Implementation and Application of PSF-Based EPI Distortion Correction to High Field Animal Imaging

The purpose of this work is to demonstrate the functionality and performance of a PSF-based geometric distortion correction for high-field functional animal EPI. The EPI method was extended to measure the PSF and a postprocessing chain was implemented in Matlab for offline distortion correction. The correction procedure was applied to phantom and in vivo imaging of mice and rats at 9.4T using different SE-EPI and DWI-EPI protocols. Results show the significant improvement in image quality for single- and multishot EPI. Using a reduced FOV in the PSF encoding direction clearly reduced the acquisition time for PSF data by an acceleration factor of 2 or 4, without affecting the correction quality

Hybrid methods for reducing specific Absorption rate in neuroradiologic MRI-examinations at high-field MR-systems

Die klinische Magnetresonanztomografie (MRT) operiert meist bei einer Magnetfeldstärke von 1,5 Tesla (T). Es halten jedoch immer mehr 3T MRT-Systeme Einzug im klinischen Alltag und seit kurzem auch 7T Ganzkörper-MRT-Systeme in die Grundlagenforschung. Höhere Magnetfeldstärken führen grundsätzlich zum einem verbesserten Signal-zu-Rausch- Verhältnis, welches sich gewinnbringend in eine erhöhte Ortsauflösung oder schnellere Bildaufnahme äußert. Ein Nachteil ist aber die dabei im Patienten deponierte Hochfrequenz-Energie (HF-Energie), welche quadratisch mit ansteigender Feldstärke zusammenhängt. Charakterisiert wird diese durch die spezifische Absorptionsrate (SAR) und ist durch vorgegebene gesetzliche Grenzwerte beschränkt. Moderne, SAR-intensive MRT-Techniken (z.B. Multispinecho-Verfahren) sind bereits bei 1,5T nahe den zulässigen SAR-Grenzwerten und somit nicht unverändert auf Hochfeld-Systeme übertragbar. In dieser Arbeit soll das Potential modularer Hybrid-MRT-Techniken genutzt werden, um das SAR bei besonders SAR-intensiven MRT-Verfahren ohne signifikante Einbußen in der Bildqualität erheblich zu verringern. Die Hybrid-Techniken sollen in Verbindung mit zusätzlichen Methoden der SAR-Reduzierung den breiteren Einsatz SAR-intensiver MRT-Techniken an hohen Magnetfeldern ermöglichen. Ziel dieser Arbeit ist es, routinefähige und SAR-reduzierte MRT-Standard-Protokolle für neuroanatomische Humanuntersuchungen mit räumlicher Höchstauflösung bei Magnetfeldern von 3T und 7T zu etablieren.Spin echo based MRI sequences builds one of the main priorities in the medical/-morphological MRI imaging. Especially T2-weighted spin-echo sequences and the multi-spin-echo RARE imaging sequence represents one of the basic methods, which allows to minimize measurement times down to minutes or seconds. Modern MRI systems usually have a magnetic field strength beyond 1.5T. In order to acquire high resolution images with acceptable acquisition times, high RF power is needed. In many cases the intrinsic safety limits of the radiated power are already exeeded at field strengths of 3T. This leads often to restrictions for the full performance. In research systems of 7T and more T2-weighted images are only feasible with significant amount of time. Especially in RARE imaging methods, which require a large number of refocusing pulses, this is a significant restriction factor. Therefore, in recent years there were further development of hybrid sequences which combines different acquisition methods together into one, to exploit their fully advantages of the basic methods and to minimize ..

Combined Acquisition Technique (CAT) for Neuroimaging of Multiple Sclerosis at Low Specific Absorption Rates (SAR)

<div><p>Purpose</p><p>To 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.</p><p>Material and Methods</p><p>The 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.</p><p>Results</p><p>Every MS lesion revealed on TSE was detected by CAT according to both raters (Cohen’s kappa of within-rater/across-CAT/TSE lesion detection κ_CAT = 1.00, at an inter-rater lesion detection agreement of κ_LES = 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.</p><p>Conclusion</p><p>T2−/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.</p></div

Image artifacts and distortions: CAT artifacts and distortions in phase-encoding direction (here from right to left, R>>L) as detected at the skull base level.

