Instrument visualization using conventional and compressed sensing SEMAC for interventional MRI at 3T.

BACKGROUND: Interventional magnetic resonance imaging (MRI) at 3T benefits from higher spatial and temporal resolution, but artifacts of metallic instruments are often larger and may obscure target structures. PURPOSE: To test that compressed sensing (CS) slice-encoding metal artifact correction (SEMAC) is feasible for 3T interventional MRI and affords more accurate instrument visualization than turbo spin echo (TSE) and gradient echo (GRE) techniques, and facilitates faster data acquisition than conventional SEMAC. STUDY TYPE: Prospective. PHANTOM AND SUBJECTS: Cadaveric animal and 20 human subjects. FIELD STRENGTH/SEQUENCE: TSE (acquisition time 31 sec), GRE (28-33 sec), SEMAC (128 sec), and CS-SEMAC (57 sec) pulse sequences were evaluated at 3T. ASSESSMENT: Artifact width and length, signal-to-noise (SNR), and contrast-to-noise (CNR) ratios of 14-22G MR-conditional needles were measured in a phantom. Subsequently, high-bandwidth TSE and CS-SEMAC sequences were assessed in vivo with 20 patient procedures for the size of the metal artifact, image sharpness, image noise, motion artifacts, image contrast, and target, instrument, and structural visibility. STATISTICAL TESTS: Repeated-measures-analysis-of-variances and Mann-Whitney U-tests were applied. P ≤ 0.05 was considered statistically significant. RESULTS: CS-SEMAC and SEMAC created the smallest needle artifact widths (3.2-3.3 ± 0.4 mm, P = 1.0), whereas GRE showed the largest needle artifact widths (8.5-8.6 ± 0.4 mm) (P < 0.001). The artifact width difference between high-bandwidth TSE and CS-SEMAC was 0.8 ± 0.6 mm (P < 0.01). SEMAC and CS-SEMAC created the lowest average needle tip errors (0.3-0.4 ± 0.1 mm, P = 1.0). The average tip error difference between high-bandwidth TSE and SEMAC/CS-SEMAC was 2.0 ± 1.7 mm (P < 0.01). SNR and CNR were similar on TSE, SEMAC, and CS-SEMAC, and lowest on GRE. CS-SEMAC yielded smaller artifacts, less noise, less motion, and better instrument visibility (P < 0.001); high-bandwidth TSE showed better sharpness (P < 0.001) and targets visibility (P = 0.007); whereas image contrast (P = 0.273) and structural visibility (P = 0.1) were similar. DATA CONCLUSION: CS-SEMAC is feasible for interventional MRI at 3T, visualizes instruments with higher accuracy than high-bandwidth TSE and GRE, and can be acquired 55% faster than conventional SEMAC. LEVEL OF EVIDENCE: 2 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2018;47:1306-1315.

MR-guided percutaneous sclerotherapy of low-flow vascular malformations: Clinical experience using a 1.5 tesla MR system.

PURPOSE: To demonstrate the feasibility, safety, and effectiveness of image-guided sclerotherapy of low-flow vascular malformations using a 1.5 Tesla (T) MR scanner with real-time imaging capability and in-suite fluoroscopy. MATERIALS AND METHODS: Thirty-three procedures were performed with real-time 1.5T MR-guidance on 22 patients with a vascular malformation in the neck (n = 2), chest (n = 6), abdomen and pelvis (n = 15), and extremities (n = 11). Quantitative analysis was performed for changes in (a) planning time, (b) targeting time (interval between needle skin puncture and lesion access), (c) intervention time (interval between needle skin puncture and needle removal), and (d) total procedure time. Qualitative analysis was performed for (a) success of therapy and (b) occurrence of complications. RESULTS: Technical success was achieved in 29 of 33 procedures. The average planning time did not significantly change between the first seven procedures and the last seven procedures (P = 0.447). The average targeting time decreased by 0:24:45 (hours:minutes:seconds) (P = 0.043), the average intervention time decreased by 0:26:58 (P = 0.022), and the average procedure time decreased by 0:28:41 (P = 0.046) when comparing the first seven procedures and the last seven procedures. Overall, there was an improvement in the patients' predominant symptoms following 82% of procedures, including a significant decrease in average pain following therapy (P < 0.001). There was a minor complication rate of 3% with no major complications. CONCLUSION: MR-guided percutaneous sclerotherapy seems to be a safe, effective, and versatile technique for treating low-flow vascular malformations. LEVEL OF EVIDENCE: 3 J. Magn. Reson. Imaging 2017;45:1154-1162.

CAIPIRINHA accelerated SPACE enables 10-min isotropic 3D TSE MRI of the ankle for optimized visualization of curved and oblique ligaments and tendons.

