Mechanisms of contrast enhancement in magnetic resonance imaging

The use of contrast agents has increased the sensitivity and specificity of magnetic resonance imaging (MRI). Contrast in MRI is multifactorial, depending not only on T1 and T2 relaxation rates, but also on flow, proton density and, in gradient-echo sequences, on the angle of the induced field. The use of contrast agents in MRI changes the T1 and T2 relaxation rates, producing increased signal intensity on T1-weighted images or decreased signal intensity on T2-weighted images, or both. All contrast agents produce changes in magnetic susceptibility by enhancing local magnetic fields. These effects are caused by interactions between nuclear and paramagnetic substance magnet moments, which produce accentuated transitions between spin states and cause shortening of T1; the paramagnetic substance causes accentuated local fields, which lead to increased dephasing and thus shortening of T2 or T2* relaxation time. The efficacy of shortening of T1, T2 or T2* relaxation time depends on the distance between the proton nucleus and the electronic field of the paramagnetic compound, the time of their interaction (correlation time) and the paramagnetic concentration. The MRI contrast agents currently in use cause shortening of T1, T2 or T2* relaxation time. Metal chelates (e.g., gadolinium-diethylene triamine penta-acetic acid [Gd-DTPA]) in low concentration cause shortening of T1 relaxation times, and the superparamagnetics (e.g., ferrite) cause shortening of T2 relaxation times.     

Diffusion-weighted and conventional magnetic resonance imaging in cerebral cryptococcoma

A case of cryptococcoma involving the right cerebral hemisphere is reported. The diffusion-weighted image shows hypointensity in the central cavity of the cryptococcoma, while apparent diffusion coefficient maps show hyperintensity. The imaging features of an intracerebral cryptococcoma mimic that of a central necrotic brain tumor, rather than a pyogenic brain abscess.     

Comparison of magnetic resonance angiography,magnetic resonance imaging and conventional angiography in cerebral arteriovenous malformation

Summary 10 patients with an AVM of the brain were examined by magnetic resonance angiography (MRA), magnetic resonance imaging (MRI) and conventional cerebral angiography (CCA). From MRA in 7/10 patients important information about vascular supply could be provided; in 3 patients, all with small AVM's it could be only suspected. CCA revealed the vascularisation of the AVM's in all patients and showed additional hemodynamic aspects better than MRA. With MRI in all cases the AVM could be diagnosed, but only the involvement of the main cerebral vessels could be demonstrated; however, MRI is superior to MRA and CCA in showing the nidus and the involved brain structures.     

Magnetic resonance imaging and histopathology of cerebral gliomas

Summary The correlation of magnetic resonance imaging (MRI) with histopathological findings was analysed in 26 patients with untreated cerebral gliomas. In low-grade gliomas, T2-weighted images demonstrated relatively homogeneous high-intensity lesions involving both the grey and the white matter. In high-grade gliomas, especially grade IV, T2-weighted images demonstrated prominent heterogeneity in signal intensity, which consisted of a hyperintense core, less hyperintense or normal intensity rim and surrounding finger-like areas of high intensity. Marked and irregular contrast enhancement was evident in all but one case of these high-grade gliomas in which gadolinium-DTPA was used. Histological examination revealed tumour cells extending as far as the borders of the high-intensity areas shown on T2-weighted images in both high-and low-grade gliomas, but in 5 of 8 low-grade and 4 of 18 high-grade gliomas, isolated tumour cells extended beyond the hyperintense areas shown on T2-weighted images.     

Diffusion-weighted magnetic resonance imaging and magnetic resonance spectroscopy in the evaluation of focal cerebral tubercular lesions

Purpose: To review magnetic resonance (MR) diffusion-weighted imaging (DWI) and spectroscopy findings in patients with focal cerebral tuberculosis and to assess whether these techniques can adequately characterize focal cerebral tubercular lesions.

Material and Methods: Sixteen patients with single or multiple lesions were evaluated on a 1.5T MR system. DWI was performed with three 'b' values of 50, 500, and 1000 s/mm² and the apparent diffusion coefficient maps were calculated. MR spectroscopy was performed using the point-resolved single-voxel technique with 2 echo time values of 135 ms and 270 ms. The signal intensities of the tubercular lesions on diffusion images and the apparent diffusion coefficients (ADCs) of their centers, along with MR spectroscopy findings, were analyzed in relation to their T2-weighted MR appearances.

