Single-shot Echo-planar Diffusion-weighted Imaging Research Articles

Hepatic Metastases: Diffusion-weighted Sensitivity-encoding versus SPIO-enhanced MR Imaging

To retrospectively compare accuracy of diffusion-weighted (DW) single-shot echo-planar imaging with sensitivity encoding (SENSE) with that of superparamagnetic iron oxide (SPIO)-enhanced magnetic resonance (MR) imaging in the evaluation of hepatic metastases due to extrahepatic malignancies. Patients provided informed consent; ethics committee approval was not required. The data of 24 patients (16 men, eight women; age range, 41-68 years; mean age, 61.9 years) with 40 resected hepatic metastases were retrospectively reviewed. Before SPIO administration, DW SENSE imaging and T2-weighted fast spin-echo (SE) and T1-weighted dual-echo fast field-echo (FFE) MR imaging were performed. After SPIO administration, T2-weighted fast SE, T1-weighted dual-echo, and T2*-weighted FFE MR examinations were performed. Images were divided into two sets: The SPIO-enhanced MR image set consisted of pre- and postcontrast T2-weighted fast SE and dual-echo T1-weighted FFE images and postcontrast T2*-weighted FFE images. The DW SENSE image set included DW SENSE images and precontrast T2-weighted fast SE and dual-echo T1-weighted FFE images. Three radiologists individually interpreted these images and sorted the confidence levels for presence of hepatic metastasis in each section into five grades. Area under the receiver operating characteristic (ROC) curve (A(z)) was calculated for each image set. Hepatic metastases showed higher signal intensity on DW SENSE images than on T2-weighted fast SE images. Conversely, signals from vessels and cysts were suppressed with DW SENSE imaging. ROC analysis showed higher A(z) values when the DW SENSE image set was interpreted (0.90) than when the SPIO-enhanced MR image set was interpreted (0.81). The sensitivity and specificity, respectively, of total cases were 0.66 and 0.90, for the SPIO-enhanced MR image set and 0.82 and 0.94 for the DW SENSE image set. During SPIO-enhanced MR image interpretation, lesions 1 cm in diameter or smaller showed significantly lower sensitivity than lesions larger than 1 cm in diameter. During both interpretation sessions, left lobe lesions showed significantly lower sensitivity than right lobe lesions. Combined reading of DW SENSE images and T2-weighted fast SE and dual-echo T1-weighted FFE MR images showed higher accuracy in the detection of hepatic metastases than did reading of SPIO-enhanced MR images.

Apparent diffusion coefficient in cervical cancer of the uterus: comparison with the normal uterine cervix

A relation between apparent diffusion coefficient (ADC) values and tumor cellular density has been reported. The purpose of this study was to measure the ADC values of cervical cancers in the uterus and compare them with those of normal cervical tissues, and to test whether ADC could differentiate between normal and malignant cervical tissues in the uterus. Twelve consecutive female patients with cervical cancer of the uterus and ten female patients with other pelvic abnormalities were included in this study. ADC was measured at 1.5 T with b-factors of 0, 300 and 600 s/mm2 using single-shot echo-planar diffusion-weighted imaging and a parallel imaging technique. The mean ADC value of cervical cancer lesions was 1.09+/-0.20 x 10(-3) mm2/s, and that of normal cervix tissue was 1.79+/-0.24 x 10(-3) mm2/s (P<0.0001). In nine patients treated by chemotherapy and/or radiation therapy, the mean ADC value of the cervical cancer lesion increased significantly after therapy (P<0.001). The present study showed, with a small number of patients, that ADC measurement has a potential ability to differentiate between normal and cancerous tissue in the uterine cervix. Further study is necessary to determine the accuracy of ADC measurement in monitoring the treatment response.

Comparison of single-shot echo-planar and line scan protocols for diffusion tensor imaging 1

Rationale and Objectives Both single-shot diffusion-weighted echo-planar imaging (EPI) and line scan diffusion imaging (LSDI) can be used to obtain magnetic resonance diffusion tensor data and to calculate directionally invariant diffusion anisotropy indices, ie, indirect measures of the organization and coherence of white matter fibers in the brain. To date, there has been no comparison of EPI and LSDI. Because EPI is the most commonly used technique for acquiring diffusion tensor data, it is important to understand the limitations and advantages of LSDI relative to EPI. Materials and methods Five healthy volunteers underwent EPI and LSDI diffusion on a 1.5 Tesla magnet (General Electric Medical Systems, Milwaukee, WI). Four-mm thick coronal sections, covering the entire brain, were obtained. In addition, one subject was tested with both sequences over four sessions. For each image voxel, eigenvectors and eigenvalues of the diffusion tensor were calculated, and fractional anisotropy (FA) was derived. Several regions of interest were delineated, and for each, mean FA and estimated mean standard deviation were calculated and compared. Results Results showed no significant differences between EPI and LSDI for mean FA for the five subjects. When intersession reproducibility for one subject was evaluated, there was a significant difference between EPI and LSDI in FA for the corpus callosum and the right uncinate fasciculus. Moreover, errors associated with each FA measure were larger for EPI than for LSDI. Conclusion Results indicate that both EPI- and LSDI-derived FA measures are sufficiently robust. However, when higher accuracy is needed, LSDI provides smaller error and smaller inter-subject and inter-session variability than EPI.

