Cerebral white matter: technical development and clinical applications of effective magnetization transfer (MT) power concepts for high-power, thin-section, quantitative MT examinations.

Abstract

High MT power techniques can expand the dynamic range of MT ratios, maintain a relatively pure MT effect, and be used effectively in MT imaging to evaluate white matter disorders.

Modeling the influence of TR and excitation flip angle on the magnetization transfer ratio (MTR) in human brain obtained from 3D spoiled gradient echo MRI

“…14) to conditions of average power per TR. This must include the excitation flip angle, which has been neglected in previous studies (4, 29).…”

“…The MTR is a semiquantitative parameter that is easily calculated and commonly reported in clinical applications of MT. Considerable work has been devoted to optimize the MTR in FLASH imaging via the properties of the selective MT pulse that is applied off‐resonance before each excitation (4–7). However, little attention has been paid to the influence of pulse repetition time (TR) and of the excitation flip angle, usually being 5° or 6°, as suggested in Finelli et al (4).…”

MT: Magnetization Transfer

“…The effects of altering the number of pulses per repetition time, and the offset frequency, were studied (McGowan et al, 1994). Finelli et al (1996) used a higher resolution three-dimensional spoilt gradient echo sequence (matrix 256 × 256 × 60; 2.5 mm sections; TR 25 ms; TE 4 ms; FA = 6 • ); the powerful offresonant MT pulse had a CW power equivalent (CWPE) to 270 Hz (1696 rad s −1 ), was 4 kHz offresonance and an MTR of 56-59 pu was measured in normal white matter. The contribution of direct saturation and spin-locking 6 was estimated to be < 5 pu.…”

“…Since MT saturation pulses are not selective, the degree of saturation is determined for multislice sequences by TR , the interval between saturation pulses, rather than TR. Finelli et al (1996) …”

“…Some workers used a CW power equivalent approximation to the pulse (Hua and Hurst, 1995;Finelli et al, 1996) during the time the saturating pulse is on. The most complete verification of a model of pulsed MT has been given by Sled and Pike (2000) and Henkelman et al (2001), who could model experimental values to within 2 %.…”