Total Scan Time Research Articles

Spatial versus angular resolution for tractography-assisted planning of deep brain stimulation.

Given the restricted total scanning time for clinical neuroimaging, it is unclear whether clinical diffusion MRI protocols would benefit more from higher spatial resolution or higher angular resolution. In this work, we investigated the relative benefit of improving spatial or angular resolution in diffusion MRI to separate two parallel running white matter tracts that are targets for deep brain stimulation: the anterior thalamic radiation and the supero-lateral branch of the medial forebrain bundle. Both these tracts are situated in the ventral anterior limb of the internal capsule, and recent studies suggest that targeting a specific tract could improve treatment efficacy. Therefore, we scanned 19 healthy volunteers at 3T and 7T according to three diffusion MRI protocols with respectively standard clinical settings, increased spatial resolution of 1.4mm, and increased angular resolution (64 additional gradient directions at b=2200s/mm2). We performed probabilistic tractography for all protocols and quantified the separability of both tracts. The higher spatial resolution protocol improved separability by 41% with respect to the clinical standard, presumably due to decreased partial voluming. The higher angular resolution protocol resulted in increased apparent tract volumes and overlap, which is disadvantageous for application in precise treatment planning. We thus recommend to increase the spatial resolution for deep brain stimulation planning to 1.4mm while maintaining angular resolution. This recommendation complements the general advice to aim for high angular resolution to resolve crossing fibers, confirming that the specific application and anatomical considerations are leading in clinical diffusion MRI protocol optimization.

NIMG-45. 3D SINGLE-SCAN FOR MULTI-CONTRAST-WEIGHTED MRI FOR LESION DETECTION IN PATIENTS WITH GLIOMAS

Abstract INTRODUCTION Multi-contrast-weighted (MCW) MRI allows derivation of T1-weighted, T2-weighted, and T2-FLAIR images and their parametric maps. Under current practice, these high-resolution 3D MR images are acquired separately with a total scan time is approximately 30 minutes. The goal of this study was to develop a novel sequence and processing strategy for MCW MRI and lesion detection of gliomas with 1mm isotropic resolution in 6 minutes, which can significantly improve clinical imaging workflow. The proposed whole brain, single-scan method enables the generation of segmented tumor regions that correspond the hyperintense lesion on T2-weighted or T2-FLAIR images (T2L). METHODS Our strategy involves using a highly accelerated 3D MRI sequence to achieve whole brain, MCW images that can be jointly segmented with modified k-means clustering. Six patients with enhancing gliomas were scanned with this sequence before the injection of gadolinium, 4 of which also received a repeat scan post-gadolinium. Our automatic segmentation of the T2L and manual definition of the contrast-enhancing lesion (CEL) was compared against manually segmented lesions drawn from conventional anatomical imaging. RESULTS Whole-brain MCW MRI was achieved with 1mm isotropic resolution in 6 mins. Grey-matter, white-matter, CSF, and the T2L could be automatically segmented with our clustering method. The resulting automatically-segmented T2L volume was not significantly different between our new sequence and clinically defined regions (average Dice Coefficient=0.81(+/-0.05); p=0.95). CONCLUSIONS Our novel, 6-minute scan and processing strategy was able to generate MCW anatomical images and automatically segment grey-matter, white-matter, and T2L volumes that were similar to manual segmentation of conventional anatomical imaging. We are currently extending the automatic segmentation to the CEL. This approach has the potential to improve clinical workflow by allowing time for therapy-specific advanced imaging and providing a noninvasive tool for monitoring tumor evolution. Validation is currently being performed in a larger cohort.

Highly accelerated vessel-selective arterial spin labeling angiography using sparsity and smoothness constraints.

