Origin of a signal intensity loss artifact in fat-saturation MR imaging.

Artifactual water signal intensity loss can be observed on fat-saturation magnetic resonance (MR) images of inhomogeneous regions such as the thorax. Magnetic effects of air inclusions on fat-saturation pulses were investigated as the possible origin of this artifact. Computer simulation results agreed well with observed production of water saturation by means of nominal fat suppression in MR imaging of phantoms and a representative clinical example.

Suppression of intravascular signal on fat-saturated contrast-enhanced thoracic MR arteriograms.

PURPOSE: To assess the prevalence of artifactual signal intensity loss within the aortic arch and proximal branch vessels on fat-saturated contrast material-enhanced magnetic resonance (MR) arteriograms of the thoracic aorta and to hypothesize about the cause of the loss of signal intensity. MATERIALS AND METHODS: Between January and June 1998, 105 consecutive MR arteriograms of the thoracic aorta were acquired in 103 patients at 1.5 T. Imaging included an arterial phase three-dimensional (3D) fat-saturated contrast-enhanced gradient-echo (GRE) sequence followed by a delayed two-dimensional (2D) transverse fat-saturated GRE sequence. All MR images were reviewed by two radiologists who were blinded to patient history and results of imaging studies and who evaluated the images for the presence of intraluminal loss of signal intensity in the aortic arch and the proximal branch vessels. RESULTS: Intravascular loss of signal intensity was present in at least one vessel on 23 of the 105 arterial phase 3D studies. Seventy-one of 91 left subclavian arterial segments had loss of signal intensity on the delayed 2D studies. CONCLUSION: Intravascular signal intensity loss can be present on contrast-enhanced fat-saturated images of the aortic arch and proximal branch vessels, particularly the left subclavian artery. This phenomenon, which is to the authors' knowledge previously unreported and which is hypothesized to result from undesired water saturation, should not be misinterpreted as stenotic or occlusive vascular disease.

17O-decoupled (1)H spectroscopy and imaging with a surface coil: STEAM decoupling.

(17)O-decoupled (1)H spin-echo imaging has been reported as a means of indirect (17)O detection, with potential application to measurement of blood flow and metabolism. In its current form, (17)O decoupling requires large RF amplitudes and a 180 degrees refocusing pulse, complicating its application in volume and surface coils, respectively. To overcome this problem, we have developed an (17)O-decoupled proton stimulated echo sequence ("STEAM decoupling") to allow (17)O detection with a surface coil. A high B(1) amplitude is easily generated, allowing complete decoupling of (17)O and (1)H. Slice-selective, (17)O-decoupled (1)H imaging is readily performed and the sequence is easily adapted for localized spectroscopy. Intrinsic correction for variations in B(1) and further compensation for B(1) inhomogeneity are discussed.

Subacute clot mimicking flow in a thrombosed arterial bypass graft on two-dimensional time-of-flight and three-dimensional contrast-enhanced MRA.

Subacute intravascular thrombus can contain methemoglobin, which results in very short spin-lattice (T1) relaxation times. We describe a case of a 78-year-old man with increasing right lower extremity claudication. The patient had a thrombosed arterial bypass graft showing high signal intensity that mimicked flow on both two-dimensional time-of-flight and three-dimensional contrast-enhanced MR angiography. Misinterpretation of the high signal thrombus as flowing blood can be avoided by obtaining a precontrast T1-weighted sequence.

Angiomyolipoma of the renal sinus: diagnosis by percutaneous biopsy.

We report a case of angiomyolipoma of the renal sinus discovered incidentally during an evaluation for microscopic hematuria. Diagnosis was confirmed by percutaneous aspiration biopsy performed with magnetic resonance imaging control allowing differentiation of this entity from other fatty tumors of the renal sinus including liposarcoma, lipoma, and sinus lipomatosis.

State-of-the-art magnetic resonance imaging of the kidneys and upper urinary tract.