OBJECTIVES: To test the hypothesis that a fourfold CAIPIRINHA accelerated, 10-min, high-resolution, isotropic 3D TSE MRI prototype protocol of the ankle derives equal or better quality than a 20-min 2D TSE standard protocol. METHODS: Following internal review board approval and informed consent, 3-Tesla MRI of the ankle was obtained in 24 asymptomatic subjects including 10-min 3D CAIPIRINHA SPACE TSE prototype and 20-min 2D TSE standard protocols. Outcome variables included image quality and visibility of anatomical structures using 5-point Likert scales. Non-parametric statistical testing was used. P values ≤0.001 were considered significant. RESULTS: Edge sharpness, contrast resolution, uniformity, noise, fat suppression and magic angle effects were without statistical difference on 2D and 3D TSE images (p > 0.035). Fluid was mildly brighter on intermediate-weighted 2D images (p < 0.001), whereas 3D images had substantially less partial volume, chemical shift and no pulsatile-flow artifacts (p < 0.001). Oblique and curved planar 3D images resulted in mildly-to-substantially improved visualization of joints, spring, bifurcate, syndesmotic, collateral and sinus tarsi ligaments, and tendons (p < 0.001, respectively). CONCLUSIONS: 3D TSE MRI with CAIPIRINHA acceleration enables high-spatial resolution oblique and curved planar MRI of the ankle and visualization of ligaments, tendons and joints equally well or better than a more time-consuming anisotropic 2D TSE MRI. KEY POINTS: • High-resolution 3D TSE MRI improves visualization of ankle structures. • Limitations of current 3D TSE MRI include long scan times. • 3D CAIPIRINHA SPACE allows now a fourfold-accelerated data acquisition. • 3D CAIPIRINHA SPACE enables high-spatial-resolution ankle MRI within 10 min. • 10-min 3D CAIPIRINHA SPACE produces equal-or-better quality than 20-min 2D TSE.

Advanced metal artifact reduction MRI of metal-on-metal hip resurfacing arthroplasty implants: compressed sensing acceleration enables the time-neutral use of SEMAC.

OBJECTIVE: Compressed sensing (CS) acceleration has been theorized for slice encoding for metal artifact correction (SEMAC), but has not been shown to be feasible. Therefore, we tested the hypothesis that CS-SEMAC is feasible for MRI of metal-on-metal hip resurfacing implants. MATERIALS AND METHODS: Following prospective institutional review board approval, 22 subjects with metal-on-metal hip resurfacing implants underwent 1.5 T MRI. We compared CS-SEMAC prototype, high-bandwidth TSE, and SEMAC sequences with acquisition times of 4-5, 4-5 and 10-12 min, respectively. Outcome measures included bone-implant interfaces, image quality, periprosthetic structures, artifact size, and signal- and contrast-to-noise ratios (SNR and CNR). Using Friedman, repeated measures analysis of variances, and Cohen's weighted kappa tests, Bonferroni-corrected p-values of 0.005 and less were considered statistically significant. RESULTS: There was no statistical difference of outcomes measures of SEMAC and CS-SEMAC images. Visibility of implant-bone interfaces and pseudocapsule as well as fat suppression and metal reduction were "adequate" to "good" on CS-SEMAC and "non-diagnostic" to "adequate" on high-BW TSE (p < 0.001, respectively). SEMAC and CS-SEMAC showed mild blur and ripple artifacts. The metal artifact size was 63 % larger for high-BW TSE as compared to SEMAC and CS-SEMAC (p < 0.0001, respectively). CNRs were sufficiently high and statistically similar, with the exception of CNR of fluid and muscle and CNR of fluid and tendon, which were higher on intermediate-weighted high-BW TSE (p < 0.005, respectively). CONCLUSION: Compressed sensing acceleration enables the time-neutral use of SEMAC for MRI of metal-on-metal hip resurfacing implants when compared to high-BW TSE and image quality similar to conventional SEMAC.

MR-guided sclerotherapy of low-flow vascular malformations using T2 -weighted interrupted bSSFP (T2 W-iSSFP): comparison of pulse sequences for visualization and needle guidance.