In vivo measurement of the apparent diffusion coefficient in normal and malignant prostatic tissues using echo-planar imaging.

To measure for the first time the apparent diffusion coefficient (ADC) values in anatomical regions of the prostate for normal and patient groups, and to investigate its use as a differentiating parameter between healthy and malignant tissue within the patient group. Single-shot diffusion-weighted echo-planar imaging (DW-EPI) was used to measure the ADC in the prostate in normal (N = 7) and patient (N = 19) groups. The spin-echo images comprised 96 x 96 pixels (field of view of 16 cm, TR/TE = 4000/120 msec) with six b-factor values ranging from 64 to 786 seconds/mm(2). The ADC values averaged over all patients in non-cancerous and malignant peripheral zone (PZ) tissues were 1.82 +/- 0.53 x 10(-3) (mean +/- SD) and 1.38 +/- 0.52 x 10(-3) mm(2)/second, respectively (P = 0.00045, N = 17, paired t-test). The ADC values were found to be higher in the non-cancerous PZ (1.88 +/- 0.48 x 10(-3)) than in healthy or benign prostatic hyperplasia central gland (BPH-CG) region (1.62 +/- 0.41 x 10(-3)). For the normal group, the mean values were 1.91 +/- 0.46 x 10(-3) and 1.63 +/- 0.30 x 10(-3) mm(2)/second for the PZ and CG, respectively (P = 0.011, N = 7). Significant overlap exists between individual values among all tissue types. Furthermore, ADC values for the same tissue type showed no statistically significant difference between the two subject groups. ADC is quantified in the prostate using DW-EPI. Values are lower in cancerous than in healthy PZ in patients, and in BPH-CG than PZ in volunteers.

Diffusion tensor trace mapping in normal adult brain using single-shot EPI technique. . A methodological study of the aging brain

To quantify age-related changes of the average diffusion coefficient value in normal adult brain using orientation-independent diffusion tensor trace mapping and to address the methodological influences on diffusion quantification. Fifty-four normal subjects (aged 20-79 years) were studied on a 1.5-T whole-body MR medical unit using a diffusion-weighted single-shot echo-planar imaging technique. Orientation-independent diffusion tensor trace maps were constructed for each subject using diffusion-weighted MR measurements in four different directions using a tetrahedral gradient combination pattern. The global average (including cerebral spinal fluid) and the tissue average of diffusion coefficients in adult brains were determined by analyzing the diffusion coefficient distribution histogram for the entire brain. Methodological influences on the measured diffusion coefficient were also investigated by comparing the results obtained using different experimental settings. Both global and tissue averages of the diffusion coefficient are significantly correlated with age (p<0.03). The global average of the diffusion coefficient increases 3% per decade after the age of 40, whereas the increase in the tissue average of diffusion coefficient is about 1% per decade. Experimental settings for self-diffusion measurements, such as data acquisition methods and number of b-values, can slightly influence the statistical distribution histogram of the diffusion tensor trace and its average value. Increased average diffusion coefficient in adult brains with aging are consistent with findings regarding structural changes in the brain that have been associated with aging. The study also demonstrates that it is desirable to use the same experimental parameters for diffusion coefficient quantification when comparing between different subjects and groups of interest.

Diffusion tensor trace mapping in normal adult brain using single-shot EPI technique. A methodological study of the aging brain.

To quantify age-related changes of the average diffusion coefficient value in normal adult brain using orientation-independent diffusion tensor trace mapping and to address the methodological influences on diffusion quantification. Fifty-four normal subjects (aged 20-79 years) were studied on a 1.5-T whole-body MR medical unit using a diffusion-weighted single-shot echo-planar imaging technique. Orientation-independent diffusion tensor trace maps were constructed for each subject using diffusion-weighted MR measurements in four different directions using a tetrahedral gradient combination pattern. The global average (including cerebral spinal fluid) and the tissue average of diffusion coefficients in adult brains were determined by analyzing the diffusion coefficient distribution histogram for the entire brain. Methodological influences on the measured diffusion coefficient were also investigated by comparing the results obtained using different experimental settings. Both global and tissue averages of the diffusion coefficient are significantly correlated with age (p<0.03). The global average of the diffusion coefficient increases 3% per decade after the age of 40, whereas the increase in the tissue average of diffusion coefficient is about 1% per decade. Experimental settings for self-diffusion measurements, such as data acquisition methods and number of b-values, can slightly influence the statistical distribution histogram of the diffusion tensor trace and its average value. Increased average diffusion coefficient in adult brains with aging are consistent with findings regarding structural changes in the brain that have been associated with aging. The study also demonstrates that it is desirable to use the same experimental parameters for diffusion coefficient quantification when comparing between different subjects and groups of interest.