State-of-the-art magnetic resonance imaging (MRI) is an excellent examination in several clinical contexts of interest to endourologists. First, it offers a one-stop imaging examination of prospective renal donors, obviating the need for arteriography and conventional excretory urography. Second, it reliably depicts urinary tract obstruction and can usually distinguish acute from chronic obstruction, although it is not as sensitive as helical CT in detecting small, nonobstructing stones. Third, it is an excellent examination for characterizing renal masses, especially complex cystic masses, and for surgical planning. Because MRI does not use ionizing radiation and because gadolinium contrast agents are essentially non-nephrotoxic, MRI is especially useful in children, women of childbearing age, and patients with renal insufficiency or renal allografts. Future developments will likely include MR "fluoroscopy," which will provide real-time imaging guidance for interventional procedures in the urinary tract.

Renal artery stenosis: evaluation with conventional angiography versus gadolinium-enhanced MR angiography.

PURPOSE: To evaluate the interobserver and intermodality variability of conventional angiography and gadolinium-enhanced magnetic resonance (MR) angiography in the assessment of renal artery stenosis. MATERIALS AND METHODS: Fifty-four patients underwent conventional angiography and gadolinium-enhanced three-dimensional gradient-echo MR angiography. Three angiographers blinded to each other's interpretations and the MR angiographic findings assessed the conventional angiograms for renal artery stenosis. Similarly, three blinded MR imagers evaluated the MR angiograms. RESULTS: Interobserver variability for the degree of renal artery stenosis in the 107 kidneys evaluated was not significantly different between the two modalities. The mean SD of the degree of stenosis was 6.9% at MR angiography versus 7.5% at conventional angiography (alpha < or = .05, P > .05). In 70 kidneys (65%), the average degree of stenosis reported by the readers for the two modalities differed by 10% or less. In 22 cases (21%), the degree of stenosis was overestimated with MR angiography by more than 10% relative to the results of conventional angiography. In 15 cases (14%), the degree of stenosis was underestimated with MR angiography by more than 10%. CONCLUSION: Gadolinium-enhanced MR angiography permits evaluation of renal artery stenosis with an interobserver variability comparable with that of conventional angiography.

Proton T1rho-dispersion imaging of rodent brain at 1.9 T.

Detection of H2(17)O with proton T1rho-dispersion imaging holds promise as a means of quantifying metabolism and blood flow with MRI. However, this technique requires a priori knowledge of the intrinsic T1rho dispersion of tissue. To investigate these properties, we implemented a T1rho imaging sequence on a 1.9-T Signa GE scanner. A series of T1rho images for different locking frequencies and locking durations were obtained from rat brain in vivo and compared with 5% (wt/vol) gelatin phantoms containing different concentrations of (17)O ranging from .037% (natural abundance) to 2.0 atom%. Results revealed that, although there is considerable T1rho-dispersion in phantoms doped with H2(17)O, the T1rho of rat brain undergoes minimal dispersion for spin-locking frequencies between .2 and 1.5 kHz. A small degree of T1rho dispersion is present below .2 kHz, which we postulate arises from natural-abundance H2(17)O. Moreover, the signal-to-noise ratios of T1rho-weighted images are significantly better than comparable T2-weighted images, allowing for improved visualization of tissue contrast. We have also demonstrated the feasibility of proton T1rho-dispersion imaging for detecting intravenous H2(17)O on a live mouse brain. The potential application of this technique to study brain perfusion is discussed.

Off-resonance proton T1rho dispersion imaging of 17O-enriched tissue phantoms.

Proton T1rho dispersion imaging is a recently described method for indirect detection of 17O. However, clinical implementation of this technique is hindered by the requirement for a high-amplitude spin-locking field (gammaB1 > 1 kHz) that exceeds current limitations in specific absorption rate (SAR). Here, a strategy is offered for circumventing high SAR in T1rho dispersion imaging of 17O through the use of low-amplitude off-resonance spin-locking pulses (gammaB1 < 300 Hz). Proton spin-lattice relaxation times in the off-resonance rotating frame were measured in H2(17)O-enriched tissue phantoms. On- and off-resonance T1rho dispersion imaging was implemented at 2 T using a spin-locking preparatory pulse cluster appended to a standard spin-echo sequence. On- and off-resonance dispersion images exhibited similar 17O-based image contrast. Magnetization transfer effects did not depend on 17O concentration and had no effect on image contrast. In conclusion, off-resonance proton T1rho dispersion imaging shows promise as a safe, sensitive technique for generating 17O-based T1rho contrast without exceeding SAR limitations.

