Magnetic resonance apparatus employing delayed self-refocusing RF excitation

• US 5,280,245 A
• Filed: 07/27/1992
• Issued: 01/18/1994
• Est. Priority Date: 07/27/1992
• Status: Expired due to Term

• Current Assignee: Ethyl Corporation (Newmarket Corporation)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Pauly, John M.
• Primary Examiner(s): ARANA, LOUIS M
• Application Number:
  US07/919,445
• Time in Patent Office: 540 Days
• Field of Search: 324/307, 324/308, 324/309, 324/311, 324/312, 324/314
• US Class Current: 324/307
• CPC Class Codes: G01R 33/446 Multifrequency selective RF pulses, e.g. multinuclear acquisition mode spatially selective RF pulses G01R33/4833

Abstract

MRI Apparatus for obtaining a spin-echo signal applies a self-refocusing RF pulse with variable delay in the spin-echo signals following application of the RF pulse. The RF pulse is designed using the Shinnar-LeRoux algorithm B₁ (t)=SLR^-1 {A_n (z), B_n (z) } with A_n '"'"'(z) substituted for A_n (z).

A_n '"'"'(z) is slightly non-minimum phase and equals P(z) A_n (z) where P(z) is a unit amplitude phase function chosen to compensate the phase of B_n (z). The RF pulse can be designed for a specific delay, a specific power constraint, and small tip angle.

Claims

1. A method of exciting a body with RF excitation in MRI apparatus with a self-refocusing spin echo and variable delay following the RF excitation comprising the steps of:

(a) placing said body in a magnetic field (B_O),

(b) applying an RF excitation pulse, B₁ (t), to said body, said excitation pulse being defined as

space="preserve" listing-type="equation">B.sub.1 (t)=SLR.sup.-1 {P(z) A.sub.n (z), B.sub.n (z)}

where

SLR is the Shinnar-LeRoux algorithm

B_n (z) is a polynomial defining an FIR filter

A_n (z) is a minimum phase polynomial consistent with B_n (z)

P(z) is a unit amplitude phase function which compensates the phase of B_n (z).

2. The method as defined by claim 1 wherein ##EQU3## where Z_o =reⁱφ

, and r,Φ

are parameters that define locations of poles and zeros of the all-pass filter stage defined by the equation.

3. The method as defined by claim 1 wherein said excitation pulse has minimum SAR, where SAR is proportional to (1-a_o) and a_o is the zero order coefficient of the A_n (z) polynomial.

4. The method as defined by claim 1 wherein B_n (z) is a polynomial defining a minimum phase FIR filter.

5. The method as defined by claim 1 wherein B_n (z) is scaled to produce a tip angle of less than 90°

.

6. The method as defined by claim 1 wherein B_n (z) is scaled to produce a tip angle of greater than 90°

.

7. Apparatus for obtaining MRI signals from a body comprising:

(a) means for establishing a magnetic field (B_O) through said body,

(b) means for exciting said body with an RF excitation pulse, B₁ (t), as follows;

space="preserve" listing-type="equation">B.sub.1 (t)=SLR.sup.-1 {P(z) A.sub.n (z), B.sub.n (z)}

where

SLR is the Shinnar-LeRoux algorithm

B_n (z) is a polynomial defining an FIR filter

A_n (z) is a minimum phase polynomial consistent with B_n (z)

P(z) is a unit amplitude phase function which compensates the phase of B_n (z); and

(c) means for detecting a spin echo signal from said body following application of said RF excitation pulses.

8. Apparatus as defined by claim 7 wherein ##EQU4## where Z_o =reⁱφ

, and r,Φ

are parameters that define locations of poles and zeros of the all-pass filter stage defined by the equation.

9. Apparatus as defined by claim 7 wherein said excitation pulse has minimum SAR, where SAR is proportional to (1-a_o) and a_o is the zero order coefficient of the A_n (z) polynomial.

10. Apparatus as defined by claim 7 wherein B_n (z) is a polynomial defining a minimum phase FIR filter.

11. Apparatus as defined by claim 7 wherein B_n (z) is scaled to produce a tip angle of less than 90°

.

12. Apparatus as defined by claim 7 wherein B_n (z) is scaled to produce a tip angle of greater than 90°

.