MAGNETIC RESONANCE IN MEDICINE
25,372-379 ( 1992)
Magnetization Transfer Time-of-Flight Magnetic Resonance Angiography G. BRUCEPIKE,*BOB s. HU,? GARYH. GLOVER,*AND DIETERR. ENZMANN*
* Department of Radiology and f Departments of Electrical Engineering
and Medicine,
Stanford University, Stanford, Calfornia 94305 Received November 25, 1991; revised January 10, 1992; accepted January 13, 1992 Time-of-flight (TOF)angiography based on inflow enhancement is limited by the steadystate signal differences between blood and the surrounding stationary tissues. We present a new TOF sequence in which magnetization transfer contrast is used to supplement wash. in effects. Angiograms demonstrating the superior performance of this technique are presented. Q 1992 Academic Press, Inc.
INTRODUCTION
Time-of-flight (TOF) magnetic resonance ( M R ) angiography methods rely upon signal magnitude differencesgenerated by the displacement of spins between successive excitations. In rapid gradient-echo sequences the wash-in of unsaturated spins results in an enhanced blood signal relative to the stationary tissues that have achieved a steady state ( 1-4). When combined with gradient moment nulling techniques to minimize flow dephasing, and judicious selection of sequence repetition time ( T R ) and excitation pulse angle ( a ) ,the contrast between stationary and in-flowing blood may be maximized. Typically, three dimensional (3D) volumes are acquired, either as a sequential series of 2D slices or directly as a 3D volume, and reprojected at multiple viewing angles (3, 5 ) . For regions demanding isotropic high resolution and containing flow in multiple directions, such as the cerebral vasculature, 3D volume acquisitions are preferred (6, 7). However, slow flow and a large excitation volume require a small excitation pulse angle to minimize premature saturation of the blood as it penetrates the volume and approaches its steady state. In the limit, when the longitudinal magnetization of the blood achieves its steady state, the available contrast depends solely upon NMR ( T ,, T 2 ,and p ) and sequence (TR, TE, a, sequence type) parameters. The exchange of magnetization between protons in water and those in macromolecules has a significant contribution to the observed relaxation rate of water in biological tissues (8-10). Wolf and Balaban ( I 1 ) were the first to produce images with magnetization transfer contrast (MTC) using a continuous wave ( CW) off-resonance saturation transfer experiment. Hu el al. (12) introduced a pulsed on-resonance saturation transfer method that has been demonstrated in a variety of imaging sequences (13, 1 4 ) . In each approach the orders of magnitude difference in T2between protons in thefiee water pool ( ' H f ) and protons in restricted motion sites ('H,.) is exploited for selective 'H, saturation. Magnetization exchange results in a decreased observed 0740-3194192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
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'H magnetization and TI with the magnitude of these effects being dependent upon the relaxation times of the individual pools, the relative pool sizes, and the exchange rate constant. In this paper we present a new approach to TOF MR angiography in which the contrast due to inflow enhancement is supplemented with magnetization transfer contrast. We present a rapid gradient-echo pulse sequence that produces significant MTC, while maintaining the signal from flowing blood, with a minimal increase in TR. The impact on contrast between blood and various tissues is estimated for both individual slices and reprojections. The efficacy of this technique is demonstrated for intracranial vessel imaging. METHODS
The 3D FT version of the magnetization transfer (MT) TOF pulse sequence is shown in Fig. 1. The TOF portion of the sequence consists of a slab-selective a pulse followed immediately by a velocity compensated combined refocusing and z phaseencoding gradient. An asymmetric read gradient is used with velocity compensation while the y phase-encoding gradient is uncompensated. To decrease signal from regions of fat, the onset of the read and y phase-encode gradients is delayed so as to produce an echo time (TE) of 7.3 ms. This represents the minimum TE achievable on our scanner (General Electric, 1.5 T Signa) for which water and fat are out of phase. The y and L phase-encodings are rephased after data sampling and a constant spoiler gradient is applied along the z direction. The phase of the RF pulse can be held constant to preserve transverse coherences or may be manipulated to spoil them ( 2 5 , 16). Selective saturation of the 'H, magnetization pool is achieved using a short intense 12 1 binomial zero-degree pulse ( 17). The extremely short T2(
25,372-379 ( 1992)
Magnetization Transfer Time-of-Flight Magnetic Resonance Angiography G. BRUCEPIKE,*BOB s. HU,? GARYH. GLOVER,*AND DIETERR. ENZMANN*
* Department of Radiology and f Departments of Electrical Engineering
and Medicine,
Stanford University, Stanford, Calfornia 94305 Received November 25, 1991; revised January 10, 1992; accepted January 13, 1992 Time-of-flight (TOF)angiography based on inflow enhancement is limited by the steadystate signal differences between blood and the surrounding stationary tissues. We present a new TOF sequence in which magnetization transfer contrast is used to supplement wash. in effects. Angiograms demonstrating the superior performance of this technique are presented. Q 1992 Academic Press, Inc.
