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OBJECTIVE To assess whether cardiac MRI or various biomarkers can be used to detect myocardial ischemia and fibrosis in dogs with cardiomegaly secondary to myxomatous mitral valve disease (MMVD). ANIMALS 6 dogs with cardiomegaly secondary to naturally occurring stage B2 MMVD being treated only with pimobendan with or without enalapril and 6 control dogs with no cardiac disease. All dogs were ≥ 5 years old with no systemic illness. PROCEDURES Serum cardiac troponin I and concentrations were measured, and dogs were anesthetized for cardiac MRI with ECG-triggered acquisition of native T1- and T2-weighted images. Gadolinium contrast was administered to evaluate myocardial perfusion and late gadolinium enhancement (LGE). Mean T1 and T2 values and regions of LGE were measured with dedicated software. Extracellular volume (ECV) was estimated on the basis of Hct and T1 values of myocardium and surrounding blood. Subjective analysis for myocardial perfusion deficits was performed. RESULTS Dogs with MMVD had significantly (P = .013) higher cardiac troponin I concentrations than control dogs, but galectin-3 concentrations did not differ (P = .08) between groups. Myocardial fibrosis was detected in 4 dogs with MMVD and 3 control dogs; no dogs had obvious myocardial perfusion deficits. Native T1 and T2 values, postcontrast T1 values, and ECV values were not significantly different between groups (all P > .3). CLINICAL RELEVANCE Results suggest that some dogs with cardiomegaly secondary to MMVD may not have clinically relevant myocardial fibrosis.

Computed tomography (CT) provides information for diagnosis, PET attenuation correction (AC), and radiation treatment planning (RTP). Disadvantages of CT include poor soft tissue contrast and exposure to ionizing radiation. While MRI can overcome these disadvantages, it lacks the photon absorption information needed for PET AC and RTP. Thus, an intelligent transformation from MR to CT, i.e., the MR-based synthetic CT generation, is of great interest as it would support PET/MR AC and MR-only RTP. Using an MR pulse sequence that combines ultra-short echo time (UTE) and modified Dixon (mDixon), we propose a novel method for synthetic CT generation jointly leveraging prior knowledge as well as partial supervision (SCT-PK-PS for short) on large-field-of-view images that span abdomen and pelvis. Two key machine learning techniques, i.e., the knowledge-leveraged transfer fuzzy c-means (KL-TFCM) and the Laplacian support vector machine (LapSVM), are used in SCT-PK-PS. The significance of our effort is threefold: 1) Using the prior knowledge-referenced KL-TFCM clustering, SCT-PK-PS is able to group the feature data of MR images into five initial clusters of fat, soft tissue, air, bone, and bone marrow. Via these initial partitions, clusters needing to be refined are observed and for each of them a few additionally labeled examples are given as the partial supervision for the subsequent semi-supervised classification using LapSVM; 2) Partial supervision is usually insufficient for conventional algorithms to learn the insightful classifier. Instead, exploiting not only the given supervision but also the manifold structure embedded primarily in numerous unlabeled data, LapSVM is capable of training multiple desired tissue-recognizers; 3) Benefiting from the joint use of KL-TFCM and LapSVM, and assisted by the edge detector filter based feature extraction, the proposed SCT-PK-PS method features good recognition accuracy of tissue types, which ultimately facilitates the good transformation from MR images to CT images of the abdomen-pelvis. Applying the method on twenty subjects' feature data of UTE-mDixon MR images, the average score of the mean absolute prediction deviation (MAPD) of all subjects is 140.72 ± 30.60 HU which is statistically significantly better than the 241.36 ± 21.79 HU obtained using the all-water method, the 262.77 ± 42.22 HU obtained using the four-cluster-partitioning (FCP, i.e., external-air, internal-air, fat, and soft tissue) method, and the 197.05 ± 76.53 HU obtained via the conventional SVM method. These results demonstrate the effectiveness of our method for the intelligent transformation from MR to CT on the body section of abdomen-pelvis.

