Colorectal hepatic metastases: quantitative measurements using single-shot echo-planar diffusion-weighted MR imaging

The purpose of this study was to obtain quantitative measurements of the apparent diffusion coefficient (ADC1), flow insensitive apparent diffusion coefficient (ADC2) and perfusion fraction (F) of colorectal hepatic metastases using DWI and to compare these measurements with those obtained in liver parenchyma. Forty patients with 66 hepatic metastases from colorectal carcinoma were prospectively evaluated using DWI with three b values. Quantitative maps of the ADC1 (using b=0, 150, 500 s/mm²images), ADC2 (using b=150, 500 s/mm² images) and fractional variation (F) between ADC1 and ADC2, which reflects perfusion fraction, were calculated. The ADC1, ADC2 and F derived from metastases and liver parenchyma were compared. The mean ADC1 values of liver parenchyma and metastases were significantly higher than the mean ADC2 values (P<0.0001, paired t-test). Colorectal metastases were found to have higher mean ADC1 and ADC2 values compared with liver (P<0.0001, Mann-Whitney test). However, the estimated F was found to be lower in metastases compared to liver (P=0.03, Mann-Whitney test). Colorectal hepatic metastases were characterised by higher ADC1 and ADC2 values, but lower F values compared to liver.     

Abdomen: diffusion-weighted MR imaging with pulse-triggered single-shot sequences

Magnetic resonance (MR) diffusion measurements of the abdomen were performed in 12 healthy volunteers by using a diffusion-weighted single-shot sequence both without and with pulse triggering for different trigger delays. Pulse triggering to the diastolic heart phase led to reduced motion artifacts on the diffusion-weighted MR images and to significantly improved accuracy and reproducibility of measurements of the apparent diffusion coefficients, or ADCs, of abdominal organs.     

Peritumoral brain regions in gliomas and meningiomas: investigation with isotropic diffusion-weighted MR imaging and diffusion-tensor MR imaging

PURPOSE: To retrospectively measure the diffusion-weighted (DW) imaging characteristics of peritumoral hyperintense white matter (WM) and peritumoral normal-appearing WM, as seen on T2-weighted magnetic resonance (MR) images of infiltrative high-grade gliomas and meningiomas. MATERIALS AND METHODS: Seventeen patients with biopsy-proved glioma and nine patients with imaging findings consistent with meningioma and an adjacent hyperintense region on T2-weighted MR images were examined with DW and diffusion-tensor MR imaging. Apparent diffusion coefficients (ADCs) were measured on maps generated from isotropic DW images of enhancing tumor, hyperintense regions adjacent to enhancing tumor, normal-appearing WM adjacent to hyperintense regions, and analogous locations in the contralateral WM corresponding to these areas. Fractional anisotropy (FA) was measured in similar locations on maps generated from diffusion-tensor imaging data. Changes in ADC and FA in each type of tissue were compared across tumor types by using a two-sample t test. P <.05 indicated statistical significance. RESULTS: Mean ADCs in peritumoral hyperintense regions were 1.309 x 10(-3) mm2/sec (mean percentage of 181% of normal WM) for gliomas and 1.427 x 10(-3) mm2/sec (192% of normal value) for meningiomas (no significant difference). Mean ADCs in peritumoral normal-appearing WM were 0.723 x 10(-3) mm2/sec (106% of normal value) for gliomas and 0.743 x 10(-3) mm2/sec (102% of normal value) for meningiomas (no significant difference). Mean FA values in peritumoral hyperintense regions were 0.178 (43% of normal WM value) for gliomas and 0.224 (65% of normal value) for meningiomas (P =.05). Mean FA values for peritumoral normal-appearing WM were 0.375 (83% of normal value) for gliomas and 0.404 (100% of normal value) for meningiomas (P =.01). CONCLUSION: The difference in FA decreases in peritumoral normal-appearing WM between gliomas and meningiomas was significant, and the difference in FA decreases in peritumoral hyperintense regions between these tumors approached but did not reach significance. These findings may indicate a role for diffusion MR imaging in the detection of tumoral infiltration that is not visible on conventional MR images.     

Sensitivity-encoded single-shot spiral imaging for reduced susceptibility artifacts in BOLD fMRI.

