Objectives: The aim of the presented study was to evaluate pulsed magnetization transfer contrast (MTC) effects using saturation pulses of variable off-resonance frequency and radio frequency (RF) amplitude for a variety of tissue types (white and gray matter, liver, kidney, spleen, muscle, and articular cartilage) in human subjects at field strengths of 0.2, 1.5, and 3.0 Tesla.
Materials and methods: MTC imaging studies of the head, knee, and abdomen were performed using an adapted multiple MTC (mMTC) module in 3 healthy volunteers for all field strengths. This mMTC pulse module applies a variable Gaussian shaped magnetization transfer (MT) saturation pulse in a proton-density weighted RF-spoiled gradient echo sequence. It allows for both a flexible MT pulse design and performance of consecutive measurements with variation of amplitude and off-resonance frequency, whereas keeping other MT pulse parameters unchanged. Magnetization transfer signal ratio (MTR) maps were calculated on a pixel-by-pixel basis. Additional mMTC imaging measurements were performed using an agar-water phantom. For assessment of undesired direct saturation effects of the MT pulse on the water pool, numerical simulations based on Bloch's equations were performed and analyzed.
Results: The results indicate that MTR values for given MT pulses (pulse shape, off-resonance frequency and flip angle) are larger at higher magnetic field strengths. For white matter, gray matter, cartilage, and muscle, an increase of 10% to 30% was found at 3.0 T when compared with 1.5 T. Low magnetic field strength of 0.2 T led to MTR values of one third to half the values at 1.5 T. MTR values for abdominal tissues were partly lower at 3.0 T compared with 1.5 T, which might be related to reduced B1 field strengths at 3.0 T due to dielectric effects.
Conclusions: The increased MT effect at a higher field strength can partly compensate the specific absorption rate related problems in MTC applications. It is shown that for flip angles of 700 degrees to 900 degrees and offset frequencies of 1000 Hz to 1500 Hz, high quality MTR maps could be obtained at an acceptable level of direct saturation for all field strengths. Furthermore, if the better signal-to-noise ratio at higher magnetic fields is taken into account, quality of MTR maps of the head and the knee at 3.0 T was clearly improved compared with lower fields under optimized and comparable conditions.