Extracranial atherosclerotic carotid artery disease: evaluation of non-breath-hold three-dimensional gadolinium-enhanced MR angiography.

OBJECTIVE: The purpose of this study was to compare the diagnostic information provided by a combination of two-dimensional and three-dimensional (3D) time-of-flight (TOF) techniques with that provided by non-breath-hold 3D spoiled gradient-echo gadolinium-enhanced MR angiography. MATERIALS AND METHODS: Fifty patients suspected of having extracranial atherosclerotic carotid artery disease were examined with all three imaging techniques using a 1.5-T MR imaging system. Three observers independently and retrospectively measured the degree of stenosis according to the North American Symptomatic Carotid Endarterectomy trial criteria. The observers were unaware of the results of other MR imaging pulse sequences and digital subtraction angiography. The standard of reference was established by digital subtraction angiography. Results were evaluated with receiver operating characteristic curve analysis. The degree of interobserver agreement was determined using pairwise kappa statistics. RESULTS: The grading of carotid artery stenosis as measured by the area under the receiver operating characteristic curve was less accurate with non-breath-hold 3D gadolinium-enhanced MR angiography than with TOF imaging. Interobserver variability was greater for non-breath-hold 3D gadolinium-enhanced MR angiography than for TOF techniques. CONCLUSION: Routine evaluation of carotid artery stenosis at the level of the bifurcation using non-breath-hold 3D gadolinium-enhanced MR angiography is less accurate than is TOF imaging and is therefore not recommended. The weakness of this technique may be due to problems in timing the injection of gadolinium and the masking of the carotid bifurcation by the venous jugular system.

17O-decoupled proton MR spectroscopy and imaging in a tissue model.

17O-decoupled proton MR spectroscopy and imaging were implemented at 2 T. Their sensitivity and accuracy in vitro were examined using semisolid tissue phantoms doped with H2(17)O. A double-tuned solenoidal coil was used to irradiate the same volume of 17O and 1H nuclei, as well as to facilitate direct calibration of the decoupling power. Decoupling efficiency was optimized as was 17O detection sensitivity. Decoupling was most efficient at RF amplitudes below 2.5 kHz (expressed as gamma [17O] x H1), which is within the limits of the acceptable specific absorption rate. Propagation of error analysis demonstrated that 17O detection sensitivity is optimal at a TE equal to the T2 of 17O-depleted water protons. Based on Meiboom's work, a simple theory was formulated for estimating the transverse relaxivity of H2(17)O and the proton signal enhancement produced by decoupling. There was excellent agreement between theory and experiment. Overall, 17O-decoupled spectroscopy and imaging were highly sensitive and accurate in quantifying H2(17)O in vitro.

Preoperative MR imaging in hyperparathyroidism: results and factors affecting parathyroid detection.