Sensitivity encoding (SENSE) with iterative image reconstruction was used to shorten the readout duration in single-shot spiral imaging by a factor of 2. This enabled susceptibility-related blurring and signal loss artifacts to be reduced and spatial resolution to be improved. As a beneficial side effect, the gradient duty cycle was also reduced. The spiral SENSE technique was applied to functional MRI (fMRI) with blood oxygen level-dependent (BOLD) contrast and compared to a conventional spiral acquisition. Stimulation experiments were performed in seven volunteers using motor, visual, and taste paradigms. The signal-to-noise ratio (SNR) and signal-to-fluctuation-noise ratio (SFNR) of the SENSE acquisitions were reduced by 20% and 13%, respectively, with respect to the longer readout. The overall activation detected was comparable to that of the conventional spiral acquisition, even though difficulties in reproducing the stimulation response hampered the evaluation. In some cases, the application of SENSE enabled recovery of activation in regions affected by signal loss due to field inhomogeneity.     

Traumatic brain injury: diffusion-weighted MR imaging findings.

BACKGROUND AND PURPOSE: Diffuse axonal injury (DAI) accounts for a significant portion of primary intra-axial lesions in cases of traumatic brain injury. The goal of this study was to use diffusion-weighted MR imaging to characterize DAI in the setting of acute and subacute traumatic brain injury. METHODS: Nine patients ranging in age from 26 to 78 years were examined with conventional MR imaging (including fast spin-echo T2-weighted, fluid-attenuated inversion-recovery, and gradient-echo sequences) as well as echo-planar diffusion-weighted MR imaging 1 to 18 days after traumatic injury. Lesions were characterized as DAI on the basis of their location and their appearance on conventional MR images. Trace apparent diffusion coefficient (ADC) maps were computed off-line with the diffusion-weighted and base-line images. Areas of increased signal were identified on the diffusion-weighted images, and regions of interests were used to obtain trace ADC values. RESULTS: In the nine patients studied, isotropic diffusion-weighted images showed areas of increased signal with correspondingly decreased ADC. In one case, decreased ADC was seen 18 days after the initial event. CONCLUSION: Decreased ADC can be demonstrated in patients with DAI in the acute setting and may persist into the subacute period, beyond that described for cytotoxic edema in ischemia.     

Cytotoxic brain edema: assessment with diffusion-weighted MR imaging.

To determine whether cytotoxic brain edema is associated with a decrease in diffusion, it was induced in rats, in the absence of ischemia, with an established model of acute hyponatremic encephalopathy. Cytotoxic brain edema secondary to acute hyponatremia was induced with intraperitoneal injections of 2.5% dextrose in water and subcutaneous injection of arginine-vasopressin. Coronal spin-echo magnetic resonance (MR) images were obtained with and without strong diffusion-sensitizing gradients before and after induction of acute hyponatremia. The apparent diffusion coefficient (ADC) was measured at two coronal section locations. In hyponatremic rats, the brain ADC was significantly reduced (P = .0153 and .0001) and was positively correlated with increased total brain water content (P = .0011). Plots of ADC versus total brain water showed a statistically significant inverse linear relationship between ADC and increasing brain water at the anterior coronal section location. The results indicate that the ADC may be a sensitive indicator of cytotoxic brain edema and thus may enable quantitative evaluation of such edema with diffusion-weighted MR imaging.     

Diffusion tensor mapping of the human brain using single-shot line scan imaging

A recently developed single-shot line scan imaging technique for diffusion measurements (Finsterbusch and Frahm, Magn Reson Med 1999;42:772-778) was extended to full diffusion tensor mapping of the human brain. Because the sequence acquires stimulated echoes from individual columns of magnetization ("lines"), the approach is affected neither by spatial aliasing when studying inner volumes nor by resonance offset effects or T2* dephasing as in diffusion-weighted echoplanar imaging. Experiments on healthy subjects were performed at 2.0 T using 31 single-shot images (5b values, 6 orientations, 520 msec each) at 1.5 x 1.5 mm2resolution (interpolated) and 6.0 mm section thickness. Apart from calculated images with isotropic diffusion weighting, the results include maps of the six independent diffusion tensor components, the apparent diffusion coefficient, the relative anisotropy, and the main diffusion direction. The achievable signal-to-noise ratio and resolution allow the identification of differently oriented nerve fibers in the brain stem. J. Magn. Reson. Imaging 2000;12:388-394.     

