
applying a residual magnetization (RM) gradient pulse prior to said 90° RF pulse, said RM gradient pulse having an amplitude G RM pe along a phase axis and a positive or a negative sign relative to an origin along said phase axis;
applying a shim gradient pulse of amplitude at least sufficient to cancel any net effect on said gradient-dependent residual magnetization B g produced by said RM gradient pulse;
applying, prior to said first 180° RF pulse, a single preparation gradient pulse having an amplitude G shift pe that is less than or equal to the RM gradient pulse amplitude G RM pe , whereby for a gradient pulse duration T, a k-value of the single preparation gradient pulse (k shift pe ) is equal to the single preparation gradient pulse amplitude G shift pe times the gradient pulse duration T along said phase axis;
applying, for each X−1 additional 180° RF pulses i (1≤i≤X) in said echo train:
a first encoding gradient pulse applied subsequent to said 180° pulse i and prior to echo i, said first encoding gradient pulse having an amplitude G ai pe such that a k-value for the first encoding gradient pulse k ai pe =G ai pe times T along said phase axis; and,
a second encoding gradient pulse applied subsequent to said echo i and prior to said 180° pulse i+1, said second encoding gradient pulse G bi pe having an amplitude G bi pe such that a k-value for the second encoding gradient pulse k bi pe =G bi pe times T along said phase axis, said second encoding gradient G bi pe having an identical sign to the sign of said RM gradient pulse; and,

applying, subsequent to said navigator pulse, a final gradient pulse, said final gradient pulse having a sign identical to the sign of said RM gradient pulse;
obtaining a magnetic resonance images based on the FSE sequence.

2. The method according to claim 1 , wherein said preparation gradient pulse is characterized by k pe shift ≥k pe max /2, where k pe is being incremented between [−k pe max /2, k pe max /2] k pe max is defined by the required spatial resolution, dr pe , along the phase axis, such as: k pe max =1/dr pe .

3. The method according to claim 1 , wherein for 1≤i≤X,k pe ai =G pe ai T=k pe shift +k pe i , where k pe ai , is the imposed k value after the gradient, G pe ai .

4. The method according to claim 1 , wherein for 1≤i≤X,k pe bi =G pe bi T=k pe shift −k pe i where k pe bi , is the imposed k value after the gradient, G pe bi .

5. The method according to claim 1 , wherein said final gradient pulse is characterized by k=k shift pe .

6. The method according to claim 1 , wherein said FSE sequence is a 3D FSE sequence comprising:
an embedded 2D FSE sequence;
an encoding axis k pe2 along said slice axis;
N pe2 increments along said slice axis;
j increments of k pe2 , 1≤j≤N pe2 , the j th increment of k pe2 (k j pe2 ) being restricted to values in the range of

-

k
max

pe
⁢

⁢
2

2

+
β

≤

k
j

pe
⁢

⁢
2

≤

k
max

pe
⁢

⁢
2

2

+
β

,

where

k
max

pe
⁢

⁢
2

=

1

dr
s

,

dr s is a spatial resolution along the slice axis, and β is an asymmetric k pe2 shift;
and further wherein said method additionally comprises:
applying a slice RM gradient pulse prior to said 90° RF pulse, said slice RM gradient pulse having an amplitude G RM s and a positive or negative sign along a slice axis, wherein said slice RM gradient pulse has a constant amplitude throughout said FSE sequence;
applying a slice shim gradient pulse having an amplitude that is at least sufficient to cancel any net effect on said gradient-dependent residual magnetization B g produced by said slice RM gradient pulse;
applying slice gradient pulses having an amplitude G s that is less than or equal to the amplitude of the slice RM gradient pulse G RM s along said slice axis, substantially simultaneously with said 90° RF pulse and said 180° RF pulse in said FSE pulse sequence;
applying, prior to the first 180° pulse, a preparation encoding gradient having an amplitude G 0 pe2 that is less than or equal to the amplitude of the slice RM gradient pulse, such that a k-value of the preparation encoding gradient k 0 pe2 =G 0 pe2 times T along said slice axis;
for each 180° pulse i (1≤i≤X) in said echo train:
applying, subsequent to said 180° pulse i and prior to echo i, a first encoding gradient pulse having amplitude G aj pe2 , such that a k-value of the first encoding gradient k aj pe2 =G aj pe2 times T along said slice axis; and,
applying a second encoding gradient pulse having an amplitude G bj pe2 subsequent to said echo i, such that a k-value of the second encoding gradient k bj pe2 =G bj pe2 times T along said slice axis; and,

applying, subsequent to said navigator pulse, a final gradient pulse;

and further wherein all four of said gradient pulses have a sign identical to, and an amplitude smaller than, that of said slice RM pulse.

7. The method according to claim 6 , wherein

k

Assignees

• Aspect Imaging Ltd

Inventors

• Shapira Boaz
• Rapoport Uri

Classifications

• G01R33/5617

  using RF refocusing, e.g. RARE · CPC title

• G01R33/383 Primary

  using permanent magnets · CPC title

• G01R33/56563

  caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0 (G01R33/56509, G01R33/56518, G01R33/56536 take precedence) · CPC title