Abstract
PurposeTo compare a novel combined acquisition technique (CAT) of turbo-spin-echo (TSE) and echo-planar-imaging (EPI) with conventional TSE. CAT reduces the electromagnetic energy load transmitted for spin excitation. This radiofrequency (RF) burden is limited by the specific absorption rate (SAR) for patient safety. SAR limits restrict high-field MRI applications, in particular.Material and MethodsThe study was approved by the local Medical Ethics Committee. Written informed consent was obtained from all participants. T2- and PD-weighted brain images of n = 40 Multiple Sclerosis (MS) patients were acquired by CAT and TSE at 3 Tesla. Lesions were recorded by two blinded, board-certificated neuroradiologists. Diagnostic equivalence of CAT and TSE to detect MS lesions was evaluated along with their SAR, sound pressure level (SPL) and sensations of acoustic noise, heating, vibration and peripheral nerve stimulation.ResultsEvery MS lesion revealed on TSE was detected by CAT according to both raters (Cohen’s kappa of within-rater/across-CAT/TSE lesion detection κCAT = 1.00, at an inter-rater lesion detection agreement of κLES = 0.82). CAT reduced the SAR burden significantly compared to TSE (p<0.001). Mean SAR differences between TSE and CAT were 29.0 (±5.7) % for the T2-contrast and 32.7 (±21.9) % for the PD-contrast (expressed as percentages of the effective SAR limit of 3.2 W/kg for head examinations). Average SPL of CAT was no louder than during TSE. Sensations of CAT- vs. TSE-induced heating, noise and scanning vibrations did not differ.ConclusionT2−/PD-CAT is diagnostically equivalent to TSE for MS lesion detection yet substantially reduces the RF exposure. Such SAR reduction facilitates high-field MRI applications at 3 Tesla or above and corresponding protocol standardizations but CAT can also be used to scan faster, at higher resolution or with more slices. According to our data, CAT is no more uncomfortable than TSE scanning.
Highlights
High-field MRI at 3 Tesla and beyond promises unprecedented signal-to-noise ratios (SNR), image resolutions and acquisition speed
Every Multiple Sclerosis (MS) lesion revealed on turbo spin-echo (TSE) was detected by Combined acquisition techniques (CAT) according to both raters (Cohen’s kappa of within-rater/ across-CAT/TSE lesion detection kCAT = 1.00, at an inter-rater lesion detection agreement of kLES = 0.82)
Multiple Sclerosis (MS) Lesion Detection For all patients, every hyperintense demyelinating MS lesion recorded on axial to CAT were 0.82 (T2)- and sagittal proton density (PD-)TSE images was detected in the corresponding CAT images by both raters (A.B. and A.J.B.) who were blinded to the used sequence (TSE vs. CAT) upon their assessment
Summary
High-field MRI at 3 Tesla and beyond promises unprecedented signal-to-noise ratios (SNR), image resolutions and acquisition speed. SAR limits constitute a problem especially for fast spin-echo (FSE) based sequences, such as turbo spin-echo (TSE) or rapid acquisitions with relaxation enhancement (RARE) [4]. These are frequently used to obtain T2and proton density (PD-) contrasts to determine, for instance, the lesion load in Multiple Sclerosis (MS). Combined acquisition techniques (CAT) by hybrid pulse sequences of TSE and EPI reduce the SAR [5,6]. A recent study investigating benefits of T2-/PD-CAT for neuroimaging at higher magnetic fields [7] showed that the hybrid EPI/TSE-combination in CAT can achieve substantial SAR reductions at equivalent image quality. Given that SNR is slightly lower for CAT compared to TSE [7,8], this is essential prior to translating CAT into clinical applications and practice
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