Interest of diffusion-weighted echo-planar MR imaging and apparent diffusion coefficient mapping in gynecological malignancies: a review

Magnetic resonance imaging (MRI) remains the standard modality for the local staging of gynecological malignancies but it has several limitations, particularly for lymph node staging or evaluating peritoneal carcinomatosis. Consequently, there has been a growing interest in functional imaging modalities. Based on molecular diffusion, diffusion-weighted imaging (DWI) is a unique, noninvasive modality that provides excellent tissue contrast and was shown to improve the radiological diagnosis of malignant tumors. Using quantitative apparent diffusion coefficient (ADC) measurement of DWI provides a new tool for better distinguishing malignant tissues from benign tumors. The aim of the present review is to report on the results of DWI for the assessment of patients with gynecological malignancies. An analysis of the literature suggests that DWI studies would improve the diagnosis of cervical and endometrial tumors. It may also improve the assessment of tumor extension in patients with peritoneal carcinomatosis from gynecological malignancies. However, since the signal intensity of some cancers can range from high intensity to low intensity, a degree of uncertainty was demonstrated due to the proximity of the normal uterine myometrium and ovaries. Interestingly, there is also evidence that ADC might improve the follow-up and monitoring of patients who receive anticancer therapies, including chemotherapy or radiation therapy.     

High-b-value diffusion-weighted MR imaging of adult brain: image contrast and apparent diffusion coefficient map features

BACKGROUND AND PURPOSE: Recent improvements in MR gradient technology allow significant increases in diffusion weighting without prohibitive signal-to-noise degradation. The purpose of our investigation was to establish normative references for the signal intensity characteristics and apparent diffusion coefficient values of the adult brain at high b values. METHODS: Fifty adults underwent diffusion-weighted single-shot spin-echo echo-planar MR imaging. Isotropic diffusion-weighted images were obtained with b values of 0, 1,000, 2,000, 2,500, 3,000, and 3,500 s/mm2. Qualitative assessments were made in multiple regions of interest in gray and white matter. Three apparent diffusion coefficient maps were generated for each of six patients with a 2-point technique at a b value of 0 and at b values of 1,000, 2,000, and 3,000 s/mm2. RESULTS: Increasing b values result in a progressive decrease in the gray to white matter signal intensity ratio. Isointensity between gray and white matter results at b values between 1,000 and 2,000 s/mm2. At b values greater than 2,000, the gray-white pattern reverses relative to the usual b value of 1,000. Apparent diffusion coefficient values were shown to decrease with increasing b values. CONCLUSION: Attention to the reversal of gray-white contrast and the dependence of apparent diffusion coefficient on the b value are important in avoiding erroneous assignment of pathologic abnormalities to normal regions. This study provides the normative data for future diffusion investigations performed at high b values.     

Apparent diffusion coefficient mapping of experimental focal cerebral ischemia using diffusion-weighted echo-planar imaging

Diffusion-weighted, echo-planar imaging (EPI) was used to map regional changes In the apparent diffusion coefficient (ADC) during experimental focal ischemia in the rat brain following permanent middle cerebral arterial occlusion (MCAO). Sixteen 64 × 64 diffusion-weighted EPIs were acquired in 32 s with successively increasing amplitudes of the diffusion-sensitive gradient pulses. A linear least-squares regression algorithm was used to fit 15 of the 16 two-dimensional matrices, on a pixel-by-pixel basis, to solve for the slope from which the ADC value was calculated. The correlation coefficient of the fit, R² was used to filter the final ADC maps, and the ADCs were then scaled appropriately to be displayed in a 256 gray level format. Ranges (bins) of 0.05 × 10⁻³ mm²/s were then grouped and color coded to qualify and quantify the evolution of ischemia in the MCA territory. The percentage of area in the ischemic and contralateral hemispheres in seven ADC bins were calculated at 30, 60, and 120 min after MCAO for 10 animals and demonstrated a significant increase in ADC bins below 0.45 × 10⁻³ mm²/s and a decrease in bins above 0.50 × 10⁻³ mm²/s over the. The postmortem infarct area, as measured by TTC staining, was highly correlated with the portion of the ischemic hemisphere falling below ADC values of 0.55 × 10⁻³ mm²/s at 2 h after stroke onset. These studies suggest that focally ischemic brain tissue can be quantitatively subdivided according to ADC values and that ADC values below 0.55 × 10⁻³ mm²/s 2 h following ischemia highly predict infarction in a rat permanent occlusion stroke model.     