Results: DWI identified 17 of the 20 lesions evaluated. Increased signal intensity was seen in 9 of the 17 lesions. The ADCs of the lesions ranged from 0.406 to 2.64×10^-3 mm²/s (mean±SD: 1.038±0.609 mm²/s). Most of the lesions with hyperintense centers on T2-weighted images were of increased intensity on diffusion images, while those with hypointense centers on T2-weighted images were of decreased signal intensity on diffusion images. However, no statistical difference in the ADCs was found between lesions with increased and those with decreased signal intensity centers on T2-weighted images. MR spectroscopy revealed a lipid peak at 0.9-1.3 ppm in all of the 14 lesions evaluated. An increase in normalized choline:creatine ratio was found in all the lesions in which the spectra were obtained with the voxel, including a variable portion of the lesion wall.

Conclusion: DWI and MR spectroscopy help in determining the nature of cerebral tubercular lesions; however, since the findings are varied, they do not help in specific characterization.     

Experimental cerebral ischemia studied using nuclear magnetic resonance imaging and spectroscopy

The effects of short-duration forebrain ischemia on cerebral metabolism in the rat have been studied using several nuclear magnetic resonance (NMR) techniques. In vivo phosphorus-31 (31P) NMR spectroscopy showed that the model produces rapid cerebral energy failure and acidosis. Reperfusion was accompanied by recovery of high-energy metabolites in about 30 minutes, with a slower recovery of pH. Proton (1H) NMR spectra of perchloric acid extracts of selected brain regions showed that levels of alanine and gamma-aminobutyric acid (GABA) were elevated and the level of glutamate was depressed immediately after the ischemic insult, returning to normal by 24 hours. The lactate level remained elevated for up to 7 days after ischemia, suggesting ongoing abnormal mitochondrial function. Postischemic cerebral glucose metabolism was monitored using carbon-13 (13C)-labelled glucose as an NMR probe. Glycolysis was impaired immediately after the ischemic insult, resulting in accumulation of glucose in the tissue and reduced formation of amino acids and tricarboxylic acid cycle intermediates. Glycolysis recovered by 1 hour, but underwent a secondary decrease at 24 hours, the time at which neuronal injury became manifest histologically and physiologically. Nuclear magnetic resonance imaging was used to follow the regional development of tissue injury in selectively vulnerable brain regions. Striatal changes were evident by 24 hours after reperfusion, increasing in intensity and accompanied by hippocampal changes by 48 hours, then becoming less pronounced by 72 hours. Histologic analysis of regional neuronal injury correlated well with the imaging results, establishing NMR imaging as a noninvasive method of visualizing the regional development of ischemic tissue injury.     

A rare cause of delayed contrast enhancement on cardiac magnetic resonance imaging: infective endocarditis

Although there have been a few reports about the magnetic resonance (MR) imaging features of infective endocarditis (IE), delayed contrast enhancement attributable to fibrosis has not been previously described. The case of a 4-year-old girl who was diagnosed with IE based on positive blood culture results and echocardiographic findings is presented. At cardiac MR imaging, late phase contrast-enhanced images revealed a significant enhancement suggesting fibrosis secondary to IE.     

Perfusion magnetic resonance imaging in pediatric brain tumors

Purpose

The use of DSC-MR imaging in pediatric neuroradiology is gradually growing. However, the number of studies listed in the literature remains limited. We propose to assess the perfusion and permeability parameters in pediatric brain tumor grading.

Methods

Thirty children with a brain tumor having benefited from a DSC-MR perfusion sequence have been retrospectively explored. Relative CBF and CBV were computed on the ROI with the largest lesion coverage. Assessment of the lesion’s permeability was also performed through the semi-quantitative PSR parameter and the K2 model-based parameter on the whole-lesion ROI and a reduced ROI drawn on the permeability maps. A statistical comparison of high- and low-grade groups (HG, LG) as well as a ROC analysis was performed on the histogram-based parameters.

