Magnetization Transfer
How is contrast generated by magnetization transfer? Also, what are MTI, MTC, and MTR?
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As described in a previous Q&A, magnetization transfer (MT) is the physical process by which macromolecules and their closely associated water molecules (the "bound" pool) cross-relax with protons in the free water pool.
Magnetization transfer imaging (MTI) is a technique by which radiofrequency (RF) energy is applied exclusively to the bound pool using specially designed MT pulse(s). Some of this energy is then transferred to the free water pool primarily via dipole-dipole interactions. Depending on the degree of coupling between these pools, the free water pool becomes partially saturated. If the free water pool is subsequently imaged using routine RF pulses and gradients, its signal will be reduced secondary to the MT effect.
The magnitude of this MT effect can be quantified by obtaining two sets of images (one with an MT pulse and one without it) and then digitally subtracting them. The magnetization transfer ratio (MTR) for a given voxel may then be defined and computed as:
Magnetization transfer imaging (MTI) is a technique by which radiofrequency (RF) energy is applied exclusively to the bound pool using specially designed MT pulse(s). Some of this energy is then transferred to the free water pool primarily via dipole-dipole interactions. Depending on the degree of coupling between these pools, the free water pool becomes partially saturated. If the free water pool is subsequently imaged using routine RF pulses and gradients, its signal will be reduced secondary to the MT effect.
The magnitude of this MT effect can be quantified by obtaining two sets of images (one with an MT pulse and one without it) and then digitally subtracting them. The magnetization transfer ratio (MTR) for a given voxel may then be defined and computed as:
MTR = (So − SMT)/So
where So is the magnitude of tissue signal before the MT pulse and SMT is the signal after the MT pulse has been applied. The relative difference in signal between two adjacent tissues (A and B) is known as magnetization transfer contrast (MTC). It should be noted that MTR measurements are not absolute, are susceptible to motion-related errors, and vary significantly as a function of the shape, bandwidth, and frequency offset of the MT pulse.
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Notwithstanding these limitations, several useful clinical applications of MTC have been demonstrated. I personally find it helpful to think of MT pulses as "macromolecule sat pulses" which selectively suppress tissues with significant water-macromolecule interactions.
This suppression of background tissue by MT pulses makes the technique especially useful as an adjunct to MR angiography (MRA). Small vessel detail and overall image contrast are significantly improved in time of flight (TOF) MRA when MT pulses are employed. A second potential use of MT pulses is in conjunction with contrast-enhanced T1-weighted images. Because gadolinium (Gd) enhancement is caused by a water-Gd ion interaction (not macromolecular cross-relaxation), MT pulses suppress the signal from background tissues and render Gd-enhanced areas more conspicuous. A third potential use of MT pulses is in conjunction with T2-weighted images when early demyelination or protein destruction may be detected before its appearance on routine images. In this application, diseased tissues with altered protein-water interactions are less suppressed by MT pulses, rendering them more conspicuous on T2-weighted images. |
Normal brain before (left) and after (right) MT saturation. Note the central sulcus (arrows) has different MT contrast than the other sulci.
Time of flight MRA without (left) and with (right) MT suppression. Note better visualization of small vessels after MT pulse.
Child with herpes encephalitis. Gadolinium enhancement is much better seen on MT image (right).
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Finally and surprisingly, mild to moderate MT effects are present in most MR images even when specific MT pulses have not been applied! Off-resonance excitation occurs in adjacent slices of 2D multi-slice imaging and is particularly prominent in fast spin-echo applications where multiple 180° pulses are used.
References
de Boer RW. Magnetization transfer contrast. Part 1: MR Physics. Philips Medical Systems MedicaMundi 1995;40:64-73.
de Boer RW. Magnetization transfer contrast. Part 2: Clinical applications. Philips Medical Systems MedicaMundi 1995;40:74-83.
Edzes HT, Samulski ET. Cross-relaxation and spin diffusion in the proton NMR of hydrated collagen. Nature 1977;265:521-523. (A famous paper first showing how the interaction between water and proteins affects relaxation times.)
Elster AD, King JC, Mathews VP, Hamilton CA. Cranial tissues: appearance at gadolinium-enhanced and nonenhanced MR imaging with magnetization transfer contrast. Radiology 1994; 190:541-546.
Elster AD, King JC, Mathews VP, Hamilton C. Improved detection of gadolinium enhancement using magnetization transfer imaging. Neuroimaging Clin N Am 1994; 4:185-192.
Mathews VP, Elster AD, King JC et al. Combined effects of magnetization transfer and gadolinium in cranial MR imaging and MR angiography. AJR Am J Roentgenol 1995; 164:169-172.
Henkelman RM, Stanisz GJ, Graham SJ. Magnetization transfer in MRI: a review. NMR Biomed 2001;14:57-64.
Ulmer JL, Mathews VP, Hamilton CA, Elster AD, Moran PR. Magnetization transfer or spin-lock? An investigation of off-resonance saturation pulse imaging with varying frequency offsets. AJNR Am J Neuroradiol 1996; 17:805-819.
Wolff SD, Balaban RS. Magnetization transfer contrast (MTC) and tissue water proton relaxation In vivo. Mag Reson Med 1989; 10: 135-144.
de Boer RW. Magnetization transfer contrast. Part 1: MR Physics. Philips Medical Systems MedicaMundi 1995;40:64-73.
de Boer RW. Magnetization transfer contrast. Part 2: Clinical applications. Philips Medical Systems MedicaMundi 1995;40:74-83.
Edzes HT, Samulski ET. Cross-relaxation and spin diffusion in the proton NMR of hydrated collagen. Nature 1977;265:521-523. (A famous paper first showing how the interaction between water and proteins affects relaxation times.)
Elster AD, King JC, Mathews VP, Hamilton CA. Cranial tissues: appearance at gadolinium-enhanced and nonenhanced MR imaging with magnetization transfer contrast. Radiology 1994; 190:541-546.
Elster AD, King JC, Mathews VP, Hamilton C. Improved detection of gadolinium enhancement using magnetization transfer imaging. Neuroimaging Clin N Am 1994; 4:185-192.
Mathews VP, Elster AD, King JC et al. Combined effects of magnetization transfer and gadolinium in cranial MR imaging and MR angiography. AJR Am J Roentgenol 1995; 164:169-172.
Henkelman RM, Stanisz GJ, Graham SJ. Magnetization transfer in MRI: a review. NMR Biomed 2001;14:57-64.
Ulmer JL, Mathews VP, Hamilton CA, Elster AD, Moran PR. Magnetization transfer or spin-lock? An investigation of off-resonance saturation pulse imaging with varying frequency offsets. AJNR Am J Neuroradiol 1996; 17:805-819.
Wolff SD, Balaban RS. Magnetization transfer contrast (MTC) and tissue water proton relaxation In vivo. Mag Reson Med 1989; 10: 135-144.
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