Magnetic Resonance Imaging

Factors Influencing Signal & Contrast

Tissue Dependent Properties
Spin-Lattice Relaxation Time, T1
Spin-Spin Relaxation Time, T2
Spin Density, r
T2*

Sequence Dependent Properties
Repetition Time, TR
Echo Time, TE
Inversion Time, TI
Rotation Angle, q
T2*

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Signal equations for imaging pulse sequences.

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Spin-Echo

S = k r (1-exp(-TR/T₁)) exp(-TE/T₂)

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Inversion Recovery (180-90)

S = k r (1-2exp(-TI/T₁)+exp(-TR/T₁))

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Inversion Recovery (180-90-180)

S = k r (1-2exp(-TI/T₁)+exp(-TR/T₁)) exp(-TE/T₂)

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Gradient Recalled Echo

S = k r (1-exp(-TR/T₁)) Sinq exp(-TE/T₂*) / (1 -Cosq exp(-TR/T₁))

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S = amplitude of the signal in the frequency domain spectrum.
k = proportionality constant that depends on the sensitivity of the signal detection circuitry.
r = Spin Density. A number proportional to the number of signal bearing spins in a voxel.
T₁, T₂, and r are specific to a tissue or pathology.

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The range T₁, T₂, and r values at 1.5T for tissues found in a magnetic resonance image of the human head.

Tissue	T1 (s)	T2 (ms)	r*
CSF	0.8 - 20	110 - 2000	70-230
White	0.76 - 1.08	61-100	70-90
Gray	1.09 - 2.15	61 - 109	85 - 125
Meninges	0.5 - 2.2	50 - 165	5 - 44
Muscle	0.95 - 1.82	20 - 67	45 - 90
Adipose	0.2 - 0.75	53 - 94	50 - 100
*Based on r=111 for 12mM aqueous NiCl2

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Contrast (C)

The difference in signal intensity of two tissues A and B.

C_AB = S_A - S_B

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Spin-Echo Sequence

C(TR)

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C(TE)

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It takes a few TR periods for the signal to equilibrate because transverse magnetization from a 90^o pulse reaches an equilibrium value after a few TR cycles.

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Weighted Image

An image whose contrast is primarily a function of the weighting variable.

Weighting	TR Value	TE Value
T1	< = T1	< < T2
T2	> > T1	> = T2
r	> > T1	< < T2

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Examples

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Spin-Echo Images

TR (ms)	TE (ms)
	20	40	60	80
250
500
750
1000
2000
TR (ms)	20	40	60	80
	TE (ms)

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Inversion Recovery Images (180-90)

	TR (ms)
TI (ms)	1000	2000
50
100
250
500
750
TI (ms)	1000	2000
	TR (ms)

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Gradient Recalled Echo Images ( TE=5 ms )

	TR (ms)
q ( o )	25	50	100	200
15
30
45
60
90
q ( o )	25	50	100	200
	TR (ms)

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Copyright © 2000 J.P. Hornak.
All Rights Reserved.