PURPOSE: Image-guided treatment of low-flow vascular (venous or lymphatic) malformations presents a challenging visualization problem, regardless of the imaging modality being used for guidance. The purpose of this study was to employ a new magnetic resonance imaging (MRI) sequence, T2 -weighted interrupted balanced steady-state free precession (T2 W-iSSFP), for real-time image guidance of needle insertion. MATERIALS AND METHODS: T2 W-iSSFP uses variable flip angle balanced steady-state free precession (bSSFP, a.k.a. SSFP) to establish T2 -weighting and fat suppression. Swine (n = 3) and patients (n = 4, three female, all with venous malformations) were enrolled in the assessment. T2 -weighted turbo spin echo (T2 -TSE) with spectral adiabatic inversion recovery (SPAIR), SPAIR-T2 -TSE or T2 -TSE for short, was used as the reference. T2 -weighted half Fourier acquired single shot turbo spin echo (T2 -HASTE) with SPAIR (SPAIR-T2 -HASTE, T2 -HASTE for short), fat saturated bSSFP (FS-SSFP), and T2 W-iSSFP were imaged. Numeric metrics, namely, contrast-to-noise ratio (CNR) efficiency (CNR divided by the square root of acquisition time) and local sharpness (the reciprocal of edge width), were used to assess image quality. MR-guided sclerotherapy was performed on the same patients using real-time T2 W-iSSFP to guide needle insertion. RESULTS: Comparing the visualization of needles in the images of swine, the local sharpness (mm(-1) ) was: 0.21 ± 0.06 (T2 -HASTE), 0.48 ± 0.02 (FS-SSFP), and 0.49 ± 0.03 (T2 W-iSSFP). T2 W-iSSFP is higher than T2 -HASTE (P < 0.001). For the patient images, their CNR efficiencies were: 797 ± 66 (T2 -HASTE), 281 ± 44 (FS-SSFP), and 860 ± 29 (T2 W-iSSFP). T2 W-iSSFP is higher than FS-SSFP (P < 0.02). The frame rate of T2 W-iSSFP was 2.5-3.5 frames per second. All MR-guided sclerotherapy procedures were successful, with all needles (six punctures) placed in the targets. CONCLUSION: T2 W-iSSFP provides effective lesion identification and needle visualization. This new pulse sequence can be used for MR-guided sclerotherapy of low-flow vascular malformations. It may have potential use in other MR-guided procedures where heavily T2 -weighted real-time images are needed.

Fused X-ray and MR imaging guidance of intrapericardial delivery of microencapsulated human mesenchymal stem cells in immunocompetent swine.

PURPOSE: To assess intrapericardial delivery of microencapsulated, xenogeneic human mesenchymal stem cells (hMSCs) by using x-ray fused with magnetic resonance (MR) imaging (x-ray/MR imaging) guidance as a potential treatment for ischemic cardiovascular disease in an immunocompetent swine model. MATERIALS AND METHODS: All animal experiments were approved by the institutional animal care and use committee. Stem cell microencapsulation was performed by using a modified alginate-poly-l-lysine-alginate encapsulation method to include 10% (wt/vol) barium sulfate to create barium-alginate microcapsules (BaCaps) that contained hMSCs. With x-ray/MR imaging guidance, eight female pigs (approximately 25 kg) were randomized to receive either BaCaps with hMSCs, empty BaCaps, naked hMSCs, or saline by using a percutaneous subxiphoid approach and were compared with animals that received empty BaCaps (n = 1) or BaCaps with hMSCs (n = 2) by using standard fluoroscopic delivery only. MR images and C-arm computed tomographic (CT) images were acquired before injection and 1 week after delivery. Animals were sacrificed immediately or at 1 week for histopathologic validation. Cardiac function between baseline and 1 week after delivery was evaluated by using a paired Student t test. RESULTS: hMSCs remained highly viable (94.8% ± 6) 2 days after encapsulation in vitro. With x-ray/MR imaging, successful intrapericardial access and delivery were achieved in all animals. BaCaps were visible fluoroscopically and at C-arm CT immediately and 1 week after delivery. Whereas BaCaps were free floating immediately after delivery, they consolidated into a pseudoepicardial tissue patch at 1 week, with hMSCs remaining highly viable within BaCaps; naked hMSCs were poorly retained. Follow-up imaging 1 week after x-ray/MR imaging-guided intrapericardial delivery showed no evidence of pericardial adhesion and/or effusion or adverse effect on cardiac function. In contradistinction, BaCaps delivery with x-ray fluoroscopy without x-ray/MR imaging (n = 3) resulted in pericardial adhesions and poor hMSC viability after 1 week. CONCLUSION: Intrapericardial delivery of BaCaps with hMSCs leads to high cell retention and survival. With x-ray/MR imaging guidance, intrapericardial delivery can be performed safely in the absence of preexisting pericardial effusion to provide a novel route for cardiac cellular regenerative therapy.

Device localization and dynamic scan plane selection using a wireless magnetic resonance imaging detector array.

PURPOSE: A prototype wireless guidance device using single sideband amplitude modulation (SSB) is presented for a 1.5T magnetic resonance imaging system. METHODS: The device contained three fiducial markers each mounted to an independent receiver coil equipped with wireless SSB technology. Acquiring orthogonal projections of these markers determined the position and orientation of the device, which was used to define the scan plane for a subsequent image acquisition. Device localization and scan plane update required approximately 30 ms, so it could be interleaved with high temporal resolution imaging. Since the wireless device is used for localization and does not require full imaging capability, the design of the SSB wireless system was simplified by allowing an asynchronous clock between the transmitter and receiver. RESULTS: When coupled to a high readout bandwidth, the error caused by the lack of a shared frequency reference was quantified to be less than one pixel (0.78 mm) in the projection acquisitions. Image guidance with the prototype was demonstrated with a phantom where a needle was successfully guided to a target and contrast was delivered. CONCLUSION: The feasibility of active tracking with a wireless detector array is demonstrated. Wireless arrays could be incorporated into devices to assist in image-guided procedures.

