Automated Measurement of Net Water Uptake From Baseline and Follow-Up CTs in Patients With Large Vessel Occlusion Stroke.

Quantifying the extent and evolution of cerebral edema developing after stroke is an important but challenging goal. Lesional net water uptake (NWU) is a promising CT-based biomarker of edema, but its measurement requires manually delineating infarcted tissue and mirrored regions in the contralateral hemisphere. We implement an imaging pipeline capable of automatically segmenting the infarct region and calculating NWU from both baseline and follow-up CTs of large-vessel occlusion (LVO) patients. Infarct core is extracted from CT perfusion images using a deconvolution algorithm while infarcts on follow-up CTs were segmented from non-contrast CT (NCCT) using a deep-learning algorithm. These infarct masks were flipped along the brain midline to generate mirrored regions in the contralateral hemisphere of NCCT; NWU was calculated as one minus the ratio of densities between regions, removing voxels segmented as CSF and with HU outside thresholds of 20-80 (normal hemisphere and baseline CT) and 0-40 (infarct region on follow-up). Automated results were compared with those obtained using manually-drawn infarcts and an ASPECTS region-of-interest based method that samples densities within the infarct and normal hemisphere, using intraclass correlation coefficient (ρ). This was tested on serial CTs from 55 patients with anterior circulation LVO (including 66 follow-up CTs). Baseline NWU using automated core was 4.3% (IQR 2.6-7.3) and correlated with manual measurement (ρ = 0.80, p < 0.0001) and ASPECTS (r = -0.60, p = 0.0001). Automatically segmented infarct volumes (median 110-ml) correlated to manually-drawn volumes (ρ = 0.96, p < 0.0001) with median Dice similarity coefficient of 0.83 (IQR 0.72-0.90). Automated NWU was 24.6% (IQR 20-27) and highly correlated to NWU from manually-drawn infarcts (ρ = 0.98) and the sampling-based method (ρ = 0.68, both p < 0.0001). We conclude that this automated imaging pipeline is able to accurately quantify region of infarction and NWU from serial CTs and could be leveraged to study the evolution and impact of edema in large cohorts of stroke patients.

Effects of motion and b-value on apparent temperature measurement by diffusion-based thermometry MRI: eye vitreous study.

PURPOSE: To make noninvasive measurements of temperature in the posterior chamber (vitreous) of the eye using diffusion-based thermometry (DBT) magnetic resonance imaging (MRI) and to explain variability in these measurements due to choice of b-value and the effects of motion. METHODS: Phantom studies of human vitreous and distilled water were performed using b-values from 0 to 1500 s/mm2 to determine the liquid-specific calibration factor for vitreous as well as to determine the temperature offsets due to sampling the diffusion curve using three higher routine clinical b-values (b = 0, 500, 1000 s/mm2 ) or four lower b-values (b = 0, 200, 400, 600 s/mm2 ), thought to be optimized for fluids. Retrospective ROI-based measurements of apparent diffusion coefficient on single slices as well as multi-slice histograms of the eyes were made in six patients with peri-orbital cellulitis and 11 age-matched controls, to assess for temperature changes in the presence of peri-orbital inflammation. A prospective study of ten repeated measurements of eye temperature using both high and lower b-value sampling was performed in ten asymptomatic volunteers to determine the reproducibility of eye temperature measurements in-vivo as well as to estimate vitreous temperature in the absence of motion. RESULTS: The diffusion coefficient of vitreous (2,088 ± 13 × 10-6 mm2 /s) was significantly lower (-1.9%, P < 0.001) compared to distilled water (2,128 ± 12 × 10-6 mm2 /s). The calibration factor for temperature measurements of vitreous using DBT is +0.74 ± 0.06°C. Temperature offsets were smaller (<-0.2°C, P < 0.01) when using larger routine clinical b-values to estimate the diffusion coefficient compared to using a series of lower b-values (<-1.0°C, P < 0.001). Two-dimensional single-slice ROI-based measurement showed significant temperature differences (ΔTI-C  = 2.5 ± 1.2°C, P < 0.001) between the eyes of patient with peri-orbital cellulitis, higher on the side of inflammation. There was no significant difference in eye temperature when using the 3D histogram (which is likely due to motion averaging as significant slice-to-slice variation was present). However, significant differences in the 3D temperature histograms between the two eyes was observed in one out of six patients. Prospective eye temperature measurements in healthy volunteers showed significant intra- and inter-subject variability (33.8-41.6°C), which was caused by eye motion. This resulted in +2.4°C cohort-wide elevation in temperature when three b-values were used and +4.7°C when four b-values were used. Using a pattern of elevated temperature at the periphery of the eye to detect motion, eye temperature is the absence of motion was estimated to be 34.5 ± 0.4°C with three higher b-values and 34.6 ± 1.9°C with four lower b-values; this temperature corresponds with prior mathematical simulations of eye temperature as well as boundary conditions. CONCLUSIONS: Globe vitreous temperature has been measured noninvasively using DBT MRI. Using routine clinical b-values of b = 0, 500 and 1000 s/mm2 produces acceptable (<-0.2°C) temperature offsets. Although DBT measurements are highly susceptible to motion, methods such as temperature differences or regression can be used to reduce or eliminate the effects of motion. Using a single clinical diffusion-weighted MRI, globe temperature difference of 1.6°C is pathological. Using a series of ten measurements, globe temperature differences larger than 0.6°C are abnormal. This study suggests CSF flow likely artifactually increases core brain temperature measured by DBT MRI.