PURPOSE: Accurate photon attenuation assessment from MR data remains an unmet challenge in the thorax due to tissue heterogeneity and the difficulty of MR lung imaging. As thoracic tissues encompass the whole physiologic range of photon absorption, large errors can occur when using, for example, a uniform, water‐equivalent or a soft‐tissue‐only approximation. The purpose of this study was to introduce a method for voxel‐wise thoracic synthetic CT (sCT) generation from MR data attenuation correction (AC) for PET/MR or for MR‐only radiation treatment planning (RTP). METHODS: Acquisition: A radial stack‐of‐stars combining ultra‐short‐echo time (UTE) and modified Dixon (mDixon) sequence was optimized for thoracic imaging. The UTE‐mDixon pulse sequence collects MR signals at three TE times denoted as UTE, Echo1, and Echo2. Three‐point mDixon processing was used to reconstruct water and fat images. Bias field correction was applied in order to avoid artifacts caused by inhomogeneity of the MR magnetic field. Analysis: Water fraction and R2* maps were estimated using the UTE‐mDixon data to produce a total of seven MR features, that is UTE, Echo1, Echo2, Dixon water, Dixon fat, Water fraction, and R2*. A feature selection process was performed to determine the optimal feature combination for the proposed automatic, 6‐tissue classification for sCT generation. Fuzzy c‐means was used for the automatic classification which was followed by voxel‐wise attenuation coefficient assignment as a weighted sum of those of the component tissues. Performance evaluation: MR data collected using the proposed pulse sequence were compared to those using a traditional two‐point Dixon approach. Image quality measures, including image resolution and uniformity, were evaluated using an MR ACR phantom. Data collected from 25 normal volunteers were used to evaluate the accuracy of the proposed method compared to the template‐based approach. Notably, the template approach is applicable here, that is normal volunteers, but may not be robust enough for patients with pathologies. RESULTS: The free breathing UTE‐mDixon pulse sequence yielded images with quality comparable to those using the traditional breath holding mDixon sequence. Furthermore, by capturing the signal before T2* decay, the UTE‐mDixon image provided lung and bone information which the mDixon image did not. The combination of Dixon water, Dixon fat, and the Water fraction was the most robust for tissue clustering and supported the classification of six tissues, that is, air, lung, fat, soft tissue, low‐density bone, and dense bone, used to generate the sCT. The thoracic sCT had a mean absolute difference from the template‐based (reference) CT of less than 50 HU and which was better agreement with the reference CT than the results produced using the traditional Dixon‐based data. CONCLUSION: MR thoracic acquisition and analyses have been established to automatically provide six distinguishable tissue types to generate sCT for MR‐based AC of PET/MR and for MR‐only RTP.

The brain is an organ of interest in high-consequence infectious disease. Subtle blood-brain-barrier disruption and edema can be identified and quantified using T2 relaxation time as a biomarker. Accuracy is required to correlate changes in imaging findings with biological processes and therapeutic effectiveness. Dual echo time Fluid-Attenuated Inversion Recovery images, which minimizes the partial volume effects from cerebrospinal fluid, can be used to quantify T2 relaxation. This sequence also meets the acquisition time requirements of biosafety level-4 facilities, where cohorts of animals are imaged on a tight schedule until terminal stages, when the animals may not survive long scans. However, the echo time used to compute T2 varies among slices due to k-space mapping in fast spin-echo sequence. Therefore, an effective echo time instead of the prescribed echo time should be considered. We hypothesize that accurate effective echo time can be estimated by optimization for each slice ordering. A longer and more accurate T2-weighted spin-echo (multiecho) sequence to compute T2 maps from mono-exponential fitting was validated using gel phantoms and used as ground truth. A brain of a non-human primate was imaged in vivo using both T2 sequences. T2 maps were generated using both methods and a correction factor was computed by linear fitting for each set of echo times and slice ordering. The coefficient of variation within phantoms and the Pearson’s correlation coefficient between in vivo ground truth T2 and each map were used to assess its accuracy and quality.

The aim of this study was to demonstrate the feasibility of in vivo perforator visualization by a newly proposed magnetic resonance-based perforator phase contrast angiography (pPCA) technique for deep inferior epigastric perforator (DIEP) flap surgery and to prospectively compare its image quality and clinical value with computed tomographic angiography (CTA), the state-of-the-art perforator imaging technique.Institutional review board approval and informed consent were obtained. DIEP pPCA and CTA data were acquired in 10 female patients before DIEP flap surgery. Image findings were compared between the two techniques and with literature reports.The overall image quality is negatively correlated with patient BMI for CTA, but positively correlated with BMI for pPCA. Compared with CTA, pPCA has significantly better image quality (P = 0.005), signal-to-noise ratio (P0.001), and contrast-to-noise ratio (perforator-to-muscle, P0.001; perforator-to-fat, P = 0.014). It also has preferable clinical value ratings, although not statistically significant (P = 0.388). There is a good agreement (84%) between perforators detected by pPCA and CTA. Perforator location deviations between pPCA and CTA are compatible with the precision required for plastic surgery. Perforator size measured by pPCA seems to be more accurate than CTA, as it is 0.8 ± 0.3 mm smaller (P0.001), consistent with the reported 0.5 to 1.2 mm overestimation by CTA. There is no significant difference in perforator intramuscular course assessment (P = 0.415).The developed magnetic resonance-based pPCA technique presents superior image quality, better vessel contrast, and more accurate perforator anatomy than the x-ray-based CTA. pPCA has the potential to emerge as the preferred preoperative planning tool for perforator flap reconstructive surgery.