Magnetic resonance-guided, real-time targeted delivery and imaging of magnetocapsules immunoprotecting pancreatic islet cells.

In type I diabetes mellitus, islet transplantation provides a moment-to-moment fine regulation of insulin. Success rates vary widely, however, necessitating suitable methods to monitor islet delivery, engraftment and survival. Here magnetic resonance-trackable magnetocapsules have been used simultaneously to immunoprotect pancreatic beta-cells and to monitor, non-invasively in real-time, hepatic delivery and engraftment by magnetic resonance imaging (MRI). Magnetocapsules were detected as single capsules with an altered magnetic resonance appearance on capsule rupture. Magnetocapsules were functional in vivo because mouse beta-cells restored normal glycemia in streptozotocin-induced diabetic mice and human islets induced sustained C-peptide levels in swine. In this large-animal model, magnetocapsules could be precisely targeted for infusion by using magnetic resonance fluoroscopy, whereas MRI facilitated monitoring of liver engraftment over time. These findings are directly applicable to ongoing improvements in islet cell transplantation for human diabetes, particularly because our magnetocapsules comprise clinically applicable materials.

Percutaneous punctures with MR imaging guidance: comparison between MR imaging-enhanced fluoroscopic guidance and real-time MR Imaging guidance.

PURPOSE: To evaluate and compare the technical accuracy and feasibility of magnetic resonance (MR) imaging-enhanced fluoroscopic guidance and real-time MR imaging guidance for percutaneous puncture procedures in phantoms and animals. MATERIALS AND METHODS: The experimental protocol was approved by the institutional animal care and use committee. Punctures were performed in phantoms, aiming for markers (20 each for MR imaging-enhanced fluoroscopic guidance and real-time MR imaging guidance), and pigs, aiming for anatomic landmarks (10 for MR imaging-enhanced fluoroscopic guidance and five for MR imaging guidance). To guide the punctures, T1-weighted three-dimensional (3D) MR images of the phantom or pig were acquired. Additional axial and coronal T2-weighted images were used to visualize the anatomy in the animals. For MR imaging-enhanced fluoroscopic guidance, phantoms and pigs were transferred to the fluoroscopic system after initial MR imaging and C-arm computed tomography (CT) was performed. C-arm CT and MR imaging data sets were coregistered. Prototype navigation software was used to plan a puncture path with use of MR images and to superimpose it on fluoroscopic images. For real-time MR imaging, an interventional MR imaging prototype for interactive real-time section position navigation was used. Punctures were performed within the magnet bore. After completion, 3D MR imaging was performed to evaluate the accuracy of insertions. Puncture durations were compared by using the log-rank test. The Mann-Whitney U test was applied to compare the spatial errors. RESULTS: In phantoms, the mean total error was 8.6 mm ± 2.8 with MR imaging-enhanced fluoroscopic guidance and 4.0 mm ± 1.2 with real-time MR imaging guidance (P < .001). The mean puncture time was 2 minutes 10 seconds ± 44 seconds with MR imaging-enhanced fluoroscopic guidance and 37 seconds ± 14 with real-time MR imaging guidance (P < .001). In the animal study, a tolerable distance (<1 cm) between target and needle tip was observed for both MR imaging-enhanced fluoroscopic guidance and real-time MR imaging guidance. The mean total error was 7.7 mm ± 2.4 with MR imaging-enhanced fluoroscopic guidance and 7.9 mm ± 4.9 with real-time MR imaging guidance (P = .77). The mean puncture time was 5 minutes 43 seconds ± 2 minutes 7 seconds with MR imaging-enhanced fluoroscopic guidance and 5 minutes 14 seconds ± 2 minutes 25 seconds with real-time MR imaging guidance (P = .68). CONCLUSION: Both MR imaging-enhanced fluoroscopic guidance and real-time MR imaging guidance demonstrated reasonable and similar accuracy in guiding needle placement to selected targets in phantoms and animals.

X-ray-visible microcapsules containing mesenchymal stem cells improve hind limb perfusion in a rabbit model of peripheral arterial disease.

