Anil N. Shetty

Penetrating atherosclerotic ulcers of the aorta

PURPOSE This study investigates the natural history and optimal imaging modality of penetrating atherosclerotic ulcers of the aorta. METHODS We reviewed our experience with 29 penetrating ulcers in 18 patients. Computed tomography (17 patients), magnetic resonance imaging (nine patients), and aortography (five patients) were used for diagnosis and follow-up. Patients were typically elderly (average age 74 years) and had hypertension and coronary artery disease. Ulcers were most common in the distal descending thoracic aorta (31%) and were characterized by a discrete ulcer crater (100%) and thickened aortic wall (89%). Modes of presentation included chest or back pain in four patients, distal embolization in two patients, and abnormal chest radiography results in one; the remaining were incidental findings. RESULTS Follow-up was available in ten patients with 17 ulcers from 1 to 7 years. Recurrent pain occurred in two patients, recurrent embolization occurred in one patient, and seven patients remained symptom free. Progression to saccular pseudoaneurysm occurred in five ulcers, and fusiform aneurysm occurred in two ulcers. Two ulcers were associated with an increase in aortic diameter, and nine ulcers did not change. There were no cases of aortic dissection or rupture in the follow-up period. There were no deaths and only one patient underwent resection. CONCLUSION The natural history of penetrating atherosclerotic ulcers is one of progressive aortic enlargement, with saccular and fusiform aneurysms the result if follow-up is sufficient. Aortic dissection, aortic rupture, and embolization can also occur but are less common. Contrast-enhanced computed tomography is the primary imaging modality.

High resolution gel-dosimetry by optical-CT and MR scanning.

The increased intricacy of Intensity-Modulated-Radiation-Therapy (IMRT) delivery has created the need for a high-resolution 3D-dosimetry (three-dimensional) system capable of measuring and verifying the complex delivery. Present clinical methods are inadequate being restricted to single points (e.g., ion-chambers) or to 2D planes (e.g., film), and are labor intensive. In this paper we show that gel-dosimetry in conjunction with optical-CT scanning can yield maps of dose that are of sufficient accuracy, resolution and precision to allow verification of complex radiosurgery deliveries, and by extension IMRT deliveries. The radiosurgery dose-distribution represents the most challenging case encountered in external beam therapy by virtue of the steep dose-gradients and high resolution of delivery. We characterize the stringent radiosurgery requirements by the RTAP (Resolution-Time-Accuracy-Precision) criteria defined as < or = 1 mm3 spatial resolution, < or = 1 hour imaging time, accurate to within 3%, and within -1% precision. The RTAP criteria is applied to an in-house laser-based optical-CT scanning system presented here, and evaluated using gel-flasks containing BANG3 gel. The same gel flasks were subsequently imaged using the MR imaging protocol recommended by the gel manufacturer, but modified to match as closely as possible the RTAP. The resulting dose-maps demonstrate the high precision (< 1.3% noise at high dose) achievable with optical CT scanning while preserving high spatial resolution (<1 mm3). Using the sequence above, the MR gel-dose maps were found to have poorer precision by a factor of 5, under the strict conditions of the RTAP. The optical CT gel-dosimetry system was further evaluated for the verification of a complex 3-isocenter radiosurgery delivery. In conclusion, this work demonstrates that gel-dosimetry and optical-CT scanning approach an important long-term goal of radiation dosimetry, as specified by the RTAP criteria, and have potential to impact the clinic by improving and facilitating clinical dose verification for the most complex external beam radiation treatments.

Contrast‐enhanced breath‐hold three‐dimensional magnetic resonance angiography in the evaluation of renal arteries: Optimization of technique and pitfalls

The authors describe the optimization of a contrast‐enhanced, breath‐held, three‐dimensional magnetic resonance angiography (CE‐BH‐3DMRA) technique in the assessment of the renal arteries and compare its utility with conventional x‐ray angiography (XRA). Signal optimization using specific pulse sequence parameters was based on the patients circulatory conditions, injection rate, and pulse sequence timing. Fifty‐one patients (27 M, 24 F; mean age 69.7 years) were evaluated with CE‐BH‐3DMRA and XRA. All patients had an MR angiogram 3 months either before or after XRA. A test bolus study was performed for accurate assessment of transit time in each patient. A total of 51 patients (115 vessels) were studied in which the sensitivity and specificity for all renal artery stenoses including the proximal and mid‐renal arterial segments were 96% and 92%, respectively. In‐stent stenosis could only be diagnosed by quantifying flow beyond the stent using an additional triggered phase contrast cine pulse sequence. A total of 11 accessory renal arteries were correctly identified. In addition, fibromuscular dysplasia in two patients and stents in three patients were correctly identified on MRA. J. Magn. Reson. Imaging 2000;12:912–923.

