Introduction to the Technical Aspects of Computed Diffusion-weighted Imaging for Radiologists

Abstract

Diffusion-weighted (DW) imaging is a magnetic resonance (MR) imaging method. It is an indispensable sequence for the diagnosis of acute cerebral infarction and is recognized as a standard tool in oncologic imaging. Computed DW imaging refers to the synthesizing of arbitrary b-value DW images from a set of measured b-value images by voxelwise fitting. Computed DW imaging is advantageous because it generates DW images with a higher diffusion effect than that achievable by using the MR imaging units in use today. Additionally, computed DW imaging can reduce imaging time while producing images characterized by a higher signal-to-noise ratio than what the acquired DW images would display at the corresponding b values. By fitting input images acquired at a lower b value and correspondingly a shorter echo time, the signal intensity of the resulting computed DW image is closer to the ideal case. Computed DW images are generated by employing mathematical models that use mono-, bi-, or triexponential equations. To generate accurate computed DW images, the appropriate model must be selected, and the image parameters for the input data must be chosen accordingly. In addition, to reduce artifacts on computed DW images, the misalignment of input data must be corrected with the aid of image registration techniques. RSNA, 2018.

Clinical Feasibility of Reduced Field-of-View Diffusion-Weighted Magnetic Resonance Imaging with Computed Diffusion-Weighted Imaging Technique in Breast Cancer Patients

“…Moreover, the computed DWI technique enables a reduction of the image acquisition time by the simple technique of deriving a higher b-value DWI by extrapolating information obtained at two or more lower b-values from the calculated ADC map [22,23]. In this study, the image quality of the rFOV DWI with computed DWI technique was equivalent to that obtained with the acquired rFOV DWI with higher b-value, despite the short image acquisition time.…”

“…Computed DWI is a computation technique that uses DWI acquired with at least two different lower b-values to obtain a new DWI with a higher b-value, using calculation on a voxel-by-voxel basis (Figure 1). The technique is based on the principle of DWI in which the values of the apparent diffusion coefficient (ADC) of various b-values are fit into an exponential curve represented by the following equation [20][21][22]:…”

“…The technique is based on the principle of DWI in which the values of the apparent diffusion coefficient (ADC) of various b -values are fit into an exponential curve represented by the following equation [ 20 , 21 , 22 ]: where is the signal intensity at a b = 0 s/mm 2 . Once the value of ADC is known, it can be used to extrapolate the expected signal intensity for each imaging voxel to any computed b- value, b c , as follows: where and are the per-voxel estimates of and ADC, respectively.…”

Clinical applications of diffusion-weighted sequence in brain imaging: beyond stroke

“…Higher b values result in greater contrast; however, they also result in lost absolute signal creating a low Fig. 1 Application of 2 gradients in equal but opposite directions in a T2 sequence results in net signal loss in moving water molecules but bright signal from water with restriction of motion signal-to-noise ratio [7]. Diffusion bright signal is considered "true" when there is a corresponding dark ADC signal (suggesting less T2 effects).…”

Linear Intensity-Based Image Registration

“…2) Apparent diffusion coefficient (ADC): Diffusion weighted imaging (DWI) measures the diffusion of water molecules within different tissues [9]. Normal prostate gland tissue has a higher water diffusion rate than cancer tissue [9].DWI is inherently T2WI but, unlike conventional T2WI, prostate cancer usually demonstrates increased SI on DWI effecting the lesion difficult to visualize within the normal high signal peripheral zone [9].To minimize the effect of this T2WI sine through effect that may result in the depiction of a false positive high intensity lesion [10], the ADC is calculated [8], which is calculated based on diffusionweighted imaging(DWI) using various b values, is useful for differentiate existence lesion either benign or malignant [11].Furthermore, Prostate lesion visible as a high signal region on DWI but as low signal region on ADC map. Fig.…”