The therapeutic goal in peripheral arterial disease (PAD) patients is to restore blood flow to ischemic tissue. Stem cell transplantation offers a new avenue to enhance arteriogenesis and angiogenesis. Two major problems with cell therapies are poor cell survival and the lack of visualization of cell delivery and distribution. To address these therapeutic barriers, allogeneic bone marrow-derived mesenchymal stem cells (MSCs) were encapsulated in alginate impregnated with a radiopaque contrast agent (MSC-Xcaps). In vitro MSC-Xcap viability by a fluorometric assay was high (96.9% ± 2.7% at 30 days postencapsulation) and as few as 10 Xcaps were visible on clinical x-ray fluoroscopic systems. Using an endovascular PAD model, rabbits (n = 21) were randomized to receive MSC-Xcaps (n = 6), empty Xcaps (n = 5), unencapsulated MSCs (n = 5), or sham intramuscular injections (n = 5) in the ischemic thigh 24 hours postocclusion. Immediately after MSC transplantation and 14 days later, digital radiographs acquired on a clinical angiographic system demonstrated persistent visualization of the Xcap injection sites with retained contrast-to-noise. Using a modified TIMI frame count, quantitative angiography demonstrated a 65% improvement in hind limb perfusion or arteriogenesis in MSC-Xcap-treated animals versus empty Xcaps. Post-mortem immunohistopathology of vessel density by anti-CD31 staining demonstrated an 87% enhancement in angiogenesis in Xcap-MSC-treated animals versus empty Xcaps. MSC-Xcaps represent the first x-ray-visible cellular therapeutic with enhanced efficacy for PAD treatment.

Rapid freehand MR-guided percutaneous needle interventions: an image-based approach to improve workflow and feasibility.

PURPOSE: To develop and evaluate software-based methods for improving the workflow of magnetic resonance (MR)-guided percutaneous interventions. MATERIALS AND METHODS: A set of methods was developed that allows the user to: 1) plan an entire procedure, 2) directly apply this plan to skin entry site localization without further imaging, and 3) place a needle under real-time MR guidance with automatic alignment of three orthogonal slices along a planned trajectory with preference to the principal patient axes. To validate targeting accuracy and time, phantom experiments (96 targets) and in vivo paraspinal and kidney needle punctures in two pigs (55 targets) were performed. The influence of trajectory obliquity, level of experience, and organ motion on targeting accuracy and time was analyzed. RESULTS: Mean targeting error was 1.8 ± 0.9 mm (in vitro) and 2.9 ± 1.0 mm (in vivo) in all directions. No statistically significant differences in targeting accuracy between single- and double-oblique trajectories, novice and expert users, or paraspinal and kidney punctures were observed. The average time (in vivo) from trajectory planning to verification of accurate needle placement was 6 minutes. CONCLUSION: The developed methods allow for accurate needle placement along complex trajectories and are anticipated to reduce table time for MR-guided percutaneous needle interventions.

Automated detection and characterization of SPIO-labeled cells and capsules using magnetic field perturbations.

Understanding how individual cells behave inside living systems will help enable new diagnostic tools and cellular therapies. Superparamagnetic iron oxide particles can be used to label cells and theranostic capsules for noninvasive tracking using MRI. Contrast changes from superparamagnetic iron oxide are often subtle relative to intrinsic sources of contrast, presenting a detection challenge. Here, we describe a versatile postprocessing method, called Phase map cross-correlation Detection and Quantification (PDQ), that automatically identifies localized deposits of superparamagnetic iron oxide, estimating their volume magnetic susceptibility and magnetic moment. To demonstrate applicability, PDQ was used to detect and characterize superparamagnetic iron oxide-labeled magnetocapsules implanted in porcine liver and suspended in agarose gel. PDQ was also applied to mouse brains infiltrated by MPIO-labeled macrophages following traumatic brain injury; longitudinal, in vivo studies tracked individual MPIO clusters over 3 days, and tracked clusters were corroborated in ex vivo brain scans. Additionally, we applied PDQ to rat hearts infiltrated by MPIO-labeled macrophages in a transplant model of organ rejection. PDQ magnetic measurements were signal-to-noise ratio invariant for images with signal-to-noise ratio > 11. PDQ can be used with conventional gradient-echo pulse sequences, requiring no extra scan time. The method is useful for visualizing biodistribution of cells and theranostic magnetocapsules and for measuring their relative iron content.

Diffusion tensor imaging of the maturing paediatric cervical spinal cord: from the neonate to the young adult.

OBJECTIVE: Normative apparent diffusion coefficient (ADC) and fractional anisotropy (FA) metrics of the brain have been published previously. However, no larger studies evaluated the normal evolution of ADC/FA metrics of the maturing paediatric spinal cord. Goal of this study is to evaluate the age-dependent evolution of the ADC/FA values of the developing/maturing normal cervical spinal cord (CSC). PATIENTS AND METHODS: Forty-one subjects, aged less than 18 years with a negative spinal MRI study and no systemic central nervous disease, underwent diffusion tensor imaging (DTI) of the CSC. DTI metrics were measured in the centre of the CSC. Regression and ANCOVA analyses were performed to evaluate the association between ADC/FA values and age and its potential modification by sex. RESULTS: A linear model emerged as the best fit for our data. ADC showed a continuous decrease with age; FA showed a continuous increase with age. CONCLUSION: The simultaneous age-related ADC decrease and FA increase likely reflect progressive maturation, myelination and fibre packing within the CSC similar to that observed in the brain. Collection of age-dependent normative DTI metrics may be helpful in the early identification and quantification of altered water diffusion in a variety of pathologies affecting the developing paediatric spinal cord.