We propose a new method for generating synthetic CT images from modified Dixon (mDixon) MR data. The synthetic CT is used for attenuation correction (AC) when reconstructing PET data on abdomen and pelvis. While MR does not intrinsically contain any information about photon attenuation, AC is needed in PET/MR systems in order to be quantitatively accurate and to meet qualification standards required for use in many multi-center trials. Existing MR-based synthetic CT generation methods either use advanced MR sequences that have long acquisition time and limited clinical availability or use matching of the MR images from a newly scanned subject to images in a library of MR-CT pairs which has difficulty in accounting for the diversity of human anatomy especially in patients that have pathologies. To address these deficiencies, we present a five-phase interlinked method that uses mDixon MR acquisition and advanced machine learning methods for synthetic CT generation. Both transfer fuzzy clustering and active learning-based classification (TFC-ALC) are used. The significance of our efforts is fourfold: 1) TFC-ALC is capable of better synthetic CT generation than methods currently in use on the challenging abdomen using only common Dixon-based scanning. 2) TFC partitions MR voxels initially into the four groups regarding fat, bone, air, and soft tissue via transfer learning; ALC can learn insightful classifiers, using as few but informative labeled examples as possible to precisely distinguish bone, air, and soft tissue. Combining them, the TFC-ALC method successfully overcomes the inherent imperfection and potential uncertainty regarding the co-registration between CT and MR images. 3) Compared with existing methods, TFC-ALC features not only preferable synthetic CT generation but also improved parameter robustness, which facilitates its clinical practicability. Applying the proposed approach on mDixon-MR data from ten subjects, the average score of the mean absolute prediction deviation (MAPD) was 89.78±8.76 which is significantly better than the 133.17±9.67 obtained using the all-water (AW) method (p=4.11E-9) and the 104.97±10.03 obtained using the four-cluster-partitioning (FCP, i.e., external-air, internal-air, fat, and soft tissue) method (p=0.002). 4) Experiments in the PET SUV errors of these approaches show that TFC-ALC achieves the highest SUV accuracy and can generally reduce the SUV errors to 5% or less. These experimental results distinctively demonstrate the effectiveness of our proposed TFC-ALC method for the synthetic CT generation on abdomen and pelvis using only the commonly-available Dixon pulse sequence.

Background: To evaluate whether parallel radiofrequency transmission (mTX) can improve the symmetry of the left and right femoral arteries in dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) of prostate and bladder cancer. Methods: Eighteen prostate and 24 bladder cancer patients underwent 3.0 Tesla DCE-MRI scan with a single transmission channel coil. Subsequently, 21 prostate and 21 bladder cancer patients were scanned using the dual channel mTX upgrade. The precontrast signal (S0) and the maximum enhancement ratio (MER) were measured in both the left and the right femoral arteries. Within the patient cohort, the ratio of S0 and MER in the left artery to that in the right artery (S0 LR, MER LR) was calculated with and without the use of mTX. Left to right asymmetry indices for S0 ( S0 LRasym) and MER (MER LRasym) were defined as the absolute values of the difference between S0 LR and 1, and the difference between MER LR and 1, respectively. Results: S0 LRasym, and MER LRasym were 0.21 and 0.19 for prostate cancer patients with mTX, and 0.43 and 0.45 for the ones imaged without it (P < 0.001). Also, for the bladder cancer patients, S0 LRasym, and MER LRasym were 0.11 and 0.9 with mTX, while imaging without it yielded 0.52 and 0.39 (P < 0.001). Conclusion: mTX can significantly improve left-to-right symmetry of femoral artery precontrast signal and contrast enhancement.