OBJECTIVE: To determine the sensitivity of MR imaging for the detection of abnormal parathyroid glands in patients with biochemical evidence of hyperparathyroidism and to identify the factors affecting detection. SUBJECTS AND METHODS: Between 1985, 82 patients with biochemical proof of hyperparathyroidism were referred for MR imaging of the parathyroid glands prior to surgery. Axial T1- (600/20 [TR/TE]) and T2-weighted (2500/40, 80) spin-echo images were obtained using an anterior neck surface coil. The interpretation of the MR image was compared with the findings at surgery and also correlated with gland histology, volume, and weight. Cases in which a false-positive or false-negative diagnosis was made were reviewed to determine the factors affecting detection. RESULTS: MR imaging detected 71 of 92 (77%) surgically proven abnormal glands (sensitivity, 77%; 95% confidence interval (CI), 68-86%) and misdiagnosed five (1.6%) of 314 normal glands as abnormal. There was no difference in the detection of enlarged glands in patients presenting for the first time (n = 71) (sensitivity, 77%; 95% CI, 66-86%) compared with patients with recurrent hyperparathyroidism (n = 12) (sensitivity, 77%; 95% CI, 46-95%). There was no significant difference in the detection of adenomas (sensitivity, 77%; 95% CI, 65-86%) compared with hyperplasia (sensitivity, 71%; 95% CI, 42-92%). Of five patients with ectopic parathyroid glands (1.6%), four had had previous surgery. All five glands were successfully located (three mediastinal, two in the neck). Factors contributing to a false-negative MR imaging diagnosis included small gland size and thyroid disease. Four of five false-positive diagnoses were due to enlarged lymph nodes being mistaken for parathyroid glands. CONCLUSIONS: MR imaging is an accurate technique for investigation of hyperparathyroidism. Pitfalls include low sensitivity for the identification of small glands, misinterpretation of enlarged lymph nodes as parathyroid adenomas, and misinterpretation because of thyroid disease. MR imaging is particularly useful in the investigation of patients who remain hypercalcemic following initial surgery.

Fatty tissue on opposed-phase MR images: paradoxical suppression of signal intensity by paramagnetic contrast agents.

PURPOSE: To evaluate paradoxically decreased signal intensity on gadolinium-enhanced opposed-phase magnetic resonance (MR) images of fatty tissues. MATERIALS AND METHODS: Unenhanced and gadolinium-enhanced axial, opposed-phase, gradient-echo images were analyzed visually and with region-of-interest measurements. Tissues measured included adipose tissue (n = 10), angiomyolipomas (n = 8), and vertebral hemangiomas (n = 7). Additionally, a phantom of mayonnaise, soybean oil, agarose, and water (63% lipid signal) with variable concentrations of gadolinium chelate was imaged with similar technique. RESULTS: After administration of gadolinium chelate, signal intensity reduction averaged 18% for adipose tissue, 34% (72-48 units) for predominately fatty angiomyolipomas, and 39% (85-52 units) for vertebral hemangiomas. Imaging of the phantom showed a maximum of 79% reduction in signal intensity with gadolinium chelate (227-47 units). DISCUSSION: Gadolinium-enhanced opposed-phase images depict a significant loss in signal intensity in tissues with MR signal predominately from lipid. Gadolinium chelate increases the signal of water within fatty tissues, which increases the amount of lipid signal suppression due to destructive interference between water and lipid proton magnetizations.

17O-decoupled 1H detection using a double-tuned coil.

17O-decoupled proton MR spectroscopy imaging with a double-tuned radiofrequency (RF) coil at 2 T was used to detect and quantify H2 17O in tissue containing various concentrations of 17O-enriched water in 5% gelatin. The pulse sequence used in these experiments consisted of a conventional proton spin-echo sequence with RF irradiation at the 17O resonance frequency applied between the proton 90 degrees pulse and the signal acquisition window. The double-tuned coil provided several advantages over systems using separate RF coils for 17O decoupling and proton excitation/detection, including ensuring that the same (or similar) sample volumes are excited and decoupled and permitting accurate calibration of the 17O decoupling pulse amplitude. The efficiency of 17O decoupling as a function of decoupling RF amplitude, decoupling duration, and decoupling resonance offset was investigated. Finally, the specific absorption rate of the 17O decoupled pulse sequence was investigated and found to lie within federal guidelines at 1.5 T.

Magnetic resonance angiography of the distal lower extremity.

OBJECTIVE: To assess magnetic resonance angiography (MRA) for demonstration of arterial patency in the ankle and foot of patients with peripheral vascular disease. METHODS: Peripheral MRA of the ankle and foot was performed on 34 limbs of 31 insulin-dependent diabetics. 2-D time-of-flight MRA (TR 33 ms/TE 7.7 ms/inferior saturation band) was performed with 16 cm field of view. Pre- or intra-operative angiographic correlation was available in all cases. RESULTS: In 24 limbs MRA was compared to conventional angiography. MRA showed more patient run-off vessel segments (120) than angiography (100). In 10 limbs MRA was compared to intraoperative angiography and for the detection of patent vessel segments showed a sensitivity of 87.5% (42/48) with a 95% confidence interval of 75% to 95% and a specificity of 95% (38/40) with a 95% confidence interval of 83% to 99%. Pitfalls included difficulty in visualizing flow at the bifurcation of the peroneal artery, in the plantar arch and retrograde flow in the lateral plantar artery. CONCLUSIONS: MRA is sensitive for the detection of patent arteries in the ankle and foot but artefacts may cause overdiagnosis of focal stenoses or occlusions.