Results

Our results showed a statistically significant difference between LG and HG groups for mean rCBV (p < 10^-3), rCBF (p < 10^-3), and for PSR (p = 0.03) but not for the K2 factor (p = 0.5). However, the ratio K2/PSR was shown to be a strong discriminating factor between the two groups of lesions (p < 10^-3). For rCBV and rCBF indicators, high values of ROC AUC were obtained (> 0.9) and mean value thresholds were observed at 1.07 and 1.03, respectively. For K2/PSR in the reduced area, AUC was also superior to 0.9.

Conclusions

The implementation of a dynamic T2* perfusion sequence provided reliable results using an objective whole-lesion ROI. Perfusion parameters as well as a new permeability indicator could efficiently discriminate high-grade from low-grade lesions in the pediatric population.

     

Reproducibility of total cerebral blood flow measurements using phase contrast magnetic resonance imaging

PURPOSE: To evaluate reproducibility of total cerebral blood flow (CBF) measurements with phase contrast magnetic resonance imaging (pcMRI). MATERIALS AND METHODS: We repeated total CBF measurements in 15 healthy volunteers with and without cardiac triggering, and with and without repositioning. In eight volunteers measurements were performed at two different occasions. In addition, measurement of flow in a phantom was performed to validate MR measurements. RESULTS: A difference of 40.4 ml/minute was found between CBF measurements performed with and without triggering (P < 0.05). For repeated triggered measurements, the coefficient of variation (CV) was 7.1%, and for nontriggered measurements 10.3%. For repeated measurements with repositioning, the CV was 7.1% with and 11.2% without triggering. Repeated measurements at different occasions showed a CV of 8.8%. Comparing measured with real flow in the phantom, the triggered differed 4.9% and the nontriggered 8.3%. CONCLUSION: The findings of this study demonstrate that pcMRI is a reliable method to measure total CBF in terms of both accuracy and reproducibility.     

Phenylketonuria: diffusion magnetic resonance imaging and proton magnetic resonance spectroscopy

Two patients with phenylketonuria are reported with white matter lesions. Diffusion magnetic resonance (MR) imaging revealed restricted diffusion patterns (high signal) on b = 1000 s/mm2 images associated with low apparent diffusion coefficient values ranging between 0.44 x 10-3 mm2/s and 0.56 x 10-3 mm2/s. On proton MR spectroscopy obtained in 1 of the patients, a prominent peak resonating at approximately 3.80 ppm was consistently present attributable to the peak rising from the alpha-proton of the phenylalanine molecule.     

Measurement of cerebral mean transit time by dynamic susceptibility contrast magnetic resonance imaging

OBJECTIVE: To determine whether direct measurement of mean transit time from pixels over in-plane vessels on high spatial resolution echo planar imaging is a reliable method for quantitative assessment of cerebral circulation. METHODS AND MATERIALS: Dynamic susceptibility contrast studies were performed using high spatial resolution echo planar imaging (echo time, 60 ms; field of view, 256 x 192-270 x 203 mm; matrix size, 256 x 192; slice thickness, 4 mm) in ten healthy subjects. Forty sequential measurements of five images between the level of the middle cerebral arteries and that of the centrum semiovale were acquired every 1.5 s before, during, and after intravenous injection of 0.12 mmol/kg of gadopentetate dimeglumine. Mean transit times were calculated from the results of gamma variate fitting to the measured deltaR2* data of the middle cerebral arteries, cerebral cortex and white matter. RESULTS: The calculated true mean transit times for cerebral cortex and white matter varied greatly among individuals and from side to side even in a given individual. The fitness of regression models for the deltaR2* curves of the middle cerebral arteries was significantly lower than those for cerebral cortex and white matter. CONCLUSION: Direct measurement of mean transit time from pixels over in-plane vessels was not sufficiently accurate for quantitative assessment of cerebral circulation, probably because the echo planar imaging we used had spatial resolution and dynamic range insufficient for determination of mean transit time for in-plane vessels.     

Advances in magnetic resonance imaging and spectroscopy

Over the past decade, MR imaging and MR spectroscopy have provided a classic example of the rapid progress that can be accomplished by closely coupled technological advancement and clinical application. This article reviews some of the technical and clinical advances in MR imaging and MR spectroscopy that have occurred over the past year.     