PurposeTo demonstrate that vessel selectivity in dynamic arterial spin labeling angiography can be achieved without any scan‐time penalty or noticeable loss of image quality compared with conventional arterial spin labeling angiography.MethodsSimulations on a numerical phantom were used to assess whether the increased sparsity of vessel‐encoded angiograms compared with non‐vessel‐encoded angiograms alone can improve reconstruction results in a compressed‐sensing framework. Further simulations were performed to study whether the difference in relative sparsity between nonselective and vessel‐selective dynamic angiograms was sufficient to achieve similar image quality at matched scan times in the presence of noise. Finally, data were acquired from 5 healthy volunteers to validate the technique in vivo. All data, both simulated and in vivo, were sampled in 2D using a golden‐angle radial trajectory and reconstructed by enforcing image domain sparsity and temporal smoothness on the angiograms in a parallel imaging and compressed‐sensing framework.ResultsRelative sparsity was established as a primary factor governing the reconstruction fidelity. Using the proposed reconstruction scheme, differences between vessel‐selective and nonselective angiography were negligible compared with the dominant factor of total scan time in both simulations and in vivo experiments at acceleration factors up to R = 34. The reconstruction quality was not heavily dependent on hand‐tuning the parameters of the reconstruction.ConclusionThe increase in relative sparsity of vessel‐selective angiograms compared with nonselective angiograms can be leveraged to achieve higher acceleration without loss of image quality, resulting in the acquisition of vessel‐selective information at no scan‐time cost.

Extracting more for less: multi-echo MP2RAGE for simultaneous T1 -weighted imaging, T1 mapping, mapping, SWI, and QSM from a single acquisition.

To demonstrate simultaneous T1 -weighted imaging, T1 mapping, mapping, SWI, and QSM from a single multi-echo (ME) MP2RAGE acquisition. A single-echo (SE) MP2RAGE sequence at 7 tesla was extended to ME with 4 bipolar gradient echo readouts. T1 -weighted images and T1 maps calculated from individual echoes were combined using sum of squares and averaged, respectively. ME-combined SWI and associated minimum intensity projection images were generated with TE-adjusted homodyne filters. A QSM reconstruction pipeline was used, including a phase-offsets correction and coil combination method to properly combine the phase images from the 32 receiver channels. Measurements of susceptibility, , and T1 of brain tissue from ME-MP2RAGE were compared with those from standard ME-gradient echo and SE-MP2RAGE. The ME combined T1 -weighted, T1 map, SWI, and minimum intensity projection images showed increased SNRs compared to the SE results. The proposed coil combination method led to QSM results free of phase-singularity artifacts, which were present in the standard adaptive combination method. T1 -weighted, T1 , and susceptibility maps from ME-MP2RAGE were comparable to those obtained from SE-MP2RAGE and ME-gradient echo, whereas maps showed increased blurring and reduced SNR. T1 , , and susceptibility values of brain tissue from ME-MP2RAGE were consistent with those from SE-MP2RAGE and ME-gradient echo. High-resolution structural T1 weighted imaging, T1 mapping, mapping, SWI, and QSM can be extracted from a single 8.5-min ME-MP2RAGE acquisition using a customized reconstruction pipeline. This method can be applied to replace separate SE-MP2RAGE and ME-gradient echo acquisitions to significantly shorten total scan time.

Analysis of the Possibility of Determining the Internal Structure of Oil and Gas Pipes by CT Method

A mathematical model has been developed to simulate the initial projections for computed tomography of large-sized oil and gas pipes and to transform the projections into the mass thickness distributions. The simulated projections of steel pipes with a diameter from 530 mm to 1420 mm are given with variation of ADC capacity and the number of photons incident on the front surface of the multichannel bremsstrahlung detector. The necessity of correct selection of these parameters is proved. A possibility to estimate the density distribution over a pipe cross section based on the inverse Abel transform for the fan-beam of bremsstrahlung is illustrated. The issues related to the technical feasibility of the computed tomography method for inspecting of large-sized pipes and concerning the estimation of the total scan time and the choice of ADC capacity are considered. Recommendations on the adjustment of the analog and digital signals ranges are given. The obtained results make it possible to evaluate the feasibility of designing a computer tomography system for monitoring large-sized oil and gas pipes.

2-D magnetic resonance spectroscopic imaging of the pediatric brain using compressed sensing.

