Volume Shim Research Articles

Implementation and prospective clinical validation of AI‐based planning and shimming techniques in cardiac MRI

PurposeCardiovascular magnetic resonance (CMR) is a vital diagnostic tool in the management of cardiovascular diseases. The advent of advanced CMR technologies combined with artificial intelligence (AI) has the potential to simplify imaging, reduce image acquisition time without compromising image quality (IQ), and improve magnetic field uniformity. Here, we aim to implement two AI‐based deep learning techniques for automatic slice alignment and cardiac shimming and evaluate their performance in clinical cardiac magnetic resonance imaging (MRI).MethodsTwo deep neural networks were developed, trained, and validated on pre‐acquired cardiac MRI datasets (>500 subjects) to achieve automatic slice planning and shimming (implemented in the scanner) for CMR. To examine the performance of our automated cardiac planning (EasyScan) and AI‐based shim (AI shim), two prospective studies were performed subsequently. For the EasyScan validation, 10 healthy subjects underwent two identical CMR protocols: with manual cardiac planning and with AI‐based EasyScan to assess protocol scan time difference and accuracy of cardiac plane prescriptions on a 1.5 T clinical MRI scanner. For the AI shim validation, a total of 20 subjects were recruited: 10 healthy and 10 cardio‐oncology patients with referrals for a CMR examination. Cine images were obtained with standard cardiac volume shim and with AI shim to assess signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), overall IQ (sharpness and MR image degradation), ejection fraction (EF), and absolute wall thickening. A hybrid statistical method using of nonparametric (Wilcoxon) and parametric (t‐test) assessments was employed for statistical analyses.ResultsCMR protocol with AI‐based plane prescriptions, EasyScan, minimized operator dependence and reduced overall scanning time by over 2 min (∼13 % faster, p < 0.001) compared to the protocol with manual cardiac planning. EasyScan plane prescriptions also demonstrated more accurate (less plane angulation errors from planes manually prescribed by a certified cardiac MRI technologist) cardiac planes than previously reported strategies. Additionally, AI shim resulted in improved B0 field homogeneity. Cine images obtained with AI shim revealed a significantly higher SNR (12.49%; p = 0.002) than those obtained with volume shim (volume shim: 32.90 ± 7.42 vs. AI shim: 37.01 ± 8.87) for the left ventricle (LV) myocardium. LV myocardium CNR was 12.48% higher for cine imaging with AI shim (149.02 ± 39.15) than volume shim (132.49 ± 33.94). Images obtained with AI shim resulted in sharper images than those obtained with volume shim (p = 0.012). The LVEF and absolute wall thickening also showed that differences exist between the two shimming methods. The LVEF by AI shim was shown to be slightly larger than LVEF by volume shim in two groups: 2.87% higher with AI shim for the healthy group and 1.70% higher with AI shim for the patient group. The LV absolute wall thickening (in mm) also showed that differences exist between shimming methods for each group with larger changes observed in the patient group (healthy: 3.31%, p = 0.234 and patient group: 7.29%, p = 0.059).ConclusionsCMR exams using EasyScan for cardiac planning demonstrated accelerated cardiac exam compared to the CMR protocol with manual cardiac planning. Improved and more uniform B0 magnetic field homogeneity also achieved using AI shim technique compared to volume shimming.

Integrated slice-specific dynamic shimming for whole-body diffusion-weighted MR imaging at 1.5\xa0T

ObjectiveTo compare integrated slice-specific dynamic shim (iShim) with distortion correction post-processing to conventional 3D volume shim for the reduction of artefacts and signal loss in 1.5 T whole-body diffusion-weighted imaging (WB-DWI).MethodsTen volunteers underwent WB-DWI using conventional 3D volume shim and iShim. Forty-eight consecutive patients underwent WB-DWI with either volume shim (n = 24) or iShim (n = 24) only. For all subjects, displacement of the spinal cord at imaging station interfaces was measured on composed b = 900 s/mm2 images. The signal intensity ratios, computed as the average signal intensity in a region of high susceptibility gradient (sternum) divided by the average signal intensity in a region of low susceptibility gradient (vertebral body), were compared in volunteers. For patients, image quality was graded from 1 to 5 (1 = Poor, 5 = Excellent). Signal intensity discontinuity scores were recorded from 1 to 4 (1 = 2 + steps, 4 = 0 steps). A p value of < 0.05 was considered significant.ResultsSpinal cord displacement artefacts were lower with iShim (p < 0.05) at the thoracic junction in volunteers and at the cervical and thoracic junctions in patients (p < 0.05). The sternum/vertebra signal intensity ratio in healthy volunteers was higher with iShim compared with the volume shim sequence (p < 0.05). There were no significant differences between the volume shim and iShim patient groups in terms of image quality and signal intensity discontinuity scores.ConclusioniShim reduced the degree of spinal cord displacement artefact between imaging stations and susceptibility-gradient-induced signal loss.

