Method and apparatus for estimating oxygen saturation of blood using magnetic resonance

• US 5,233,991 A
• Filed: 08/08/1991
• Issued: 08/10/1993
• Est. Priority Date: 08/08/1991
• Status: Expired due to Term

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Wright, Graham A.
• Primary Examiner(s): JAWORSKI, FRANCIS J
• Application Number:
  US07/742,249
• Time in Patent Office: 733 Days
• Field of Search: 128/653.1-653.5, 324/306-309, 324/312, 324/318, 324/321
• US Class Current: 600/410
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: A61B 5/145 Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissuemeasuring of blood pressure or blood flow A61B5/02; non-radiation detecting or locating of foreign bodies in blood A61B5/06
• CPC Class Codes: G01R 33/465 applied to biological material, e.g. in vitro testing

Abstract

The oxygen saturation of blood is determined by obtaining spin-spin relaxation time (T2_b) of the fluid using magnetic resonance imaging techniques. A simplified model relates oxygen saturation of blood to measured spin-spin relaxation time. A pulse sequence is given for in vivo estimation of T2_b including a plurality of 180° refocusing RF signals which are equally spaced in time. The last refocusing pulse is slice selective. Fat signal is suppressed by initially using a short inversion recovery sequence followed by a frequency selective 90° pulse that excites only water protons.

Claims

1. A method of determining a characteristic of a moving fluid based on measurements of spin-spin relaxation time of the fluid comprising the steps of

a) placing said moving fluid in a static magnetic field (B₀) along a first axis (z),

b) applying an excitation RF signal to said moving fluid thereby establishing transverse nuclei spins,

c) applying a plurality (0,1,2, . . . N) of 180°

refocusing pulses to said moving fluid with said pulses being equally spaced, the number and spacing of said pulses determining echo time,

d) applying after said plurality of 180°

refocusing pulses a slice selective pulse (that is slice selective), the time between said slice selective pulse and an echo being constant,

e) detecting signals from said nuclei spins,

f) determining the spin-spin relaxation time of said nuclei spins from said signals, and

g) determining said characteristic from said spin-spin relaxation time.

2. The method as defined in claim 1 wherein said moving fluid is blood and said characteristic is oxygen saturation of the blood.

3. The method as defined in claim 2 wherein the relationship of measured spin-spin relaxation time, T2_b, to calculated oxygen saturation of blood, %HbO₂, is given by:

##EQU3## where T2₀ is the spin-spin relaxation time of fully oxygenated blood,

τ

₁₈₀ is time between successive pulses in said plurality of 180°

refocusing pulses,

ω

₀ is the resonant nuclei frequency, and

K is a constant.

4. The method as defined by claim 3 wherein step b) includes at least one of i) applying an inversion pulse, then applying a magnetic gradient (Gz) along said first axis, and ii) applying a frequency selective 90°

pulse that excites water protons, thereby suppressing signals from fat.

5. The method as defined by claim 3 wherein step c) includes applying a pair of spoiling gradients with said last pulse to dephase any out of section signal.

6. The method as defined by claim 3 wherein step d) includes spatially encoding the signals using two-dimensional Fourier transform encoding.

7. The method as defined by claim 3 wherein step d) includes spatially encoding the signals using spiral gradients in k space during data acquisition.

8. The method as defined by claim 1 wherein step b) includes applying an inversion pulse, then applying a magnetic gradient (Gz) along said first axis, and then applying a frequency selective 90°

pulse that excites water protons, thereby suppressing signals from fat.

9. The method as defined in claim 1 wherein step c) includes applying a pair of spoiling gradients with said last pulse to dephase any out of section signal.

10. The method as defined in claim 1 wherein step d) includes spatially encoding the signals using two-dimensional Fourier transform encoding.

11. The method as defined in claim 1 wherein step d) includes spatially encoding signals using spiral gradients in k space durnig data acquisition.

12. Apparatus for determining a characteristic of a moving fluid based on measurements of spin-spin relaxation time of the fluid comprising

a) means for establishing a static magnetic field (B₀) through said fluid along a first axis (z),

b) means for applying an inversion RF signal to said moving fluid thereby establishing transverse nuclei spins,

c) means for applying a plurality of 180°

refocusing pulses to said moving fluid with said pulses being equally spaced,

d) means for applying a slice selective pulse after said plurality of 180°

pulses, the time between said slice selective pulse and an echo being constant,

e) means for detecting free induction decay signals from said nuclei spins,

f) means for determining the spin-spin relaxation times of said nuclei spins from said signals, and

g) means for determining said characteristic from said spin-spin relaxation time.

13. Apparatus as defined by claim 12 wherein said moving fluid is blood and said characteristic is oxygen saturation of blood, the relationship of measured spin-spin relaxation time, T2_b, to calculated oxygen saturation of blood, %HbO₂, is given by:

##EQU4## where T2₀ is the spin-spin relaxation time of fully oxygenated blood,

τ

₁₈₀ is time between successive pulses in said plurality of 180°

refocusing pulses,

ω

₀ is the resonant nuclei frequency, and

K is a constant.

14. Apparatus as defined by claim 13 wherein said means for detecting free induction decay signals includes means for spatially encoding the signals using two-dimensional Fourier transform encoding.

15. Apparatus as defined by claim 14 wherein said means for detecting free induction decay signals includes means for applying spiral gradients in k space during data acquisition.