Magnetic resonance spectroscopic imaging helps to determine abnormal brain tissue conditions by evaluating metabolite concentrations. Although a powerful technique, it is underutilized in routine clinical studies because of its long scan times. In this study, we evaluated the feasibility of scan time reduction in metabolic imaging using compressed-sensing-based MR spectroscopic imaging in pediatric patients undergoing routine brain exams. We retrospectively evaluated compressed-sensing reconstructions in MR spectroscopic imaging datasets from 20 pediatric patients (11 males, 9 females; average age: 5.4±4.5years; age range: 3days to 16years). We performed retrospective under-sampling of the MR spectroscopic imaging datasets to simulate accelerations of 2-, 3-, 4-, 5-, 7- and 10-fold, with subsequent reconstructions in MATLAB. Metabolite maps of N-acetylaspartate, creatine, choline and lactate (where applicable) were quantitatively evaluated in terms of the root-mean-square error (RMSE), peak amplitudes and total scan time. We used the two-tailed paired t-test along with linear regression analysis to statistically compare the compressed-sensing reconstructions at each acceleration with the fully sampled reference dataset. High fidelity was maintained in the compressed-sensing MR spectroscopic imaging reconstructions from 50% to 80% under-sampling, with the RMSE not exceeding 3% in any dataset. Metabolite intensities and ratios evaluated on a voxel-by-voxel basis showed no statistically significant differences and mean metabolite intensities showed high correlation compared to the fully sampled reference dataset up to an acceleration factor of 5. Compressed-sensing MR spectroscopic imaging has the potential to reduce MR spectroscopic imaging scan times for pediatric patients, with negligible information loss.

In vivo potassium MRI of the human heart.

Potassium ions (K+ ) play a critical role in cardiac electrophysiology, and changes in their concentration reflect pathophysiological processes related to cardiovascular diseases. Here, we investigated the feasibility of in vivo 39 K MRI of the human heart. To achieve this, we developed, evaluated, and applied a 39 K/1 H RF coil, which is tailored for 39 K MRI of human heart at 7.0T. The performance of the 39 K/1 H RF coil was evaluated by electromagnetic field and specific absorption ratio simulations using 2 (male/female) human voxel models. The RF coil was evaluated at the bench and applied in an in vivo proof-of-principle study involving 7 healthy volunteers. The experiments were performed using a 7.0T whole-body MR system in conjunction with a 3D density-adapted projection reconstruction imaging technique. For in vivo 39 K MRI of the human heart, a nominal spatial resolution of 14.5 × 14.5 × 14.5 mm3 within a total scan time of 30 min was achieved. The average SNR within the heart was 9.6 ± 2.4. This work validates the design of a 39 K/1 H RF coil for cardiac MR at 7.0T and demonstrates for the first time in vivo the feasibility of 39 K MRI of the human heart.

MRI-based iPAGAT polymer gel dosimetry using fast recovery spin echo sequences

In an estimation of iPAGAT polymer gel dosimeters using a magnetic resonance imaging (MRI) system, the dosimetric characteristics were investigated with two echo sequences: a spin echo (SE) and a fast recovery fast spin echo (FRFSE) sequence. FRFSE can shorten total scan time compared to SE because of its short repetition time (TR) and long echo train length (ETL). Although the R2-dose response measured from FRFSE was decreased compared to SE, both responses were fitted quadratic curve with a high correlation coefficient. In addition, their calibrated dose distributions showed high conformity to the planned data in the three-dimensional (3D) gamma index, dose volume histogram (DVH) and surface rendering analyses; however, results of SE were a little superior to FRFSE. In conclusion, it is suggested that FRFSE can present the accurate characteristics of polymer gel dosimeters under the optimum parameters.

Dual cardiac and respiratory gated thoracic imaging via adaptive gantry velocity and projection rate modulation on a linear accelerator: A Proof-of-Concept Simulation Study.