MR-guided portal vein delivery and monitoring of magnetocapsules: assessment of physiologic effects on the liver.

PURPOSE: The authors previously developed magnetic resonance (MR)-trackable magnetocapsules (MCs) that can simultaneously immunoprotect human islet cells and noninvasively monitor portal delivery and engraftment in real time with MR imaging. This study was designed to assess the physiologic effects of the delivery of a clinical dose of MCs (140,000 capsules) into the portal vein (PV) in swine over a 1-month period. MATERIALS AND METHODS: MCs were formed by using clinical-grade alginate mixed with a clinically applicable dosage of ferumoxide. Percutaneous access into the PV was obtained by using a custom-built, MR-trackable needle, and 140,000 MCs were delivered under MR guidance in five swine. Portal pressures and liver function data were obtained over a 4-week period. RESULTS: A transient increase in portal pressure occurred immediately after MC delivery that returned to normal levels by 4 weeks after MC delivery. Liver function test results were normal during the entire period, and the appearance of the MCs on MR imaging did not change. CONCLUSIONS: A clinically applicable dose of 140,000 MCs has no adverse effects on portal pressures or liver function in this normal swine model during the first month after delivery.

Off-resonance angiography: a new method to depict vessels--phantom and rabbit studies.

PURPOSE: To evaluate the utility of inversion recovery with on-resonant water suppression (IRON) in combination with injection of the long-circulating monocrystalline iron oxide nanoparticle (MION)-47 for contrast material-enhanced magnetic resonance (MR) angiography. MATERIALS AND METHODS: Experiments were approved by the institutional animal care committee. Eleven rabbits were imaged at baseline before injection of a contrast agent and then serially 5-30 minutes, 2 hours, 1 day, and 3 days after a single intravenous bolus injection of 80 micromol of MION-47 per kilogram of body weight (n = 6) or 250 micromol/kg MION-47 (n = 5). Conventional T1-weighted MR angiography and IRON MR angiography were performed on a clinical 3.0-T imager. Signal-to-noise and contrast-to-noise ratios were measured in the aorta of rabbits in vivo. Venous blood was obtained from the rabbits before and after MION-47 injection for use in phantom studies. RESULTS: In vitro blood that contained MION-47 appeared signal attenuated on T1-weighted angiograms, while characteristic signal-enhanced dipolar fields were observed on IRON angiograms. In vivo, the vessel lumen was signal attenuated on T1-weighted MR angiograms after MION-47 injection, while IRON supported high intravascular contrast by simultaneously providing positive signal within the vessels and suppressing background tissue (mean contrast-to-noise ratio, 61.9 +/- 12.4 [standard deviation] after injection vs 1.1 +/- 0.4 at baseline, P < .001). Contrast-to-noise ratio was higher on IRON MR angiograms than on conventional T1-weighted MR angiograms (9.0 +/- 2.5, P < .001 vs IRON MR angiography) and persisted up to 24 hours after MION-47 injection (76.2 +/- 15.9, P < .001 vs baseline). CONCLUSION: IRON MR angiography in conjunction with superparamagnetic nanoparticle administration provides high intravascular contrast over a long time and without the need for image subtraction.

Hind limb ischemia in rabbit model: T2-prepared versus time-of-flight MR angiography at 3 T.

PURPOSE: To prospectively compare various parameters of vessels imaged at 3 T by using time-of-flight (TOF) and T2-prepared magnetic resonance (MR) angiography in a rabbit model of hind limb ischemia. MATERIALS AND METHODS: Experiments were approved by the institutional animal care and use committee. Endovascular occlusion of the left superficial femoral artery was induced in 14 New Zealand white rabbits. After 2 weeks, MR angiography and conventional (x-ray) angiography were performed. Vessel sharpness was evaluated visually in the ischemic and nonischemic limbs, and the presence of small collateral vessels was evaluated in the ischemic limbs. Vessel sharpness was also quantified by evaluating the magnitude of signal intensity change at the vessel borders. RESULTS: The sharpness of vessels in the nonischemic limbs was similar between the TOF and the T2-prepared images. In the ischemic limbs, however, T2-prepared imaging, as compared with TOF imaging, generated higher vessel sharpness in arteries with diminished blood flow (mean vessel sharpness: 44% vs 30% for popliteal arteries, 45% vs 28% for saphenous arteries; P < .001 for both comparisons) and enabled better detection of small collateral vessels (93% vs 36% of vessels, P < .001). CONCLUSION: T2-prepared imaging can facilitate high-spatial-resolution MR angiography of small vessels with low blood flow and thus has potential as a tool for noninvasive evaluation of arteriogenic therapies, without use of contrast material. SUPPLEMENTAL MATERIAL: http://radiology.rsnajnls.org/cgi/content/full/2452062067/DC1.

