Importance of Fractional b Value for Calculating Apparent Diffusion Coefficient in DWI.

OBJECTIVE: The purpose of this study is to examine how the fractional b value affects the calculation of apparent diffusion coefficient (ADC) using DWI. The fractional b value is the point of intersection between the fast and slow components of biexponential decay in DWI. SUBJECTS AND METHODS: Human brains were imaged using multiple b values on echo-planar DWI. The ADCs of white matter, gray matter, and thalamus were calculated using the combination of b values by two-point and multipoint methods, and the characteristics of each ADC value were compared. RESULTS: When the two selected points for calculation were smaller than the fractional b value (b = 1700 s/mm2), the ADC value was 0.0007-0.0008 mm2/s, but when the two points used for calculation were greater than the fractional b value, the ADC value was 0.0003-0.0004 mm2/s. When a range of b values was included in the fast and slow components by use of the multipoint method, the ADC value showed a statistically significant increase as the number of multiple b values increased. CONCLUSION: The ADC value fluctuated when the b values used for calculation were higher than the fractional b value. Therefore, it is important to determine the fractional b value of the target tissue.

Optimal b Values for Generation of Computed High-b-Value DW Images.

OBJECTIVE: The purpose of this study was to investigate the optimal b values required for the generation of computed high-b-value DW images. SUBJECTS AND METHODS: Brain DWI was performed for eight subjects using 16 b values, ranging from 0 s/mm(2) to 5000 s/mm(2). The fractional value as the turning point between the fast and slow components was determined using a diffusion decay curve. Image contrast in the white matter, gray matter, thalamus, putamen, and lateral ventricle was compared between the acquired high-b-value images and the computed high-b-value images derived from various b value images. In addition, the signal-to-noise ratio of each computed image and acquired image was measured and compared. RESULTS: The fractional values obtained from the diffusion decay curve were between 1200 and 1800 s/mm(2). Image contrast in all regions was nearly equivalent between the acquired and computed high-b-value images when b values higher than the fractional value were used, whereas image contrast was significantly different between the two sets of images when b values lower than the fractional value were used (p < 0.05). The computed images derived from low-b-value images showed a higher signal-to-noise ratio, compared with the acquired images. CONCLUSION: Optimal b values should be considered when acquiring images for the derivation of computed DW images.

Evaluation of Imaging Quality and ADC for the Tetrahedral Gradients of Diffusion Weighted Imaging.

The diffusion weighted images (DWIs) are used widely in clinical practice. Recently, the gradient overplus imaging was developed in addition to orthogonal technique for the duration method of motion-probing gradient pulses. The gradient overplus technique can reduce the load of a gradient strength, and enables setting of the shortest TE compared with the orthogonal gradients, and moreover it has the information on diffusion tensor. The apparent diffusion coefficient (ADC) values, signal-to-noise ratio (SNR), and geometric distortion were compared for the two duration methods of DWI using four different phantoms of coefficient of viscosity. In the gradient overplus, the time of smallest TE became smaller than the orthogonal gradients according to the increase of the b value. As a result, SNR increased by shortening of TE, and thus the geometric distortion was improved. Moreover, the ADC value did not show any difference in the two duration methods of DWI. The gradient overplus technique is a more useful technique in a clinical study than the orthogonal gradients because the quality of image is improved, and the ADC value is the same as the orthogonal gradients.