Differentiation between intra-axial metastatic tumor progression and radiation injury following fractionated radiation therapy or stereotactic radiosurgery using MR spectroscopy, perfusion MR imaging or volume progression modeling

OBJECTIVE To determine the accuracy of magnetic resonance spectroscopy (MRS), perfusion MR imaging (MRP), or volume modeling in distinguishing tumor progression from radiation injury following radiotherapy for brain metastasis. METHODS Twenty-six patients with 33 intra-axial metastatic lesions who underwent MRS (n=41) with or without MRP (n=32) after cranial irradiation were retrospectively studied. The final diagnosis was based on histopathology (n=4) or magnetic resonance imaging (MRI) follow-up with clinical correlation (n=29). Cho/Cr (choline/creatinine), Cho/NAA (choline/N-acetylaspartate), Cho/nCho (choline/contralateral normal brain choline) ratios were retrospectively calculated for the multi-voxel MRS. Relative cerebral blood volume (rCBV), relative peak height (rPH) and percentage of signal-intensity recovery (PSR) were also retrospectively derived for the MRPs. Tumor volumes were determined using manual segmentation method and analyzed using different volume progression modeling. Different ratios or models were tested and plotted on the receiver operating characteristic curve (ROC), with their performances quantified as area under the ROC curve (AUC). MRI follow-up time was calculated from the date of initial radiotherapy until the last MRI or the last MRI before surgical diagnosis. RESULTS Median MRI follow-up was 16 months (range: 2-33). Thirty percent of lesions (n=10) were determined to be radiation injury; 70% (n=23) were determined to be tumor progression. For the MRS, Cho/nCho had the best performance (AUC of 0.612), and Cho/nCho >1.2 had 33% sensitivity and 100% specificity in predicting tumor progression. For the MRP, rCBV had the best performance (AUC of 0.802), and rCBV >2 had 56% sensitivity and 100% specificity. The best volume model was percent increase (AUC of 0.891); 65% tumor volume increase had 100% sensitivity and 80% specificity. CONCLUSION Cho/nCho of MRS, rCBV of MRP, and percent increase of MRI volume modeling provide the best discrimination of intra-axial metastatic tumor progression from radiation injury for their respective modalities. Cho/nCho and rCBV appear to have high specificities but low sensitivities. In contrast, percent volume increase of 65% can be a highly sensitive and moderately specific predictor for tumor progression after radiotherapy. Future incorporation of 65% volume increase as a pretest selection criterion may compensate for the low sensitivities of MRS and MRP.

Diagnostic accuracy of ultrasonography and magnetic resonance imaging for the detection of fetal anomalies: a blinded case–control study

To compare the accuracy of two‐dimensional ultrasound (2D‐US), three‐dimensional ultrasound (3D‐US) and magnetic resonance imaging (MRI) for the diagnosis of congenital anomalies without prior knowledge of indications and previous imaging findings.

Contrast-enhanced three-dimensional MR angiography of the pulmonary vascular tree

Contrast-enhanced three-dimentional MR angiography has evolved into a promising technique in the study of the pulmonary vasculature. Both congenital and acquired entities can be now morphologically demonstrated in a non-invasive manner obviating the need for conventional pulmonary angiography. Due to spatial resolution limitations, however, it is still premature to routinely apply the method in the detection of small subsegmental emboli, in cases of suspected pulmonary embolism, and further technical developments will be required. In this paper we present a spectrum of congenital and acquired disorders affecting the pulmonary vascular tree as demonstrated with contrast-enhanced three-dimensional MR angiography.