Pulse triggering for improved diffusion-weighted MR imaging of the abdomen

The aim of this work was to reduce the influence of motion on diffusion-weighted MR images of the abdomen by pulse triggering of single-shot sequences. Five healthy volunteers were examined both without and with finger pulse-triggering of a diffusion-weighted single-shot echo planar MR imaging sequence at 1.5 T. Series of diffusion-weighted images were acquired at different phases of the cardiac cycle by varying the time delay between finger pulse and sequence acquisition. The measurements were repeated three times. The diffusion weighted images were analysed by measuring the signal intensities and by determining the ADC values within the spleen, kidney and liver. The magnitude of motion artifacts on diffusion weighted images shows a strong dependence on the trigger delay. The optimum trigger delay is found to be between 500 and 600 ms. For these values the abdominal organs appear homogeneous on all diffusion weighted images and the strongest signal intensities are detected. At optimum triggering the accuracy of the apparent diffusion coefficients is up to 10 times better than without triggering. Moreover, the standard deviation of the repeated measurements is smaller than 12% for all volunteers and for all organs. Without triggering the standard deviation is larger by a factor of 4 on average. Pulse triggering of single-shot sequences leads to significant reduction of motion related artifacts on diffusion weighted images of the abdomen and provides more accurate and reproducible ADC values.

Diffusion-weighted single-shot echoplanar MR imaging for liver disease.

The aims of this study were to determine apparent diffusion coefficients (ADCs) of the abdominal organs and liver lesions, to determine the effect of the magnitude of b values on the ADCs, and to determine whether measured ADCs of liver tumors help differentiate benign from malignant lesions. Six healthy volunteers and 126 patients were examined with diffusion-weighted single-shot echo-planar imaging using multiple b values (maximum, 846 sec/mm2). The ADCs of the liver, spleen, kidney, 49 malignant liver lesions (33 hepatocellular carcinomas, 15 metastatic liver tumors, and one cholangiocellular carcinoma), and 30 benign lesions (17 cysts, 12 hemangiomas, and one angiomyolipoma) were calculated. The ADCs of the abdominal organs and liver lesions showed smaller values when calculated with the greater maximum b values. The ADCs of the benign lesions calculated with all the b values of less than 850 sec/mm2 (2.49+/-1.39 x 10(-3) mm2/sec) were significantly (p = .0024) greater than those of the malignant lesions (1.01+/-0.38 x 10(-3) mm2/sec). When the maximum b value is 846 sec/mm2, use of a threshold ADC of 1.6 x 10(-3) mm2/sec would result in a sensitivity of 98% and a specificity of 80% for differentiation of malignant liver lesions from benign lesions. Measurement of ADC has good potential for characterizing liver lesions, but the calculated ADCs could be affected by the magnitude of the maximum b value.

Acute cerebral infarction: quantification of spin-density and T2 shine-through phenomena on diffusion-weighted MR images.

To quantify the relative contributions of spin density and T2 effects ("shine through") on diffusion-weighted (DW) magnetic resonance (MR) images of acute and subacute cerebral infarction. In 30 patients, 1.5-T imaging was performed within the first 7 days after onset of cerebral infarction. Estimates of T2, spin density, and apparent diffusion coefficient (ADC) in the region of stroke and contralateral normal brain were computed by means of standard regression techniques after quadruple-echo conventional MR imaging and single-shot echo-planar DW imaging with a maximum b value of 1,000 sec/mm2. Expected signal intensity (S) enhancement ratios resulting from independent changes in T2, spin density, and ADC were then calculated for the DW sequence. The overall SI of cerebral infarction on DW images was significantly higher than that of normal brain throughout the 1st week after stroke (mean relative SI enhancement ratio, 2.29; P < .001). During the first 2 days after stroke, decreased ADC within the stroke region made the dominant contribution to increased SI on DW images. By day 3, increased T2 values in the stroke region became equally important, and, from days 3-7, the contribution to SI from T2 effects became dominant. A slight increase of spin density in the stroke region made a relatively small and constant contribution to DW SI over the 1st week. The increased SI of subacute cerebral infarction on DW images reflects not only a shortening of ADC but a prolongation of T2 and spin-density values.

The effect of residual Nyquist ghost in quantitative echo-planar diffusion imaging

Single-shot diffusion-weighted echo-planar imaging (EPI) is typically used for most clinical diffusion studies due to its low sensitivity to patient motion. Although the Nyquist ghost artifact in EPI can be substantially reduced, there is frequently a residual ghost with low signal intensity. As reported in this study, this residual ghost can produce severe artifacts when maps of the apparent diffusion coefficient (ADC) are calculated from single-shot echo-planar images. The artifacts presented in this paper appear as regions of apparently low ADC which simulate regions of reduced diffusion, but are in fact generated by b-value dependent Nyquist ghosts of the orbits. Data acquired in vivo were used to demonstrate that these artifacts can be avoided by including standard methods of spatial presaturation or fluid-suppression in the diffusion-weighted EPI protocol. In addition, phantom studies were used to illustrate how phase and amplitude variations in the ghost generate the artifacts and theoretical expressions, derived elsewhere, were used to provide a detailed understanding of the artifacts observed in vivo. The level of Nyquist ghost reported for the current generation of commercial scanners suggests that this is a general phenomenon which should be a consideration in all EPI-based diffusion studies. Magn Reson Med 42:385-392, 1999.