Prostate MRI is useful in the detection and staging of prostate cancer. Prostate cancer demonstrates decreased signal on T2*-weighted (T2W) images. However, post-biopsy hemorrhage, prostatitis, granulomatous disease, and hyperplastic nodules can all show similar decreased signal on T2W images (1). The most commonly encountered prostate cancer mimickers are hemorrhage and inflammation. Current clinical prostate MRI uses a multi-parametric imaging approach to increase the sensitivity and specificity of diagnosis, including T1W-weighted turbo-spin echo sequence (TSE) and T2W TSE, diffusion weighted imaging (DWI), dynamic contrast enhancement (DCE) using gadolinium, and MR spectroscopic imaging (MRS). High-grade prostate cancer demonstrates decreased signal on T2W TSE images, diffusion restriction, early/intense contrast enhancement with early contrast washout, and increased choline/decreased citrate on MRS (2–4). Two general categories of field susceptibility variations can be identified in prostate MRI, those related to hemorrhage or nonhemorrhagic. Post-biopsy hemorrhage is commonly encountered in prostate MRI. The test of choice to identify hemorrhage in clinical practice is T1W TSE imaging with hemorrhage showing increased signal on T1W TSE images (5–7). The appearance of hemorrhage is very dependent on the time from biopsy, and hemorrhage is generally not visualized after 8 weeks after biopsy. The true sensitivity of 3 Tesla (T) MRI in hemorrhage detection is unknown. A sensitive technique is needed to identify hemorrhage; this can be accomplished by means of B0 susceptibility mapping. In addition to identifying regions of hemorrhage, local B0 mapping technique may also identify other regions of susceptibility artifact that can influence the DWI and MRS sequences. MRS has potential to improve the specificity of prostate MRI as shown at 1.5T (8) and in a small series showed promise at 3T (9,10). The increased field strength at 3T should improve the signal to noise ratio (SNR) and increase chemical shift between the metabolite peaks, allowing for improved spectral differentiation capability. Despite the theoretical advantages at 3T, there are many additional difficulties in performing MRS at 3T. Particularly, local field inhomogeneity can impede data quality and reproducibility of spectroscopy (11). Moreover, the susceptibility gradient between prostate tissue and hemorrhage is known to have deleterious effects on MRS of the prostate at 1.5T (12). Accurate mapping of these areas of local inhomogeneity may allow for better planning of MRS. We herein propose a fast, high-resolution T2*W echo planar imaging (EPII sequence for highly sensitive local B0 susceptibility mapping of prostate at 3T. Preliminary results of the contrast difference of this sequence in post-biopsy hemorrhage are compared with traditional T1W TSE sequences. Feasibility of the T2*W maps to identify other spectrally detrimental B0 field inhomogeneities is also demonstrated.

This article reviews fundamental principles and sequence techniques that have been used successfully for imaging diseases of the abdomen and pelvis at 1.5 Tesla. This article also introduces concepts and the specific alteration of sequence parameters for optimization of abdominal-pelvic imaging at 3 Tesla.

To evaluate whether parallel radiofrequency transmission (mTX) can improve the symmetry of the left and right femoral arteries in dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) of prostate and bladder cancer.Eighteen prostate and 24 bladder cancer patients underwent 3.0 Tesla DCE-MRI scan with a single transmission channel coil. Subsequently, 21 prostate and 21 bladder cancer patients were scanned using the dual channel mTX upgrade. The precontrast signal ( S0) and the maximum enhancement ratio (MER) were measured in both the left and the right femoral arteries. Within the patient cohort, the ratio of S0 and MER in the left artery to that in the right artery ( S0_LR, MER_LR) was calculated with and without the use of mTX. Left to right asymmetry indices for S0 ( S0_LRasym) and MER ( MER_LRasym) were defined as the absolute values of the difference between S0_LR and 1, and the difference between MER_LR and 1, respectively.S0_LRasym, and MER_LRasym were 0.21 and 0.19 for prostate cancer patients with mTX, and 0.43 and 0.45 for the ones imaged without it (P0.001). Also, for the bladder cancer patients, S0_LRasym, and MER_LRasym were 0.11 and 0.9 with mTX, while imaging without it yielded 0.52 and 0.39 (P0.001).mTX can significantly improve left-to-right symmetry of femoral artery precontrast signal and contrast enhancement.

Cerebral cortical blood flow (lCoBF) and metabolic rate for oxygen (lCoMRO2) were studied in eight patients undergoing intracranial aneurysm clipping. The patients were anesthetized with fentanyl 10 micrograms/kg and 70% nitrous oxide combined with 30% oxygen. Hypotension was induced with isoflurane. A thermal diffusion probe was used to measure lCoBF, and arterial and cerebral venous blood samples were obtained for measurement of arterio-cerebral venous O2 content difference. Measurements were made prior to hypotension, during hypotension (to mean arterial pressure approximately 50 mmHg), and posthypotension. Mean lCoBF decreased from 69 +/- 20 mL/100 g/min at normotension to 59 +/- 13 mL/100 g/min during hypotension (P less than .03, NS) and was 61 +/- 18 mL/100 g/min upon return to normotension (all values mean +/- 1 SD). The lCoMRO2 averaged 3.9 +/- 1.6 mL/100 g/min and 3.1 +/- 1.5 mL/100 g/min, respectively (P less than .03, NS) for normotension upsilon hypotension. Values for cerebral venous PO2 and O2 saturation also did not differ significantly between study periods. These results indicate that isoflurane-induced hypotension during fentanyl-nitrous oxide anesthesia allows maintenance of a constant lCoBF and oxygen delivery.