IMAGE QUALITY ANALYSIS 4 CHAMBER SECTIONS OF CARDIAC MRI WITH AND WITHOUT UTILIZING SHIM VOLUME IN THE STEADY STATE FREE PRECESSION SEQUENCES

Background: Cardiac MRI examination is relatively rare in Indonesia. The effect of the use of volume shim on moving organs, such as the heart is relatively unknown and noticed by radiographers and cardiologists. Objective: To analyse the image quality of 4 chamber sections of Cardiac Magnetic Resonance Imaging with and without the use of shim volume on Steady State Free Precession (SSFP) sequences so that it can determine the most optimum 4 chamber images to maintain the diagnostic value by physician. Method: This research is designed through quantitative analytic approach with experiment method. The total samples used are 13 subjects ranging from 7 to 80 years old and 5 respondents and the sample undergone into a series of examinations of Cardiac Magnetic Resonance Imaging. Afterward, the 4 chamber sections in SSFP sequence were given different treatment, namely, with and without the use of shim volume. The result will be assessed by the respondents and then it will be done by non-parametric Wilcoxon Two-Sample Test and parametric Paired Sample T Test. Results: Thereafter, the result of statistic from the respondent assessment is that the quality of anatomy images has p=(0.113) whereas the p=(0.354) for the degree of artefact images and clarifies that the anatomy image quality and the degree of artefact is not too much different. Conclusion: The conclusion of the research is with and without the use of shim volume is not too significant to affect the quality of 4 chamber images.

A Digital Preclinical PET/MRI Insert and Initial Results.

Combining Positron Emission Tomography (PET) with Magnetic Resonance Imaging (MRI) results in a promising hybrid molecular imaging modality as it unifies the high sensitivity of PET for molecular and cellular processes with the functional and anatomical information from MRI. Digital Silicon Photomultipliers (dSiPMs) are the digital evolution in scintillation light detector technology and promise high PET SNR. DSiPMs from Philips Digital Photon Counting (PDPC) were used to develop a preclinical PET/RF gantry with 1-mm scintillation crystal pitch as an insert for clinical MRI scanners. With three exchangeable RF coils, the hybrid field of view has a maximum size of 160 mm × 96.6 mm (transaxial × axial). 0.1 ppm volume-root-mean-square B 0-homogeneity is kept within a spherical diameter of 96 mm (automatic volume shimming). Depending on the coil, MRI SNR is decreased by 13% or 5% by the PET system. PET count rates, energy resolution of 12.6% FWHM, and spatial resolution of 0.73 mm (3) (isometric volume resolution at isocenter) are not affected by applied MRI sequences. PET time resolution of 565 ps (FWHM) degraded by 6 ps during an EPI sequence. Timing-optimized settings yielded 260 ps time resolution. PET and MR images of a hot-rod phantom show no visible differences when the other modality was in operation and both resolve 0.8-mm rods. Versatility of the insert is shown by successfully combining multi-nuclei MRI ((1)H/(19)F) with simultaneously measured PET ((18)F-FDG). A longitudinal study of a tumor-bearing mouse verifies the operability, stability, and in vivo capabilities of the system. Cardiac- and respiratory-gated PET/MRI motion-capturing (CINE) images of the mouse heart demonstrate the advantage of simultaneous acquisition for temporal and spatial image registration.