Characterization of focal liver lesions by ADC measurements using a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging technique

The aim of this study was to determine apparent diffusion coefficients (ADCs) of focal liver lesions on the basis of a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging sequence (DW-SS-EPI) and to evaluate whether ADC measurements can be used to characterize lesions. One hundred and two patients with focal liver lesions [11 hepatocellular carcinomas (HCC), 82 metastases, 4 focal nodular hyperplasias (FNH), 56 hemangiomas and 51 cysts; mean size, 16.6 mm; range 5–92 mm] were examined on a 1.5-T system using respiratory triggered DW-SS-EPI (b-values: 50, 300, 600 s/mm²). Results were correlated with histopathologic data and follow-up imaging. The ADCs of different lesion types were compared, and lesion discrimination using optimal thresholds for ADCs was evaluated. Mean ADCs (×10⁻³mm²/s) were 1.24 and 1.04 for normal and cirrhotic liver parenchyma and 1.05, 1.22, 1.40, 1.92 and 3.02 for HCCs, metastases, FNHs, hemangiomas and cysts, respectively. Mean ADCs differed significantly for all lesion types except for comparison of metastases with HCCs and FNHs. Overall, 88% of lesions were correctly classified as benign or malignant using a threshold value of 1.63 × 10⁻³mm²/s. Measurements of the ADCs of focal liver lesions on the basis of a respiratory triggered DW-SS-EPI sequence may constitute a useful supplementary method for lesion characterization.     

Case report: spinal cord infarction demonstrated on diffusion-weighted MR imaging with a single-shot fast spin-echo sequence

We describe the diffusion-weighted (DW) MR imaging findings in three cases of spinal cord infarction using a recently developed single-shot fast spin-echo (SSFSE) technique. The SSFSE-DW MR images, which were obtained 20 hours, 3 days, and 18 days, respectively, after the ischemic event, demonstrated conspicuous areas of hyperintensity in the affected portions. Follow-up DW MR images, obtained in two of the patients at 17 days and 3 months, respectively, showed persistent decreased apparent diffusion coefficient values. SSFSE-DW imaging of the spinal cord may provide additional information for assessment of ischemic changes.     

Nonsuppressing normal thymus on chemical-shift MR imaging and anterior mediastinal lymphoma: differentiation with diffusion-weighted MR imaging by using the apparent diffusion coefficient

Objectives

To prospectively evaluate usefulness of the apparent diffusion coefficient (ADC) in differentiating anterior mediastinal lymphoma from nonsuppressing normal thymus on chemical-shift MR, and to look at the relationship between patient age and ADC.

Methods

Seventy-three young subjects (25 men, 48 women; age range, 9-29 years), who underwent chemical-shift MR and diffusion-weighted MR were divided into a normal thymus group (group A, 40 subjects), and a lymphoma group (group B, 33 patients). For group A, all subjects had normal thymus with no suppression on opposed-phase chemical-shift MR. Two readers measured the signal intensity index (SII) and ADC. Differences in SII and ADC between groups were tested using t-test. ADC was correlated with age using Pearson correlation coefficient.

Results

Mean SII±standard deviation was 2.7±1.8% for group A and 2.2±2.4% for group B, with no significant difference between groups (P=.270). Mean ADC was 2.48±0.38x10^-3mm²/s for group A and 1.24±0.23x10^-3mm²/s for group B. A significant difference between groups was found (P<.001), with no overlap in range. Lastly, significant correlation was found between age and ADC (r=0.935, P<.001) in group A.

Conclusions

ADC of diffusion-weighted MR is a noninvasive and accurate parameter for differentiating lymphoma from nonsuppressing thymus on chemical-shift MR in young subjects.

Key Points

? SII cannot differentiate mediastinal lymphoma from nonsuppressing normal thymus at visual assessment ? ADC is useful for distinguishing nonsuppressing normal thymus from mediastinal lymphoma ? ADC is more accurate than transverse-diameter and surface-area in this discrimination ? ADC of normal thymus is age dependent and increases with increasing age

     

Rapid MR imaging of the pediatric brain using the fast spin-echo technique.

PURPOSE: To evaluate diagnostic reliability and to establish optimal scanning techniques of a recently developed Fast Spin-echo MR pulse sequence that allows rapid proton density-weighted and T2-weighted imaging. METHODS: We compared lesion conspicuity and signal intensity measurements on Fast Spin-echo and conventional spin-echo sequences in 81 patients ranging from 1 week to 25 years in age on a 1.5-T MR unit. A total of 28 Fast Spin-echo dual-echo images (14 slice locations) were obtained in 2:08 minutes with a 256 x 128 matrix or in 3:12 minutes with a 256 x 192 matrix at a TR of 2000 msec and two excitations. RESULTS: Lesion conspicuity and characterization on Fast Spin-echo images compared favorably with conventional spin-echo images in our series when pseudo-TEs of 15 and 90 msec were employed for proton density-weighted and T2-weighted images, respectively. Fast Spin-echo images yielded diagnostic information in four nonsedated patients whose conventional spin-echo images were either degraded by motion or unobtainable. Fat signal remained bright on T2-weighted Fast Spin-echo images. Magnetic-susceptibility effects were slightly reduced with Fast Spin-echo but did not pose any diagnostic problem in our series. CONCLUSION: Diagnostically reliable rapid dual-echo brain images can be obtained with Fast Spin-echo sequences.     