<p>The straight gyrus and olfactory sulcus are slightly displaced leftwards in CAT (<i>top right</i>; depending on blip polarity, cf. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091030#pone.0091030-Choli1" target="_blank">[7]</a>) compared to TSE (<i>top left</i>). Otherwise, CAT contours (red outlines in <i>lower left</i>) overlay almost perfectly with TSE (<i>lower left</i>) and vice versa (<i>lower right</i>) upon CAT/TSE co-registration (RMS deviation ≤1.6e⁻⁶ mm). Artifacts did not significantly interfere with MS lesion detection. Even tiny multiple T2-hyperintense demyelinations (<i>arrows</i>) are well visualized despite minimal blurring (at a spatial noise ratio TSE/CAT of 0.81 in this case).</p

TSE and CAT brain images in Multiple Sclerosis (MS).

<p>Exemplary T2- (upper row) and PD-weighted images acquired by TSE (<i>left column</i>) and CAT (<i>right column</i>) sequences are shown. The data from this representative patient illustrate, along with those from another in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091030#pone-0091030-g005" target="_blank">Fig. 5</a>, the diagnostic equivalence of both MR techniques: Every lesion picked up on the TSE image is detected on the CAT image as well. Minimally reduced SNR of CAT compared to TSE which has previously been quantified <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091030#pone.0091030-Choli1" target="_blank">[7]</a> is noticeable upon close visual inspection but does not impede diagnostic accuracy (spatial noise ratio of TSE to CAT was 0.82 for T2- and 0.88 for PD-weighted images).</p

Subjective ratings.

<p>Rated sensations of RF-induced heating (<i>top row</i>), acoustic noise (<i>middle row</i>) and scanning vibrations (<i>bottom row</i>) for CAT compared to TSE (cf. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091030#pone-0091030-g003" target="_blank">Fig. 3</a>). For the temperature ratings, only n = 7 <i>asterisks</i> for PD and T2 scanning are displayed because just this few patients noticed temperature differences between CAT and TSE while the rest (n = 33 out of 40 patients; 82.5%) perceived zero difference. None of the ratings revealed significant differences between CAT and TSE, indicating that CAT is no more uncomfortable than TSE scanning. (<i>box</i>: upper and lower quartiles, <i>thick black line</i>: median, <i>whiskers</i>: most extreme values of the interquartile range, <i>circle</i>: outlier, <i>asterisk</i>: extreme value).</p

Sound waves and frequency spectra for T2-weighted TSE, CAT (λ = 0.5) and pure EPI.

<p>Peak SPLs increase the higher the EPI proportion (i.e., the lower the CAT factor λ) but average SPLs of CAT at λ = 0.5 and TSE are comparable (<i>top</i>). EPI read-outs introduce a fundamental frequency peak at the reciprocal of twice the echo spacing (here: ESP = 2.6 ms/FFT peak = 192 Hz) which increases the higher the EPI proportion (i.e., the lower λ is set; <i>bottom</i>).</p

Measurement protocol and Visual Rating Scale (VRS).

<p>The order of CAT and TSE sequences was varied by rotation according to a Latin square. Upon measurement of a CAT/TSE double (<b>A</b>) the patient rated the two sequences in comparison to each other. Ratings scored sensations of temperature (RF-induced heating), acoustic noise and scan vibrations (<b>B</b>). Negative VRS values indicate less heating, acoustic noise and scan vibrations during CAT vs. TSE imaging, positive values indicate that higher temperatures, acoustic noise and vibration levels were perceived during CAT vs. TSE imaging while zero refers to no subjective difference between the CAT and TSE.</p

Specific absorption rates (SAR).

§<p>100% corresponding to SAR limit of 3.2 W/kg for head examinations according to the IEC regulation.</p><p>*Difference significant (<i>paired t-tests</i>; α = 0.05; p<0.001).</p