MR compatibility aspects of a silicon photomultiplier-based PET/RF insert with integrated digitisation

The combination of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) into a single device is being considered a promising tool for molecular imaging as it combines the high sensitivity of PET with the functional and anatomical images of MRI. For highest performance, a scalable, MR compatible detector architecture with a small form factor is needed, targeting at excellent PET signal-to-noise ratios and time-of-flight information. Therefore it is desirable to use silicon photo multipliers and to digitize their signals directly in the detector modules inside the MRI bore.A preclinical PET/RF insert for clinical MRI scanner was built to demonstrate a new architecture and to study the interactions between the two modalities.The disturbance of the MRI’s static magnetic field stays below 2 ppm peak-to-peak within a diameter of 56 mm (90 mm using standard automatic volume shimming). MRI SNR is decreased by 14%, RF artefacts (dotted lines) are only visible in sequences with very low SNR. Ghosting artefacts are visible to the eye in about 26% of the EPI images, severe ghosting only in 7.6%.Eddy-current related heating effects during long EPI sequences are noticeable but with low influence of 2% on the coincidences count rate. The time resolution of 2.5 ns, the energy resolution of 29.7% and the volumetric spatial resolution of 1.8 mm3 in the PET isocentre stay unaffected during MRI operation.Phantom studies show no signs of other artefacts or distortion in both modalities. A living rat was simultaneously imaged after the injection with 18F-Fluorodeoxyglucose (FDG) proving the in vivo capabilities of the system.

Bilateral breast MRI by use of dual-source parallel radiofrequency excitation and image-based shimming at 3 Tesla: improvement in homogeneity on fat-suppression imaging.

In this study, we aimed to compare fat-suppression homogeneity on breast MR imaging by using dual-source parallel radiofrequency excitation and image-based shimming (DS-IBS) with single-source radiofrequency excitation with volume shim (SS-Vol) at 3 Tesla. Twenty patients were included. Axial three-dimensional T1-weighted turbo-field-echo breast images with DS-IBS and SS-Vol were obtained. Fat suppression was scored with four grade points. The contrast of the pectoral muscle and the fat in each breast area was obtained in the head medial, head lateral, foot medial, and foot lateral areas. The axillary space was calculated and compared between DS-IBS and SS-Vol. The average DS-IBS score was significantly higher than that of SS-Vol. The mean contrasts of fat in the foot lateral areas and axillary spaces on DS-IBS images were significantly higher than on SS-Vol images.

Robust myocardial T2 and T2 * mapping at 3T using image-based shimming.

Intramyocardial hemorrhage and area at risk are both prognostic markers in acute myocardial infarction (AMI). Myocardial T2 and T2 * mapping have been used to detect such tissue changes at 1.5T but these techniques are challenging at 3.0T due to additional susceptibility variation. We studied T2 and T2 * myocardial mapping techniques at 3.0T on a system employing B1 shimming and compared two different methods of B0 shimming. Fifteen volunteers and six AMI patients were scanned on a 3T system. Volume and image-based (IB) B0 shimming techniques were implemented. Single breath-hold, multiecho gradient, and spin echo sequences were employed from which T2 * and T2 maps were calculated. In volunteers, there was no significant difference in mean values obtained with volume or IB shimming for T2 mapping (39.1 ± 6.0 msec vs. 39.4 ± 6.1 msec; P > 0.05) or for T2 * mapping (24.2 ± 6.7 msec vs. 24.1 ± 5.2 msec; P > 0.05). There were no significant regional differences in mean T2 values between septal, anterior, and posterior segments with either shimming technique (all P > 0.05); but there were significant regional differences in mean T2 * values using volume shimming (27.8 ± 5.2 msec vs. 28.4 ± 5.8 msec vs. 15.9 ± 8.3 msec; P < 0.05)-but not with IB shimming (25.7 ± 5.4 msec vs. 25.3 ± 5.9 msec vs. 18.7 ± 4.6 msec; P > 0.05). At 3.0T, cardiac T2 mapping is robust. Although T2 * mapping is prone to more regional heterogeneity this can be reduced by using IB instead of conventional volume B0 shimming.