Cardiac motion is typically not accounted for during pretreatment imaging for central lung and mediastinal tumors. However, cardiac induced tumor motion averages 5.8mm for esophageal tumors and 3-5 mm for some lung tumors, which can result in positioning errors. Our aim is to reduce both cardiac- and respiratory-induced motion artifacts in thoracic cone beam computed tomography (CBCT) images through gantry velocity and projection rate modulation on a standard linear accelerator (linac). The acquisition of dual cardiac and respiratory gated CBCT thoracic images was simulated using the XCAT phantom with patient-measured respiratory and ECG traces. The gantry velocity and projection rate were modulated based on the cardiac and respiratory signals to maximize the angular consistency between adjacent projections in the gated cardiac-respiratory bin. The mechanical limitations of a gantry-mounted CBCT system were investigated. For our protocol, images were acquired during the 60%-80% window of cardiac phase and 20% displacement either side of peak exhale of the respiratory cycle. The comparator method was the respiratory-only gated CBCT acquisition with constant gantry speed and projection rate in routine use for respiratory correlated four-dimensional (4D) CBCT. All images were reconstructed using the Feldkamp-Davis-Kress (FDK) algorithm. The methods were compared in terms of image sharpness as measured using the edge response width (ERW) and contrast as measured using the contrast to noise ratio (CNR). The effects of the total number of projections acquired and magnitude of cardiac motion on scan time and image quality were also investigated. Median total scan times with our protocol ranged from 117s (40 projections) through to 296s (100 projections), compared with 240s for the conventional protocol (1320 projections). The scan times were dictated by the number of projections acquired, heart rate, length of the respiratory cycle and mechanical constraints of the CBCT system. Our protocol was able to provide between 8% and 43% decrease in the median value of the ERW in the anterior/posterior (AP) direction across the 17 traces when there was 0.5cm of cardiac motion and between 35% and 64% decrease when there was 1.0cm of cardiac motion over conventional acquisition. In the superior-inferior (SI) direction, our protocol was able to provide between 22% and 26% decrease in the median value of the ERW across the 17 traces when there was 0.5cm of cardiac motion and between 30% and 35% decrease when there was 1.0cm of cardiac motion over conventional acquisition. The magnitude of the cardiac motion did not have an observable effect on the median value of the CNR. Across all 17 traces, our adaptive protocol produced noticeably more consistent, albeit lower CNR values compared with conventional acquisition. For the first time, the potential of adapting CBCT image acquisition to changes in the patient's cardiac and respiratory rates simultaneously for applications in radiotherapy was investigated. This work represents a step towards thoracic imaging that reduces the effects of both cardiac and respiratory motion on image quality.

Whole-brain snapshot CEST imaging at 7 T using 3D-EPI.

The aim of this work is to develop a fast and robust CEST sequence in order to allow the acquisition of a whole-brain imaging volume after a single preparation block (snapshot acquisition). A 3D-CEST sequence with an optimized 3D-EPI readout module was developed, which acquires the complete k-space data following a single CEST preparation for 1 saturation offset. Whole-brain mapping of the Z-spectrum with 2mm isotropic resolution is achieved at 68 saturation frequencies in 5 minutes (4.33 s per offset). We analyzed the distribution in order to optimize correction and to provide accurate CEST quantification across the whole brain. We obtained maps for 3 different CEST contrasts from 4 healthy subjects. Based on our distribution analysis, we conclude that 3 sampling points allow for sufficient compensation of variations across most of the brain. Two brain regions, the cerebellum and the temporal lobes, are difficult to quantify at 7 T due to very low that was achieved in these regions. The proposed sequence enables robust acquisition of 2 mm isotropic whole-brain CEST maps at 7 Tesla within a total scan time of 16 minutes.

Optimization of q-space sampling for mean apparent propagator MRI metrics using a genetic algorithm

Mean Apparent Propagator (MAP) MRI is a recently introduced technique to estimate the diffusion probability density function (PDF) robustly. Using the estimated PDF, MAP MRI then calculates zero-displacement and non-Gaussianity metrics, which might better characterize tissue microstructure compared to diffusion tensor imaging or diffusion kurtosis imaging. However, intensive q-space sampling required for MAP MRI limits its widespread adoption. A reduced q-space sampling scheme that maintains the accuracy of the derived metrics would make it more practical. A heuristic approach for acquiring MAP MRI with fewer q-space samples has been introduced earlier with scan duration of less than 10 minutes. However, the sampling scheme was not optimized systematically to preserve the accuracy of the model metrics. In this work, a genetic algorithm is implemented to determine optimal q-space subsampling schemes for MAP MRI that will keep total scan time under 10 min. Results show that the metrics derived from the optimized schemes more closely match those computed from the full set, especially in dense fiber tracts such as the corpus callosum.