Dynamic imaging of allogeneic mesenchymal stem cells trafficking to myocardial infarction.

BACKGROUND: Recent results from animal studies suggest that stem cells may be able to home to sites of myocardial injury to assist in tissue regeneration. However, the histological interpretation of postmortem tissue, on which many of these studies are based, has recently been widely debated. METHODS AND RESULTS: With the use of the high sensitivity of a combined single-photon emission CT (SPECT)/CT scanner, the in vivo trafficking of allogeneic mesenchymal stem cells (MSCs) colabeled with a radiotracer and MR contrast agent to acute myocardial infarction was dynamically determined. Redistribution of the labeled MSCs after intravenous injection from initial localization in the lungs to nontarget organs such as the liver, kidney, and spleen was observed within 24 to 48 hours after injection. Focal and diffuse uptake of MSCs in the infarcted myocardium was already visible in SPECT/CT images in the first 24 hours after injection and persisted until 7 days after injection and was validated by tissue counts of radioactivity. In contrast, MRI was unable to demonstrate targeted cardiac localization of MSCs in part because of the lower sensitivity of MRI. CONCLUSIONS: Noninvasive radionuclide imaging is well suited to dynamically track the biodistribution and trafficking of mesenchymal stem cells to both target and nontarget organs.

Three-Dimensional CAIPIRINHA SPACE TSE for 5-Minute High-Resolution MRI of the Knee.

OBJECTIVE: The aim of this study was to prospectively test the hypothesis that a 2-dimensional (2D) CAIPIRINHA (controlled aliasing in parallel imaging results in higher acceleration) sampling pattern facilitates 5-minute high spatial resolution 3-dimensional (3D) sampling perfection with application optimized contrast using different flip angle evolutions (SPACE) magnetic resonance imaging (MRI) of the knee with image quality similar or better than current 2D turbo spin echo (TSE) and 3D SPACE standards. MATERIALS AND METHODS: The study was approved by our institutional review board. Twenty asymptomatic subjects (12 men, 8 women; mean age, 42 years; age range, 24-65 years) underwent 3 T MRI of the knee. A 4-fold accelerated 3D SPACE TSE prototype with 2D CAIPIRINHA sampling pattern and 5-minute acquisition time was compared with commercially available 2-fold and 4-fold accelerated 3D SPACE and 2D TSE pulse sequences with acquisition times of 11 minutes and 15 seconds, 6 minutes and 30 seconds, as well as 9 minutes and 48 seconds, respectively. Outcome variables included image quality, anatomic visibility, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR). Statistical analysis included Friedman, repeated measures analysis of variances, and Cohen's weighted κ tests. Bonferroni-corrected P values of 0.005 and less were considered statistically significant. RESULTS: Overall, image quality, visibility of anatomic structures, SNR, and CNR of 3D CAIPIRINHA SPACE were statistically similar to 2-fold accelerated 3D SPACE and significantly better than 4-fold accelerated 3D SPACE, which exhibited degrading parallel imaging artifacts. Compared with 2.5-mm 2D TSE images, 0.5-mm 3D CAIPIRINHA SPACE images showed statistically similar good edge sharpness and very good contrast resolution, and significantly less partial volume as well as absent chemical shift and pulsatile flow artifacts. Visibility of menisci, anterior cruciate ligament, posterior cruciate ligament, medial collateral ligament, and lateral collateral ligament was good to very good on 0.5-mm 3D CAIPIRINHA SPACE images as compared with good on 2.5-mm 2D TSE image (P < 0.005). The SNR of fat, fluid, and cartilage as well as CNR between cartilage, fluid, fat, posterior cruciate ligament, and menisci were minimally higher on 2.5-mm 2D TSE image (P < 0.005). Image quality, visibility of anatomic structures, SNR, and CNR of 2.5-mm 3D CAIPIRINHA SPACE and 2.5-mm 2D TSE images were good to very good without significant differences. CONCLUSIONS: Three-dimensional SPACE with 2D CAIPIRINHA sampling pattern enables high-quality 3D TSE MRI of the knee at an acquisition time of 5 minutes and image quality, visibility of anatomic structures, SNR, and CNR similar to conventional 3D SPACE and 2D TSE, both of which require approximately 10-minute acquisition times.

Compressed Sensing SEMAC: 8-fold Accelerated High Resolution Metal Artifact Reduction MRI of Cobalt-Chromium Knee Arthroplasty Implants.