Mapping vascular and glomerular pathology in a rabbit model of neonatal acute kidney injury using MRI.

Acute kidney injury (AKI) in premature neonates is common due to the administration of life-saving therapies. The impact of AKI on renal morphology and susceptibility to further renal damage is poorly understood. Recent advances in radiological imaging have allowed integration of soft tissue morphology in the intact organ, facilitating a more complete understanding of changes in tissue microstructure associated with pathology. Here, we applied magnetic resonance imaging (MRI) to detect both glomerular and vascular changes in a rabbit model of neonatal AKI, induced by indomethacin and gentamicin. Using combined spin-echo MRI and cationic ferritin enhanced gradient-echo MRI (CFE-MRI), we observed (a) an increased cortical arterial diameter in the AKI cohort compared to healthy controls, and (b) focal loss of vascular density and glomerular loss in a circumferential band ~1 mm from the cortical surface. This combined use of vascular and glomerular imaging may give insight into the etiology of AKI and its impact on renal health later in life.

Prevalence and quantitative analysis of indium-111 pentetreotide (Octreoscan) uptake in the pancreatic head on SPECT/CT imaging: establishing a region of interest-based pathological uptake threshold.

OBJECTIVE: Determine the prevalence of benign indium-111 (In) pentetreotide uptake in the pancreatic head and determine if a semi-quantitative method can be used to differentiate physiologic from pathologic uptake. PATIENTS AND METHODS: Institutional Review Board-approved, HIPAA-compliant retrospective review of 197 somatostatin receptor scintigraphy studies performed in 136 patients, from December 2012 to November 2013 at a large academic medical center. The pancreatic head uptake was visually graded and for all positive cases, two-dimensional and three-dimensional ratios of the pancreatic head to normal liver uptake were calculated. Statistical analysis using paired and two-sample t-tests was performed. RESULTS: Nineteen of one hundred twenty-nine (14.7%) patients had benign In pentetreotide uptake in the pancreatic head. Seven of seven (100%) patients with neuroendocrine (NE) tumors had definite visual uptake. Uptake was 2.7× more likely benign than malignant. Using a three-dimensional region of interest (ROI) method, the pancreatic head-to-liver ratio was 0.91±0.38 (0.37-1.63) for benign uptake and 8.2±7.3 (1.79-23.6) for pathologic uptake (P<0.001). A threshold of 1.67 provided 100% accuracy for determining the presence or absence of a pancreatic head NE tumor. Using a two-dimensional ROI method, the uptake ratio was 0.88±0.37 (0.28-1.73) for benign and 7.5±6.2 (1.85-19.6) for pathologic uptake (P<0.001); a ratio threshold of 1.62 provided 97% accuracy. There was no difference between the uptake ratios at 4 and 24 h. CONCLUSION: In pentetreotide uptake in the pancreatic head is common and more frequently benign than malignant. Using simple ROI ratiometric methods helps to differentiate benign physiologic from malignant NE tumor uptake.