A comparison of shimming techniques for optimizing fat suppression in MR mammography

We evaluated the degree of inhomogeneities of fat suppression using the fully automated three-dimensional breast shimming technique (Image Based-Smart: IB-Smart) and manual setting of a rectangular parallelepiped shim (volume shimming) in MR mammography. Information on breast shape was collected from 9 patients whose images were insufficiently fat-suppressed. A breast phantom made of a thermoplastic sheet was used. Shimming of the magnetic field was done with IB-Smart and various dimensions of volume shims: the anterior to posterior/right to left/head to foot directions were set to 75-150/150-350/50-150 mm. The volumes of inhomogeneously suppressed fat were measured. The calculated volume with inhomogeneous fat suppression with use of IB-Smart was 13.3 × 10(4) mm(3). The smallest volume of inhomogeneous fat suppression with volume shimming was 5.4 × 10(4) mm(3) when the anterior-posterior/right-left/head-foot directions were set to 75/350/50 mm. Our results show that using optimized dimensions of volume shims enables better fat suppression than does IB-Smart.

Robust myocardial T2 and T2* mapping at 3T

Summary Myocardial hemorrhage can be assessed implementing T2 and T2* mapping techniques, however robust myocardial T2 and T2* mapping is challenging at 3T. The goal of this study was to test T2 and T2* myocardial mapping techniques at 3T, with potential improvement in image quality on a system employing B1 shimming, with two methods of for B0 shimming. Background Hemorrhage in the core of acute myocardial infarction (AMI) and area at risk appear to be markers of prognosis, enabling targeted therapy and potentially changing outcome. Myocardial T2 and T2* mapping have been used to detect such tissue changes at 1.5T. Such investigation at 3T is challenging due to additional susceptibility variation (e.g. inferolateral artefact at the heart-lungliver interface). We studied T2 and T2* myocardial mapping techniques at 3T on a system employing B1 shimming, with two methods of B0 shimming. Methods Fifteen volunteers and three STEMI patients were scanned on a 3T Philips Achieva TX system with a 32channel cardiac coil. Triggered, single breath-hold, multi-echo sequences were employed from which T2* and T2 maps were calculated. For the T2 map, the shot of 24 refocused spin echoes was subdivided into six groups, with a linear k-space order within each group contributing to a separate k-space. This strategy allows data acquisition within a breath hold. The echoes used for the centre of k-space for each image/group had consistent parity. Conventional first-order volume B0 shimming (over a cuboid encompassing the whole heart and descending aorta, from which the on-resonance signal is maximised) was compared with image based (IB) B0 shimming, for which first- and second-order field adjustments are calculated from B0 maps, to maximise B0 map homogeneity over an arbitrarily-shaped volume prescribed by the user which can be drawn around the heart (ShimTool; Schar et al MRM 2010). In all cases, septal, anterior and posterior ROIs were manually drawn to obtain average T2 or T2* values. Results Figure 1 shows typical T2 and T2* maps obtained in healthy volunteers. T2 and T2* values are reported in Table 1. In 9 of the total of 15 volunteers, IB shimming reduced T2* map heterogeneity, particularly in the inferolateral wall. In the remaining 6 volunteers, conventional volume shim and IB shimming performed equally. For the T2 mapping, no difference in artefact power between conventional volume shim and IB B0 shimming was detected, although homogeneity improved away from susceptibility artefact. The patient data reflected a similar pattern with additional increase in T2 and T2* values in the affected MI territory.

Cardiac T2* and lipid measurement at 3.0 T-initial experience

This study was designed to assess whether breath-hold cardiac multiecho imaging at 3.0 T is achievable without significant image artefacts and if fat/water phase interference modulates the exponential T2* signal decay. Twelve healthy volunteers (mean age 39) were imaged on a Philips Intera 3.0 T MRI scanner. Multiecho imaging was performed with a breath-hold spoiled gradient echo sequence with a seven echo readout (echo times 1.15-8.05 ms, repetition time 11 ms) using a black-blood prepulse and volume shimming. T2* values were calculated with both mono- and biexpoential fits from the mean signal intensity of the interventricular septum. The global mean T2* was 27.3 ms +/- 6.4. The mean signal-to-noise ratio (SNR) of the septum was 22.8 +/- 9.9, and the contrast-to-noise ratio (CNR) of the septum to the left ventricular cavity 20.3 +/- 9.4. A better fit was obtained with a biexponential model and the mean fat fraction derived was 3.7%. Cardiac functional parameters were in the normal range and showed no correlation with T2*. Cardiac T2* estimation with gradient multiecho imaging at 3.0 T can be achieved with minimal artefact and modelling the signal decay with a biexponential function allows estimation of myocardial lipid content as well as T2* decay.