Contrast agent dose effects in cerebral dynamic susceptibility contrast magnetic resonance perfusion imaging

Purpose

To study the contrast agent dose sensitivity of hemodynamic parameters derived from brain dynamic susceptibility contrast MRI (DSC‐MRI).

Materials and Methods

Sequential DSC‐MRI (1.5T gradient‐echo echo‐planar imaging using an echo time of 61–64 msec) was performed using contrast agent doses of 0.1 and 0.2 mmol/kg delivered at a fixed rate of 5.0 mL/second in 12 normal subjects and 12 stroke patients.

Results

1) Arterial signal showed the expected doubling in relaxation response (ΔR2*) to dose doubling. 2) The brain signal showed a less than doubled ΔR2* response to dose doubling. 3) The 0.2 mmol/kg dose studies subtly underestimated cerebral blood volume (CBV) and cerebral blood flow (CBF) relative to the 0.1 mmol/kg studies. 4) In the range of low CBV and CBF, the 0.2 mmol/kg studies overestimated the CBV and CBF compared with the 0.1 mmol/kg studies. 5) The 0.1 mmol/kg studies reported larger ischemic volumes in stroke.

Conclusion

Subtle but statistically significant dose sensitivities were found. Therefore, it is advisable to carefully control the contrast agent dose when DSC‐MRI is used in clinical trials. The study also suggests that a 0.1 mmol/kg dose is adequate for hemodynamic measurements. J. Magn. Reson. Imaging 2009;29:52–64. © 2008 Wiley‐Liss, Inc.     

Simultaneous multinuclear magnetic resonance imaging and spectroscopy

A technique has been developed to perform simultaneous multinuclear magnetic resonance imaging and spatially localized spectroscopy. It is inherently superior in terms of time efficiency over current approaches which use sequential or interleaved methods. The pulse sequence uses a parallel excitation and acquisition scheme to acquire multislice proton images concurrently with phosphorus-31 spectroscopic images. Because the phosphorus signal is necessarily collected in the presence of a gradient, an essential element of the technique is an algorithm to extract pure chemical-shift information.     

Perfusion characteristics of radiation-injured lung on Gd-DTPA-enhanced dynamic magnetic resonance imaging

RATIONALE AND OBJECTIVES: A contrast-enhanced dynamic magnetic resonance (MR) study was performed experimentally and clinically to describe perfusion characteristics of radiation-injured lung according to pathologic phases. METHODS: The MR study was performed before and at 0.5, 1, 2, 3, 4, and 7 months after 40 Gy-dose irradiation to the right hemithorax in 8 dogs, and clinically in 12 lung lesions of 9 patients with acute or fibrotic radiation pneumonitis. Altered Gd-DTPA kinetics in the affected lungs was assessed by time-signal intensity curves. MR findings were correlated with lung histology and CT images. RESULTS: Within 1 month after irradiation, the irradiated animal lungs showed focal and persistent contrast enhancement relative to nonirradiated lungs. This abnormality was pronounced during the next 2 months. After 4 months, irradiated lungs conversely showed lower enhancement during the Gd-DTPA first-pass but were followed by persistently greater enhancement during Gd-DTPA redistribution phase. Similar differences in enhancement abnormalities between acute and fibrotic radiation pneumonitis were clinically observed. CONCLUSION: These findings indicate that Gd-DTPA kinetics can be altered according to the histopathologic change in early/acute radiation pneumonitis and radiation fibrosis and that the contrast-enhanced perfusion MRI may help differentiate the phases of radiation pneumonitis.     

Safety of magnetic resonance imaging contrast agents.

Many of the MR examinations performed in the world each year are accompanied by administration of one of these frequently used intravenous contrast agents: Magnevist, Omniscan, and ProHance. Accordingly, MR practitioners must understand the basic pharmacokinetics, side effects, and the potential for adverse events for these contrast agents. Additionally, MR practitioners must know how to manage side effects and adverse events that may occur in association with the administration of contrast agents. Notably, the use of MR imaging contrast agents in special patient populations must be understood. This article discusses each of these important issues.