Characterizing and eliminating errors in enhancement and subtraction artifacts in dynamic contrast-enhanced breast MRI: Chemical shift artifact of the third kind.

PURPOSE: To characterize errors in enhancement in breast dynamic contrast-enhanced (DCE) MRI studies as a function of echo time and determine the source of dark band artifacts in clinical subtraction images. METHODS: Computer simulations, oil and water substitute (methylene chloride), as well as an American College of Radiology quality control phantom were tested. Routine clinical DCE breast MRI study was bracketed with (accelerated) in-phase DCE acquisitions in five patients. RESULTS: Simulation results demonstrated up to -160% suppression of the expected enhancement caused by differential enhancement of fat and water. Two-dimensional gradient-recalled echo and fat-suppressed 3D GRE phantom imaging confirmed the simulation results and showed that fat suppression does not eliminate the artifact. In vivo in-phase DCE images showed increased enhancement consistent with predictions and also confirmed increased spatial blurring on in-phase 3D gradient-recalled echo images. Combined multi-dimensional partial Fourier and parallel imaging provided a time-equivalent in-phase DCE MRI acquisition. CONCLUSION: Errors in expected enhancement occur in DCE breast MRI subtraction images because of differential enhancement of fat and water and incomplete fat signal suppression. These errors can lead to artificial suppression of enhancement as well as dark band artifacts on subtraction images. These artifacts can be eliminated with a time-equivalent in-phase fat-suppressed 3D gradient-recalled echo sequence. Understanding chemical shift artifact of the third kind, a unique artifact of artificial enhancement suppression in the presence of intravoxel fat and water signal, will aid DCE breast MRI image interpretation. In-phase acquisitions (combined with simultaneous minimum echo time or opposed-phase echoes) may facilitate qualitative, quantitative and longitudinal analysis of contrast enhancement. Magn Reson Med 79:2277-2289, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Resolution enhanced T1-insensitive steady-state imaging.

Resolution enhanced T(1)-insensitive steady-state imaging (RE-TOSSI) is a new MRI pulse sequence for the generation of rapid T(2) contrast with high spatial resolution. TOSSI provides T(2) contrast by using nonequally spaced inversion pulses throughout a balanced steady-state free precession (SSFP) acquisition. In RE-TOSSI, these energy and time intensive adiabatic inversion pulses and associated magnetization preparation are removed from TOSSI after acquisition of the data around the center of k-space. Magnetization evolution simulations demonstrate T(2) contrast in TOSSI as well as reduction in the widening of the point spread function width (by up to a factor of 4) to a near ideal case for RE-TOSSI. Phantom experimentation is used to characterize and compare the contrast and spatial resolution properties of TOSSI, RE-TOSSI, balanced SSFP, Half-Fourier Acquisition Single-Shot Turbo Spin Echo (HASTE), and turbo spin echo and to optimize the fraction of k-space acquired using TOSSI. Comparison images in the abdomen and brain demonstrate similar contrast and improved spatial resolution in RE-TOSSI compared with TOSSI; comparison balanced SSFP, HASTE, and turbo spin echo images are provided. RE-TOSSI is capable of providing high spatial resolution T(2)-weighted images in 1 s or less per image.

Characterization and reduction of saturation banding in multiplanar coherent and incoherent steady-state imaging.

Hypointense band artifacts occur at intersections of nonparallel imaging planes in rapidly acquired MR images; quantitative or numerical analysis of these bands and strategies to mitigate their appearance have largely gone unexplored. The magnetization evolution in the different regions of multiplanar images was simulated for three common rapid steady-state techniques (spoiled gradient echo, steady state free precession, balanced steady state free precession). Saturation banding was found to be highly dependent on the pulse sequence, acquisition time, and phase-encoding order. Encoding the center of k-space at the end of the acquisition of each slice (i.e., reverse centric phase encoding) is demonstrated to be a simple and robust method for significantly reducing the relative saturation in all imaging planes. View ordering and resolution dependence were confirmed in multiplanar abdominal images. The added importance of reducing the artifact in accelerated acquisition techniques (e.g., parallel imaging) is particularly notable in multiplanar balanced steady state free precession images in the brain.