Evaluation of brain in myotonic dystrophy using diffusion tensor MR imaging

In many cases of myotonic dystrophy, high-intensity areas are seen in the cerebral white matter on T2-weighted imaging. Brain MRI was performed in 15 patients with myotonic dystrophy using diffusion tensor imaging, which is sensitive to the detailed structure of white matter, and the results were compared with those of normal controls. FA (anisotropic diffusion) values in the cerebral white matter of myotonic dystrophy patients were significantly lower than those of normal controls (p< 0.01), even if the hyperintense lesion was not seen on T2-weighted imaging. Values of trace (isotropic diffusion) in myotonic dystrophy patients were significantly higher than those of normal controls (p< 0.05), except in the posterior limb of the internal capsule. Diffusion tensor imaging could detect pathological change of the cerebral white matter in myotonic dystrophy patients, and may be useful for quantification and detection of subtle pathological change.     

MR line-scan diffusion-weighted imaging of term neonates with perinatal brain ischemia.

BACKGROUND AND PURPOSE: MR diffusion-weighted imaging provides early demonstration of neonatal brain infarction. The evolution and limitations of diffusion-weighted imaging findings in newborns, however, have not been evaluated. Using line-scan diffusion imaging (LSDI), we investigated perinatal ischemic brain injury. METHODS: Nineteen term newborns (age, 9 hours to 8 days; mean age, 2.6 days) with perinatal brain ischemia were evaluated using LSDI (1520/62.5/1 [TR/TE/excitations]) (b maximum = 750 s/mm2) and T1- and T2-weighted spin-echo (conventional) MR imaging. Follow-up examinations were performed in seven patients and autopsy in one. Apparent diffusion coefficients (ADCs) were measured in deep gray matter, white matter, the cortex, and focal lesions. RESULTS: Based on conventional MR imaging or pathologic findings, patients were divided into two groups. Group 1 (n = 12) had symmetric/diffuse injury consistent with global hypoperfusion. Group 2 (n = 7) had focal/multifocal injury suggesting cerebrovascular occlusion. ADCs were abnormal at initial examination in 10 newborns in group 1 and in all newborns in group 2. The results of LSDI were abnormal before conventional MR imaging was performed in three newborns in group 1. ADCs were maximally decreased between days 1 and 3 in deep gray matter, perirolandic white matter, and focal lesions. Delayed decreases in ADCs were observed in subcortical white matter from days 4 through 10 in three patients in group 1. CONCLUSION: After global hypoperfusion, LSDI showed deep gray matter and perirolandic white matter lesions before conventional MR imaging. LSDI may underestimate the extent of injury, however, possibly because of variations in the compartmentalization of edema, selective vulnerability, and delayed cell death. Differences in LSDI of symmetric/diffuse and focal/multifocal lesions may reflect differences in pathophysiology or timing of the injury. These findings may have implications for acute interventions.     

Improved MR imaging of the brain by using compensating gradients to suppress motion-induced artifacts

Sixty patients were examined with and without extra gradient pulses, which compensate for motion-induced phase errors, in order to determine the effect those gradients had in suppressing the motion artifacts frequently present in the brainstem, temporal lobes, and basal ganglionic regions on routine T2-weighted brain MR imaging. Two comparative studies were performed: (1) in 50 patients the motion-artifact suppression technique (MAST) was compared with a single-echo MR examination, and (2) in 10 patients the MAST technique was compared with the second echo of a symmetric dual-echo sequence. In the first study 39 patients were examined at 1.5 T and 11 patients were examined at 0.5 T with the same pulse sequences. We found that MAST resulted in a significant improvement of image quality in 24 of 39 patients on the high-field-strength system and in two of 11 patients on the mid-field-strength system. In the second study, we found that in four of the 10 patients, MAST resulted in a suppression of artifacts greater than that achieved by even-echo rephasing alone. With MAST, artifacts were eliminated that not only obscured normal structures but that could have left doubt about the presence of a true signal abnormality. There was, however, marked suppression of the CSF flow-void phenomenon and increased signal from flowing blood, particularly in the cortical veins and dural sinuses. Because of this, the use of additional pulse sequences in which these motion-compensating gradients were not used was necessary under certain clinical circumstances. We conclude that, with these motion-compensating gradients, artifacts are reduced or eliminated, and a marked improvement in image quality can be obtained without the need for cardiac gating.