Off-resonance correction for pseudo-continuous arterial spin labeling using the optimized encoding scheme

Pseudo-continuous arterial spin labeling (PCASL) MRI has become a popular tool for non-invasive perfusion imaging and angiography. However, it suffers from sensitivity to off-resonance effects within the labeling plane, which can be exacerbated at high field or in the presence of metallic implants, leading to spatially varying signal loss and cerebral blood flow underestimation. In this work we propose a prospective correction technique based on the optimized encoding scheme, which allows the rapid calculation of transverse gradient blips and RF phase modulations that best cancel phase offsets due to off-resonance at the locations of the feeding arteries within the labeling plane. This calculation is based upon a rapidly acquired single-slice fieldmap and is applicable to any number and arrangement of arteries. In addition, this approach is applicable to both conventional PCASL and a vessel-selective variant known as vessel-encoded PCASL (VEPCASL). Through simulations and experiments in healthy volunteers it was shown that in the presence of off-resonance effects a strong bias in the strength of the perfusion signal across vascular territories can be introduced, the signal-to-noise ratio (SNR) efficiency of PCASL and VEPCASL can be severely compromised (∼40% reduction in vivo), and that vessel-selective signal in VEPCASL can be incorrectly assigned. Distortion of the spatial regions placed in the label or control conditions in the presence of off-resonance effects was confirmed in phantom experiments. The application of the proposed correction restored SNR efficiency to levels present in the absence of off-resonance effects and corrected errors in the vascular territory maps derived from VEPCASL. Due to the rapid nature of the required calculations and fieldmap acquisition, this approach could be inserted into protocols with minimal effect on the total scan time.

Deep residual network for off-resonance artifact correction with application to pediatric body MRA with 3D cones.

To enable rapid imaging with a scan time-efficient 3D cones trajectory with a deep-learning off-resonance artifact correction technique. A residual convolutional neural network to correct off-resonance artifacts (Off-ResNet) was trained with a prospective study of pediatric MRA exams. Each exam acquired a short readout scan (1.18 ms ± 0.38) and a long readout scan (3.35 ms ± 0.74) at 3 T. Short readout scans, with longer scan times but negligible off-resonance blurring, were used as reference images and augmented with additional off-resonance for supervised training examples. Long readout scans, with greater off-resonance artifacts but shorter scan time, were corrected by autofocus and Off-ResNet and compared with short readout scans by normalized RMS error, structural similarity index, and peak SNR. Scans were also compared by scoring on 8 anatomical features by two radiologists, using analysis of variance with post hoc Tukey's test and two one-sided t-tests. Reader agreement was determined with intraclass correlation. The total scan time for long readout scans was on average 59.3% shorter than short readout scans. Images from Off-ResNet had superior normalized RMS error, structural similarity index, and peak SNR compared with uncorrected images across ±1 kHz off-resonance (P < .01). The proposed method had superior normalized RMS error over -677 Hz to +1 kHz and superior structural similarity index and peak SNR over ±1 kHz compared with autofocus (P < .01). Radiologic scoring demonstrated that long readout scans corrected with Off-ResNet were noninferior to short readout scans (P < .05). The proposed method can correct off-resonance artifacts from rapid long-readout 3D cones scans to a noninferior image quality compared with diagnostically standard short readout scans.

Implementation of the FLORET UTE sequence for lung imaging.

Magnetic resonance imaging of lungs is inherently challenging, but it has become more common with the use of UTE sequences and their relative insensitivity to motion. Spiral UTE sequences have been touted recently as having greater k-space sampling efficiencies than radial UTE, but few are designed for the shorter T2 * of the lung. In this study, FLORET (Fermat looped, orthogonally encoded trajectories), a recently developed spiral 3D-UTE sequence designed for the short T2 * species, was implemented in human lungs for the first time and the images were compared with traditional radial UTE images. The FLORET sequence was implemented with parameters optimized for lung imaging on healthy and diseased (cystic fibrosis) subjects. On healthy subjects, radial UTE images (3D-radial and 2D-radial with phase encoding) were acquired for comparison to FLORET. Various metrics including SNR, vasculature contrast, diaphragm sharpness, and parenchymal density ratios were acquired and compared among the separate UTE sequences. The FLORET sequence performed similarly to traditional radial UTE methods with a much shorter total scan time for fully sampled images (FLORET: 1 minute 55 seconds, 3D-radial: 3 minutes 25 seconds, 2D-radial with phase encoding: 7 minutes 22 seconds). Additionally, the FLORET image obtained on the cystic fibrosis subject resulted in the observation of cystic fibrosis lung pathology similar or superior to that of the other UTE-MRI techniques. The FLORET sequence allows for faster acquisition of high diagnostic-quality lung images and its short T2 * components without sacrificing SNR, image quality, or tissue/disease quantification.