Halting the effects of flow enhancement with effective intermittent zeugmatographic encoding (HEFEWEIZEN) in SSFP.

PURPOSE: To describe a new method for performing dark blood (DB) magnetization preparation in TrueFISP (bSSFP) and apply the technique to high-resolution carotid artery imaging. MATERIALS AND METHODS: The developed method (HEFEWEIZEN) provides directional flow suppression, while preserving bSSFP contrast, by periodically applying spatial saturation in short repetition time (TR) TrueFISP. Steady-state free precession (SSFP) conditions are maintained throughout the acquisition for the imaging slice magnetization. HEFEWEIZEN was implemented on a 1.5 T scanner with standard receiver coils. Studies were performed in phantoms, eight asymptomatic volunteers, and two patients with low- and high-grade carotid artery stenosis. RESULTS: Average flow suppression was 88% +/- 4% (arterial) and 85% +/- 3% (venous) in a multislice study. Stationary signal, contrast, and fine details were maintained with only slight signal suppression (11% +/- 11%). Comparison to diffusion-prepared SSFP in the common carotid artery demonstrated significant improvement in wall-lumen contrast-to-noise ratio efficiency (P = 0.024). DB contrast was achieved with only 13% increased acquisition time (14.3 sec). Further acceleration was possible by confining the DB preparation to the central 60% of k-space. CONCLUSION: A fast, short TR, DB TrueFISP pulse sequence was developed and tested in the carotid arteries of asymptomatic volunteers and patients.

A technique for MRI-guided transrectal deep pelvic abscess drainage.

OBJECTIVE: The purpose of this article is to introduce a technique for transrectal drainage of deep pelvic abscesses performed under interactive MRI guidance. CONCLUSION: A new method for triorthogonal image plane MRI guidance was developed and used to interactively monitor the puncture needle on continuously updated sets of adjustable three-plane images. The merits and limitations of the technique are highlighted and the patient population that is likely to benefit from this approach is suggested.

Update to pulse sequences for interventional MR imaging.

The motivations for developing MR-guided minimally invasive therapy include its excellent soft tissue contrast, tomographic imaging in any direction (as opposed to projection imaging as in fluoroscopy), the absence of ionizing radiation,the abundance of contrast mechanisms (including bright blood pulse sequences that lead to excellent vessel conspicuity without exogenous contrast agent injection), the ability to obtain physiologic information such as perfusion, and an overall excellent safety profile. The main pulse sequences used today for interventional MR imaging are T1/T2-weighted FISP and TrueFISP, T2-weighted turbo spin-echo, and T1-weighted FLASH. The specific clinical question, the underlying pathophysiology,and the procedure to be performed dictate which sequence is used. Each of these sequences has been written to acquire data in conventional rectilinear trajectories, radial k-space paths, or even spirals. In many ways, the questions being researched in interventional MR imaging have been dictated by the primary issues in greatest need of resolution or that most directly facilitate new clinical development. A decade ago, research focused on exploration of new scan strategies for contrast and temporal resolution. Advancements in the last decade have made it possible to acquire and display greater than 10 images per second in realtime with millimeter resolution in all three directions. This temporal and spatial resolution is considered high enough to guide most interventions. With this capability, other research has focused on instrument tracking. The field has gone from the capability to track a single coil and superimpose it on a previously acquired roadmap to systems that follow, adapt, and provide high-resolution images due to the advent of multichannel receiver systems, improved graphics, higher processor speeds, and increases in speed and quantity of memory. Hence, instruments can be reliably identified and tracked and the information can be used to update pulse sequence parameters in real time, thereby opening new opportunities for interventional MR imaging that extend from biopsy and thermal therapy to image-guided vascular and cardiac procedures. Today, we see such issues as RF heating of wires used for device localization and the noise generated by rapid switching of MR gradients being significant obstacles yet to overcome to allow the full strength of MR-guided interventions to be realized clinically. It is anticipated that these topics will emerge as critical concepts in the next decade of interventional MR imaging research.