In a retrospective comparison of 116 fetal MRI exams—half performed at 3T, half at 1.5T—the higher field strength produced the higher imaging scores.
Grading the depictions from 0 to 4, with 4 denoting the best, Teresa Victoria, MD, PhD, of the Children’s Hospital of Philadelphia and colleagues found that fetal imaging at 3T produced higher imaging scores in the evaluation of cartilage and spine when single-shot turbo spin-echo (SSTSE) and steady-state free precession (SSFP) sequences were used.
It also proved superior to the 1.5T in the assessment of most other anatomic structures, including bowel, liver and kidney, when SSFP sequences were used.
When the ratings for all structures evaluated using SSFP sequences were averaged to obtain a composite measurement, one rater came back with a mean imaging score of 3.1 ± 0.6 on the 3T studies versus 2 ± 0.5 for the 1.5T studies.
Another rater’s overall scores were 2.0 ± 0.6 for the 3T studies and 1.8 ± 0.3 for those obtained with the 1.5T.
On this scale,
4 = Excellent depiction of anatomy and high tissue contrast, with the image being optimal for diagnostic purposes;
3 = Clear visualization of the structure and reduced tissue contrast, with image-based diagnosis feasible;
2 = Suboptimal visualization of structures, with diagnosis still possible;
1 = Poor depiction, with the image being inadequate for diagnostic purposes; and
0 = No structure seen.
Reporting on their study in the January edition of the American Journal of Roentgenology, the researchers note that transitioning from 1.5T to 3T may challenge those who seek to make the switch, as signal loss and artifact conspicuity are inherent in many applications of the higher field strength.
However, they assert that their findings, together with advancements on the way in MRI software and 3T protocols, “may allow optimal visualization and examination of fetal pathologic abnormalities, thus better identifying fetal and maternal needs both prenatally and postnatally.”
The authors acknowledge and address some potential safety concerns with the 3T, including the risk of acoustic damage to the fetus from the noise generated by the coils.
“Our own studies have shown that the frequency of 3T coil noise is greater than 1 kHz (authors’ unpublished data), a finding from which we can infer that there is little transmission through the fetal auditory canal,” they write.
Also of concern is the energy absorbed by the fetus from MRI’s excitation and refocusing pulses.
“Studies performed at The Children’s Hospital of Philadelphia have shown that the specific absorption rate (SAR) generated during fetal imaging at 3T is well below the FDA limit” of four watts per kilogram of maternal body weight, write Victoria et al. “In any case, all MRI equipment has a built-in system that prohibits exposure beyond FDA limits.”
The authors conclude, “[W]e found an overall advantage to performing fetal MRI with a magnetic field strength of 3T.”