Prostatic and periprostatic cysts: findings on MR imaging.

Cysts of the prostate or perioprostatic tissues are uncommon and include congenital müllerian or utricular cysts, prostatic retention cysts, cysts of benign prostatic hyperplasia, cystic carcinoma, parasitic and infectious cysts, as well as cysts of the ejaculatory apparatus or seminal vesicles. The radiological diagnosis of prostatic or periprostatic cysts can be difficult because of the resolution needed to define the relationship of a cyst to surrounding structures, such as the vas deferens, seminal vesicles, and ejaculatory ducts [1]. Prostatic cysts are easily identified on MR images by virtue of their high signal on T2-weighted images and can be characterized because of their typical locations and the high resolution and multiple imaging planes provided by MR [2]. Because these conditions are usually managed conservatively, pathologic proof is not possible in all cases, and the diagnosis is often made on the basis of clinical features and imaging appearance. The purpose of this essay is to illustrate the findings on MR imaging.

Tumor necrosis factor and immune interferon act in concert to slow the lateral diffusion of proteins and lipids in human endothelial cell membranes.

Vascular endothelial surface-related activities may depend on the lateral mobility of specific cell surface macromolecules. Previous studies have shown that cytokines induce changes in the morphology and surface antigen composition of vascular endothelial cells in vitro and at sites of immune and inflammatory reactions in vivo. The effects of cytokines on membrane dynamic properties have not been examined. In the present study, we have used fluorescence photobleaching recovery (FPR) to quantify the effects of the cytokines tumor necrosis factor (TNF) and immune interferon (IFN-gamma) on the lateral mobilities of class I major histocompatibility complex protein, of an abundant 96,000 Mr mesenchymal cell surface glycoprotein (gp96), and of a phospholipid probe in cultured human endothelial cell (HEC) membranes. Class I protein and gp96 were directly labeled with fluorescein-conjugated monoclonal antibodies; plasma membrane lipid mobility was examined with the phospholipid analogue fluorescein phosphatidylethanolamine (Fl-PE). In untreated, confluent HEC monolayers, diffusion coefficients were 30 x 10(-10) cm2 s-1 for class I protein, 14 x 10(-10) cm2 s-1 for gp96, and 80 x 10(-10) cm2 s-1 for Fl-PE. Fractional mobilities were greater than 80% for each probe. Cultures treated at visual confluence for 3-4 d with either 100 U/ml TNF or 200 U/ml IFN-gamma did not exhibit significant changes in protein or lipid mobilities despite significant changes in cell morphology and membrane antigen composition. In HEC cultures treated concomitantly with TNF and IFN-gamma, however, diffusion coefficients decreased by 71-79% for class I protein, 29-55% for gp96, and 23-38% for Fl-PE. Fractional mobilities were unchanged. By immunoperoxidase transmission electron microscopy, plasma membranes of untreated and cytokine-treated HEC were flat and stained uniformly for class I antigen. "Line" FPR measurements on doubly treated HEC demonstrated isotropic diffusion of class I protein, gp96, and Fl-PE. Finally, although TNF and IFN-gamma retarded the growth of HEC cultures and disrupted the organization of cell monolayers, the slow diffusion rates of gp96 and Fl-PE in confluent doubly treated monolayers were not reproduced in sparse or subconfluent untreated monolayers. We conclude that the slowing of protein and lipid diffusion induced by the combination of TNF and IFN-gamma is not due to plasma membrane corrugations, to anisotropic diffusion barriers, or to decreased numbers of cell-cell contacts.(ABSTRACT TRUNCATED AT 400 WORDS)