INTRODUCTION
Time-of-flight (TOF) magnetic resonance ( M R ) angiography methods rely upon signal magnitude differencesgenerated by the displacement of spins between successive excitations. In rapid gradient-echo sequences the wash-in of unsaturated spins results in an enhanced blood signal relative to the stationary tissues that have achieved a steady state ( 1-4). When combined with gradient moment nulling techniques to minimize flow dephasing, and judicious selection of sequence repetition time ( T R ) and excitation pulse angle ( a ) ,the contrast between stationary and in-flowing blood may be maximized. Typically, three dimensional (3D) volumes are acquired, either as a sequential series of 2D slices or directly as a 3D volume, and reprojected at multiple viewing angles (3, 5 ) . For regions demanding isotropic high resolution and containing flow in multiple directions, such as the cerebral vasculature, 3D volume acquisitions are preferred (6, 7). However, slow flow and a large excitation volume require a small excitation pulse angle to minimize premature saturation of the blood as it penetrates the volume and approaches its steady state. In the limit, when the longitudinal magnetization of the blood achieves its steady state, the available contrast depends solely upon NMR ( T ,, T 2 ,and p ) and sequence (TR, TE, a, sequence type) parameters. The exchange of magnetization between protons in water and those in macromolecules has a significant contribution to the observed relaxation rate of water in biological tissues (8-10). Wolf and Balaban ( I 1 ) were the first to produce images with magnetization transfer contrast (MTC) using a continuous wave ( CW) off-resonance saturation transfer experiment. Hu el al. (12) introduced a pulsed on-resonance saturation transfer method that has been demonstrated in a variety of imaging sequences (13, 1 4 ) . In each approach the orders of magnitude difference in T2between protons in thefiee water pool ( ' H f ) and protons in restricted motion sites ('H,.) is exploited for selective 'H, saturation. Magnetization exchange results in a decreased observed 0740-3194192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
372
373
COMMUNICATIONS
'H magnetization and TI with the magnitude of these effects being dependent upon the relaxation times of the individual pools, the relative pool sizes, and the exchange rate constant. In this paper we present a new approach to TOF MR angiography in which the contrast due to inflow enhancement is supplemented with magnetization transfer contrast. We present a rapid gradient-echo pulse sequence that produces significant MTC, while maintaining the signal from flowing blood, with a minimal increase in TR. The impact on contrast between blood and various tissues is estimated for both individual slices and reprojections. The efficacy of this technique is demonstrated for intracranial vessel imaging. METHODS
The 3D FT version of the magnetization transfer (MT) TOF pulse sequence is shown in Fig. 1. The TOF portion of the sequence consists of a slab-selective a pulse followed immediately by a velocity compensated combined refocusing and z phaseencoding gradient. An asymmetric read gradient is used with velocity compensation while the y phase-encoding gradient is uncompensated. To decrease signal from regions of fat, the onset of the read and y phase-encode gradients is delayed so as to produce an echo time (TE) of 7.3 ms. This represents the minimum TE achievable on our scanner (General Electric, 1.5 T Signa) for which water and fat are out of phase. The y and L phase-encodings are rephased after data sampling and a constant spoiler gradient is applied along the z direction. The phase of the RF pulse can be held constant to preserve transverse coherences or may be manipulated to spoil them ( 2 5 , 16). Selective saturation of the 'H, magnetization pool is achieved using a short intense 12 1 binomial zero-degree pulse ( 17). The extremely short T2(