An efficient alternative to magnetic resonance (MR) composite relaxation (R2-R1ρ) mapping in human knee cartilage study at 3T

An efficient alternative to magnetic resonance (MR) composite relaxation (R2-R1ρ) mapping in human knee cartilage study at 3T

Bone Mineral Density and Microstructure of the Elbow in Baseball Pitchers: An Analysis by Second-Generation HR-pQCT

Background: Baseball pitchers’ elbows are exposed to repeated overloading during the throwing motion, which causes bone structural changes such as bone sclerosis and osteophyte formation. They have been observed by clinical CT and MRI, while the bone microstructural change has not yet been studied in vivo. The aims of this study were to establish a method of imaging the elbow joint using second-generation high-resolution peripheral quantitative CT and to investigate the bone microstructural change in baseball pitchers’ dominant elbows. Methods: The subjects were 17 baseball pitchers. The elbow was fixed using a custom-made cast and scanned by second-generation high-resolution peripheral quantitative CT. The scan conditions were as follows: voxel size 60.7 μm, integration time 43 ms, scan length 30.6 mm, and total scan time 8 min. Volumetric bone mineral density (vBMD) and trabecular bone microstructure were analyzed in the 6.5-mm3 cubic regions set in the capitellum and trochlea, and the dominant and nondominant elbows were compared. Results: vBMD, bone volume fraction (BV/TV), and trabecular thickness (Tb.Th) at the capitellum were significantly higher in the dominant elbow than in the nondominant elbow. On the other hand, no parameters at the trochlea were significantly different. Conclusions: Higher vBMD accompanied by thicker trabecular bone was observed at the capitellum. These bone microstructural changes would reflect the valgus stress generated by the pitching motion in the elbow joint.

Image quality and diagnostic accuracy of complex-averaged high b value images in diffusion-weighted MRI of prostate cancer.

To evaluate the impact of complex-averaging on image quality (IQ) and diagnostic accuracy of acquired and calculated high b value (aHBV, cHBV) images in diffusion-weighted prostate MRI. This retrospective study included 84 patients who underwent multiparametric prostate MRI at 3 Tesla without endorectal coil. DWIs were acquired at three different b values which included two lower b values (b = 50,900s/mm2) and one higher b value (aHBV at 2000s/mm2). The acquired data were postprocessed to generate two different types of trace-weighted images-using conventional magnitude-averaging and complex-averaging. Using lower b values (b = 50,900s/mm2) from both conventional and complex-averaged image sets, cHBV images (b = 2000s/mm2) and ADC maps were derived. All image sets were reviewed by two radiologists in different reading sessions to assess image quality and PIRADS. The diagnostic accuracy of different image sets for the detection of prostate lesions was performed by correlating PIRADS and Gleason scores. Complex-averaging did not impact ADC values of the prostate lesions compared to magnitude-averaging (P = 0.08). Complex-averaging improved image quality of acquired high b value and calculated high b value images (P < 0.0001). Complex-averaging also improved the level of confidence (LOC) of the acquired high b value for both readers (P < 0.0001, P < 0.05), but only for reader A in calculated high b value (P < 0.0001). The image quality of calculated high b value images was not significantly different than acquired high b value images. The dataset combining complex-averaging and calculated high b value provided the highest diagnostic accuracy (but not statistically significant) for detection of the significant prostate lesion compared to the magnitude-averaged acquired high b value (79.55% vs. 72.73%; P = 0.317). The mean acquisition time for b = 2000s/mm2 sequence (aHBV) was 6 min 30 s (± 1min 16 s) out of a total of 28min 31s (± 4min 26s) for the entire mp-MRI protocol (approximately 25% of total scan time). Complex-averaging provides better image quality and level of confidence without significant impact on ADC values and diagnostic accuracy for detection of the significant prostate lesions . The calculated high b value images are also comparable to (and can substitute) the acquired high b value images which can help in reducing the imaging time.