OBJECTIVE: The aim of this study was to prospectively test the hypothesis that a compressed sensing-based slice encoding for metal artifact correction (SEMAC) turbo spin echo (TSE) pulse sequence prototype facilitates high-resolution metal artifact reduction magnetic resonance imaging (MRI) of cobalt-chromium knee arthroplasty implants within acquisition times of less than 5 minutes, thereby yielding better image quality than high-bandwidth (BW) TSE of similar length and similar image quality than lengthier SEMAC standard of reference pulse sequences. MATERIALS AND METHODS: This prospective study was approved by our institutional review board. Twenty asymptomatic subjects (12 men, 8 women; mean age, 56 years; age range, 44-82 years) with total knee arthroplasty implants underwent MRI of the knee using a commercially available, clinical 1.5 T MRI system. Two compressed sensing-accelerated SEMAC prototype pulse sequences with 8-fold undersampling and acquisition times of approximately 5 minutes each were compared with commercially available high-BW and SEMAC pulse sequences with acquisition times of approximately 5 minutes and 11 minutes, respectively. For each pulse sequence type, sagittal intermediate-weighted (TR, 3750-4120 milliseconds; TE, 26-28 milliseconds; voxel size, 0.5 × 0.5 × 3 mm) and short tau inversion recovery (TR, 4010 milliseconds; TE, 5.2-7.5 milliseconds; voxel size, 0.8 × 0.8 × 4 mm) were acquired. Outcome variables included image quality, display of the bone-implant interfaces and pertinent knee structures, artifact size, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR). Statistical analysis included Friedman, repeated measures analysis of variances, and Cohen weighted k tests. Bonferroni-corrected P values of 0.005 and less were considered statistically significant. RESULTS: Image quality, bone-implant interfaces, anatomic structures, artifact size, SNR, and CNR parameters were statistically similar between the compressed sensing-accelerated SEMAC prototype and SEMAC commercial pulse sequences. There was mild blur on images of both SEMAC sequences when compared with high-BW images (P < 0.001), which however did not impair the assessment of knee structures. Metal artifact reduction and visibility of central knee structures and bone-implant interfaces were good to very good and significantly better on both types of SEMAC than on high-BW images (P < 0.004). All 3 pulse sequences showed peripheral structures similarly well. The implant artifact size was 46% to 51% larger on high-BW images when compared with both types of SEMAC images (P < 0.0001). Signal-to-noise ratios and CNRs of fat tissue, tendon tissue, muscle tissue, and fluid were statistically similar on intermediate-weighted MR images of all 3 pulse sequence types. On short tau inversion recovery images, the SNRs of tendon tissue and the CNRs of fat and fluid, fluid and muscle, as well as fluid and tendon were significantly higher on SEMAC and compressed sensing SEMAC images (P < 0.005, respectively). CONCLUSIONS: We accept the hypothesis that prospective compressed sensing acceleration of SEMAC is feasible for high-quality metal artifact reduction MRI of cobalt-chromium knee arthroplasty implants in less than 5 minutes and yields better quality than high-BW TSE and similarly high quality than lengthier SEMAC pulse sequences.

Simultaneous evaluation of infarct size and cardiac function in intact mice by contrast-enhanced cardiac magnetic resonance imaging reveals contractile dysfunction in noninfarcted regions early after myocardial infarction.

BACKGROUND: The objective of this study was to noninvasively determine the effects of reperfused myocardial infarction (MI) on regional and global left-ventricular (LV) function 24 hours after MI in intact mice with contrast-enhanced cardiac MRI and a single, gradient-echo pulse sequence. METHODS AND RESULTS: Twenty-three mice received baseline MRI scans followed by either 60 minutes of coronary occlusion (MI group, n=15) or thoracotomy without occlusion (sham group, n=8). Gadolinium-DTPA-enhanced magnetic resonance (MR) images were acquired 24 hours after surgery. Hearts were then excised for conventional infarct size determination via 2,3,5-triphenyl tetrazolium chloride (TTC) staining. In addition to infarct size, analysis of the MR images yielded left ventricular (LV) mass, LV end-systolic volume (LVESV), LV end-diastolic volume (LVEDV), LV ejection fraction (LVEF), cardiac output, and percent LV wall thickening (%WTh). Twenty-four hours after surgery, infarct size was 28.1+/-1.8% of LV mass by MRI and 27.5+/-1.7% by TTC (P=NS). Bland-Altman analysis revealed close agreement between the results obtained by the 2 methods. MI had little effect on LVEDV but caused a 98% increase in LVESV (from 11.3 to 22.4 microL, P<0.05), which resulted in a significant reduction in LVEF (from 70% to 37%, P<0.05). Compared with LV regional function at baseline, %WTh 24 hours after MI was significantly depressed, not only in infarcted myocardium but also in regions remote from the infarct zone. In contrast, sham-operated mice showed a small but significant increase in %WTh 24 hours after surgery (P<0.05). CONCLUSIONS: MRI can accurately assess both infarct size and cardiac function in intact mice early after large, reperfused MI, revealing the existence of contractile dysfunction in noninfarcted regions of the heart.