A liposomal Gd contrast agent does not cross the mouse placental barrier

The trans-placental permeability of liposomal Gadolinium (Gd) nanoparticle contrast agents was evaluated in a pregnant mouse model. Pregnant Balb/c mice at 16.5 (±1) days of gestation were imaged using a 3D Spoiled Gradient Echo method at 9.4 T using two contrast agents: a clinically approved Gd chelate, Multihance® (gadobenate dimeglumine), and a novel experimental liposomal Gd agent. A Dynamic Contrast Enhancement (DCE) protocol was used to capture the dynamics of contrast entry and distribution in the placenta, and clearance from circulation. A blinded clinical radiologist evaluated both sets of images. A reference region model was used to measure the placental flow and physiological parameters; volume transfer constant (Ktrans), efflux rate constant (Kep). The Gd content of excised placentae and fetuses was measured, using inductively coupled plasma mass spectrometry (ICP-MS). MRI images of pregnant mice and ICP-MS analyses of placental and fetal tissue demonstrated undetectably low transplacental permeation of the liposomal Gd agent, while the clinical agent (Multihance) avidly permeated the placental barrier. Image interpretation and diagnostic quality was equivalent between the two contrast agents. Additional testing to determine both maternal and fetal safety of liposomal Gd is suggested.

3D breath-hold contrast-enhanced MRA: a preliminary experience in aorta and iliac vascular disease.

PURPOSE Our goal was to describe a 3D breath-hold (3D BH) contrast-enhanced MRA technique and apply the technique to patients with known or suspected aortic and iliac artery disease. METHOD A fat-suppressed 3D GRE pulse sequence was designed with a total of 16 partition encodings. This took < 24 s for data acquisition in the abdomen and pelvis and was easily achieved during a single breath-hold. The technique was applied to 26 patients who presented with either known or suspected abdominal aortic or iliac vascular diseases. For comparison, in 19 patients a 2D TOF MRA pulse sequence with a traveling saturation band was used. Angiographic correlation was made in 18 studies. RESULTS The 3D BH MRA was easily applicable in the evaluation of vascular anatomy and pathology. In three cases, it was superior to 2D TOF and conventional angiography for visualizing clot within the wall of an aneurysm in the abdominal aorta. In 20 cases, both MRA techniques overestimated the degree of stenosis in the lower peripheral vessels; however, this was more pronounced on 2D TOF. In five cases, the aneurysm wall was clearly defined by 3D BH MRA, whereas there was considerable signal loss in 2D TOF due to complex flow. With 3D BH MRA, the entire vessel territory both in abdominal aorta and in iliac vessels was visualized in all cases without signal falloff in the FOV. Breath-holding provided static images of the vessels that were free of blurring due to respiratory motion. CONCLUSION Preliminary experience suggests that 3D BH with its distinct advantage of speed may serve as a useful screening tool for patients who cannot have conventional angiography or tolerate a lengthy MR examination of the abdominal aorta and iliac arteries.

Non-invasive fetal lung assessment using diffusion-weighted imaging

The main goal was to develop a reproducible method for estimating the diffusion of water in human fetal lung tissue using diffusion‐weighted imaging (DWI). A secondary objective was to determine the relationship of the apparent diffusion coefficients (ADCs) in the fetal lung to menstrual age and total lung volume.

Catheter contrast-enhanced coronary CT-angiography using an aortic root injection- preliminary technical development in four patients

The feasibility of performing coronary computed tomographic angiography (CTA) with an intra-aortic injection of iodinated contrast was tested in four patients immediately following diagnostic cardiac catheterization and placement of a 6-french pigtail catheter into the aortic root. A diluted contrast mixture [75 cc Omnipaque 350 mixed with 125 cc normal saline (4-slice scanner) vs. 40 cc Omnipaque 350 mixed with 60 cc normal saline (16-slice scanner)] was injected at a rate of 6 to 7.7 cc/sec at the onset of breath-hold and helical (4 slice, n = 3, and 16-slice, n = 1) retrospective ECG-triggered acquisition. The proximal and distal coronary vasculature including small tributaries were depicted with high signal-to-noise (S/N) and free from underlying blood pool enhancement. The right coronary artery was sub optimally enhanced in two patients. No significant disease was found on CTA or XRA in two patients; however, severe multi-vessel disease was demonstrated on both modalities in two patients. Catheter contrast-enhanced coronary CTA is feasible utilizing low injection rates and low iodinated contrast volumes, however, catheter modifications are required for consistent and homogeneous opacification of the aortic root.