High temporal resolution arterial spin labeling MRI with whole‐brain coverage by combining time‐encoding with Look‐Locker and simultaneous multi‐slice imaging

PurposeThe goal of this study was to achieve high temporal resolution, multi‐time point pseudo‐continuous arterial spin labeling (pCASL) MRI in a time‐efficient manner, while maintaining whole‐brain coverage.MethodsA Hadamard 8‐matrix was used to dynamically encode the pCASL labeling train, thereby providing the first source of temporal information. The second method for obtaining dynamic arterial spin labeling (ASL) signal consisted of a Look‐Locker (LL) readout of 4 phases that are acquired with a flip‐angle sweep to maintain constant sensitivity over the phases. To obtain whole‐brain coverage in the short LL interval, 4 slices were excited simultaneously by multi‐banded radiofrequency pulses. After subtraction according to the Hadamard scheme, the ASL signal was corrected for the use of the flip‐angle sweep and background suppression pulses. The BASIL toolkit of the Oxford Centre for FMRIB was used to quantify the ASL signal.ResultsBy combining a time‐encoded pCASL labeling scheme with an LL readout and simultaneous multi‐slice acquisition, 28 time points of 16 slices with a 75‐ or 150‐ms time resolution were acquired in a total scan time of 10 minutes 20 seconds, from which cerebral blood flow (CBF) maps, arterial transit time maps, and arterial blood volume could be determined.ConclusionWhole‐brain ASL images were acquired with a 75‐ms time resolution for the angiography and 150‐ms resolution for the perfusion phase by combining the proposed techniques. Reducing the total scan time to 1 minute 18 seconds still resulted in reasonable CBF maps, which demonstrates the feasibility of this approach for practical studies on brain hemodynamics.

Towards patient connected imaging with ACROBEAT: Adaptive CaRdiac cOne BEAm computed Tomography

Robotic C-arm cone beam computed tomography (CBCT) systems are playing an increasingly pivotal role in interventional cardiac procedures and high precision radiotherapy treatments. One of the main challenges in any form of cardiac imaging is mitigating the intrinsic motion of the heart, which causes blurring and artefacts in the 3D reconstructed image. Most conventional 3D cardiac CBCT acquisition techniques attempt to combat heart motion through retrospective gating techniques, whereby acquired projections are sorted into the desired cardiac phase after the completion of the scan. However, this results in streaking artefacts and unnecessary radiation exposure to the patient. Here, we present our Adaptive CaRdiac cOne BEAm computed Tomography (ACROBEAT) acquisition protocol that uses the patient’s electrocardiogram (ECG) signal to adaptively regulate the gantry velocity and projection time interval in real-time. It enables prospectively gated patient connected imaging in a single sweep of the gantry. The XCAT digital software phantom was used to complete a simulation study to compare ACROBEAT to a conventional multi-sweep retrospective ECG gated acquisition, under a variety of different acquisition conditions. The effect of location and length of the acquisition window and total number of projections acquired on image quality and total scan time were examined. Overall, ACROBEAT enables up to a 5 times average improvement in the contrast-to-noise ratio, a 40% reduction in edge response width and an 80% reduction in total projections acquired compared to conventional multi-sweep retrospective ECG gated acquisition.

Fast quantitative 3D ultrashort echo time MRI of cortical bone using extended cones sampling.

To investigate the effect of stretching sampling window on quantitative 3D ultrashort TE (UTE) imaging of cortical bone at 3 T. Ten bovine cortical bone and 17 human tibial midshaft samples were imaged with a 3T clinical MRI scanner using an 8-channel knee coil. Quantitative 3D UTE imaging biomarkers, including T1 , , magnetization transfer ratio and magnetization transfer modeling, were performed using radial or spiral Cones sampling trajectories with various durations. Errors in UTE-MRI biomarkers as a function of sampling time were evaluated using radial sampling as a reference standard. For both bovine and human cortical bone samples, no significant differences were observed for all UTE biomarkers (single-component , bicomponent and relative fractions, T1 , magnetization transfer ratio, and magnetization transfer modeling of macromolecular fraction) for spiral sampling windows of 992 µs to 1600 µs compared with a radial sampling window of 688 µs. The total scan time can be reduced by 76% with quantification errors less than 5%. Quantitative UTE-MRI techniques can be greatly accelerated using longer sampling windows without significant quantification errors.