Apparent diffusion coefficient

Apparent diffusion coefficient (ADC) is a measure of the magnitude of diffusion (of water molecules) within tissue, and is commonly clinically calculated using MRI with diffusion-weighted imaging (DWI) ¹. 

Basics

Diffusion-weighted imaging (DWI) is widely appreciated as an indispensable tool in the examination of the CNS. It is considered useful not only for the detection of acute ischemic stroke but also for the characterization and differentiation of brain tumors and intracranial infections.

DWI exploits the random motion of water molecules. The extent of tissue cellularity and the presence of intact cell membrane help determine the impedance of water molecule diffusion. This impedance of water molecules diffusion can be quantitatively assessed using the apparent diffusion coefficient (ADC) value. This assessment can be done using different b values via changing gradient amplitude ^2,3,6. 

Measurement

ADC values are calculated automatically by the software and then displayed as a parametric map that reflects the degree of diffusion of water molecules through different tissues. Then, by use of a dedicated workstation, ADC measurements are recorded for a given region by drawing regions of interest (ROIs) on the ADC map ⁶.

An ADC of tissue is expressed in units of mm²/s. There is no unanimity regarding the boundaries of the range of normal diffusion, but ADC values less than 1.0 to 1.1 x 10^-3 mm²/s (or 1000-1100 x 10^-6 mm²/s) are generally acknowledged in adults as indicating restriction. However, this is entirely dependent on the organ being imaged and the pathology ⁷.

Some rough useful values (10^-6 mm²/s) ^8-10:

• brain
  • white matter: 670-800
  • deep grey matter: 700-850 
  • cortical grey matter: 800-1000
  • CSF: 3000-3400
  • astrocytoma
    • grade II: 1273 ± 293
    • grade III: 1067 ± 276
    • grade IV: 745 ± 135
• pelvis ^11,12
  • normal endometrial zone: 1530
  • benign endometrial lesions: 1300
    • uterine polyps: 1270-1580
  • endometrial cancer: 880-980

Diffusion changes are correlated to the clinical deficit and are potentially useful for early diagnosis and longitudinal evaluation, especially in the context of pharmacological trials.