The spatial correlation of nuclear magnetic resonance imaging (NMRI) and cerebral blood flow (CBF) may improve our ability to identify ischemic brain lesions and may provide further insight into the pathophysiology of early cerebral ischemia. Eleven pentobarbital-anesthetized adult cats underwent exposure of the common carotid arteries bilaterally and the right middle cerebral artery through a transorbital approach. Baseline NMRI images were obtained with a single spin-echo, multislice technique using a 0.6-T field, 0.4-cm slice thickness, and a surface coil. Focal ischemia was produced with right middle cerebral artery occlusion and potentiated with bilateral common carotid artery ligation. Sequential NMRI studies were then performed at 1, 2, 4, 6, and 12 hours or until CBF was determined in the same cats using [14C]iodoantipyrine at either 2 (n = 2), 4 (n = 2), 6 (n = 2), or 12 (n = 1) hours after the time of occlusion. This protocol allowed temporal and spatial correlation of NMRI and CBF. Alternate 5-mm brain slices were incubated with 1% 2,3,5-triphenyltetrazolium chloride (TTC) for 45 minutes at 37-41 degrees C and frozen in liquid Freon for later autoradiographic CBF determination. Four cats were studied only with NMRI and TTC (not CBF). The correlation between areas of increased NMRI signal intensity observed in T2-weighted images (repetition time 2,000 msec, echo time 120 msec), vital staining with TTC, low CBF, and routine histology was evaluated. During the early phase (less than 6 hours), T2-weighted NMRI changes were localized to the central ischemic gray matter areas, as defined in the later CBF images, with no involvement of the white matter. By the twelfth hour the NMRI changes involved the entire ischemic area including gray and white matter. The initial visible changes seen on T2-weighted NMRI are suggestive of cellular edema, and the later changes are characteristic of vasogenic edema. The spread of NMRI changes compared with the ischemic area determined from autoradiographic CBF is consistent with the previously described biphasic evolution of ischemic injury. These data suggest that T2-weighted NMRI could be used clinically to delineate areas of acute ischemic stroke.

The circulatory changes in the cortex around a cerebral arteriovenous malformation (AVM) were studied in 18 patients. The AVMs had rapid circulation times with early draining veins on angiography. Local cortical blood flow (ICoBF) was measured with cortically applied thermister/Peltier stack arrays. The AVMs had a more pronounced effect on ICoBF at a 2- to 4-cm distance from the AVM margin than in the adjacent cortex. Mean preexcision ICoBF was 62.9 ± 6.7 (SE) ml/100 g/minute (i.e., similar to normal controls) near the AVM margin and 43.0 ± 4.2 ml/100 g/minute far (i.e., >2 cm) from the AVM. CO2 reactivity (COR) before excision was 1.1 ± 0.3 ml/100 g/minute/torr of CO2 (i.e., similar to normal controls) at near sites and 0.6 ± 0.3 ml/100 g/minute/torr of CO2 at far sites. The mean postexcision near ICoBF remained stable at 55.8 ± 5.1 ml/100 g/minute at near sites, but the far ICoBF significantly increased (P < 0.05) to 57.2 ± 6.8 ml/100 g/minute. The cortical feeding artery pressure was substantially below the normal cortical artery pressure in 50% of the cases studied. Pressure in these arteries normalized after occlusion and AVM excision, resulting in a rapid increase in cortical artery perfusion pressure. Draining red vein pressure, which was elevated before AVM excision, also dropped after excision, contributing to the increase in perfusion pressure. Two patients who developed the normal perfusion pressure breakthrough syndrome (PBS) after operation had low ICoBF and disturbed COR before AVM excision and marked increase of ICoBF after excision. Factors that contribute to the development of PBS include: (a) low ICoBF around an AVM; (b) impaired COR; (c) low cortical artery pressure before AVM excision; (d) normalization in cortical artery pressure after AVM removal; and (e) substantial increase in ICoBF after AVM removal. Maintenance of a low systemic arterial blood pressure during the early postoperative period is essential in reducing the risk of severe cerebral edema.