ADC stands for apparent diffusion coefficient. It measures the magnitude of water molecule diffusion within various types of tissue. ADC values are calculated from magnetic resonance (MRI) images with diffusion weighted imaging. MRI software calculates ADC values automatically, and displays them as a visual map of tissue reflecting how much diffusion of water molecules is present throughout the tissues.
ADC is expressed in units of mm²/s. Following are some guidelines on ADC units for different types of brain tissue:
• White matter – 670 to 800 x 10-6 mm²/s
• Cortical gray matter – 800 to 1000 x 10-6 mm²/s
• Deep gray matter – 700 to 850 x 10-6 mm²/s
• Cerebrospinal fluid – 3000 to 3400 x 10-6 mm²/s
• Astrocytoma Grade II – 1273 +/- 293 x 10-6 mm²/s
• Astrocytoma Grade III – 1067 +/- 276 x 10-6 mm²/s
• Astrocytoma Grade IV – 745 +/- 135 x 10-6 mm²/s
Changes in ADC correlate with various clinical signs. They can be useful to clinicians for early diagnosis, and for long-term evaluation, particularly when evaluating the effect of various pharmaceuticals.
Understanding Diffusion Weighted Imaging and ADC
Understanding diffusion weighted imaging (DWI) can help with understanding ADC values. DWI is a type of MR medical imaging that measures random motion of water molecules within a specific volume of tissue. Though the relationship between diffusion and cell characteristics is complicated, there are some generalities that are helpful. In general, densely cellular tissues and tissues with cellular swelling have lower diffusion coefficients.
Diffusion of water inside a particular volume of brain tissue is typically hindered by cell membranes. Therefore, diffusion weighted imaging can offer clues about diffusion in intracellular fluid (within cytoplasm or within organelles), diffusion within extracellular fluid (such as interstitial fluid, intravascular fluid, lymphatic fluid, or the brain ventricles, for example), or diffusion between intra- and extracellular areas.
How DWI affects ADC depends upon the tissue type and its pathology. With acute stroke, for example, a decrease in ADC is believed to result from water moving into the intracellular compartment as well as cellular swelling decreasing extracellular space. The same is true in tumors that are highly cellular (such as small round blue cell tumors) and in high-grade gliomas.
Restricted Diffusion: It’s All Relative
Restricted diffusion should be expressed in terms of relativity to what is considered normal. It’s not something that’s present or absent, but is a physical parameter. For example, simply saying that epidermoids show restricted diffusion isn’t as informative as saying that they show restricted diffusion compared to arachnoid cysts.
When radiologists are familiar with normal ADC values and routinely measure them and include them in medical imaging results, they provide themselves and other clinicians with more valuable information. When radiologists and others become familiar with ADC values they expect to encounter in various situations and remember that restricted diffusion is relative, they can learn to detect patterns and be more accurate in things like predicting tumor grades and assessing tumor response to treatments.
ADC Values Can Inform Prognosis
ADC values can be important in a number of clinical scenarios, including
• Early diagnosis of ischemic stroke
• Differentiation between acute and chronic stroke
• Differentiation between acute stroke and something mimicking a stroke
• Differentiation between epidermoid and arachnoid cysts
• Differentiation between abscesses and necrotic tumors
• Characterization of cortical lesions in Creutzfeldt-Jakob disease (CJD)
• Differentiation between diffuse temporal gliomas and herpes encephalitis
• Characterization of diffuse axonal injury
• Grading of certain types of tumors
• Characterization of demyelination
As one example, a 2013 study at Johns Hopkins Medical Institution found that ADC could predict prognosis of astrocytoma independent of tumor grade. Researchers found that survival rates of malignant astrocytomas of grades 3 and 4 were worse when ADC values fell below a specific threshold, independent of grade.
With astrocytomas, tumor cellularity increases with increasing grade. Therefore, the impeding effect of cell membranes is expected to increase, reducing ADC value. So generally, the higher the tumor grade, the more cellular the tumor is, and the lower ADC. The Johns Hopkins researchers found through systemic review and meta-analysis that low ADC values in malignant astrocytomas correlate with poorer survival, regardless of their relationship with tumor grade.
ADC Values Important Outside the Brain Too
ADC values aren’t only important in medical imaging studies of the brain. Researchers in Pakistan, Saudi Arabia, and Oman studied correlation between ADC and the Gleason score in patients who had prostate cancer. Twenty-eight patients with biopsy-proven prostate cancer underwent MRI testing. ADC values from the MRIs were compared with each patient’s Gleason score using the one-way ANOVA test. Of 106 quadrants tested that were positive for malignancy, 89 lesions showed diffusion restriction. An inverse relationship was observed between Gleason score and mean ADC values, as follows:
• Gleason score 6: ADC 935 mm²/s
• Gleason score 7: ADC 837 mm²/s
• Gleason score 8: ADC 614 mm²/s
• Gleason score 9: ADC 571 mm²/s
The researchers concluded that ADC values may help differentiate between low-, intermediate-, and high-risk prostate cancers.
Diffusion weighted imaging (DWI), which depends on motion of water molecules in tissues, can offer clues about tissue integrity. ADC values in normal brain tissue and in a number of different kinds of lesions allow some classification based on ADC values when compared to normal brain tissue. Understanding ADC values for different pathologies compared to normal tissues can help radiologists use medical imaging to better inform diagnosis and prognosis. While ADC values have been studied most extensively in neuroradiology, they may also provide important information about other types of tissue, such as tissue affected by stroke. It may also provide important clues outside the brain. Prostate cancer, for example, shows different ADC values based on Gleason score.
Dr. Carl Raboi – Neuroradiologist Specialist
The Board-certified radiologists who own and operate SteleRAD have decades of combined experience, and work hard to understand the latest developments in medical imaging technology.
Dr. Carl Raboi is one of SteleRAD’s neuroradiology specialists. Neuroradiology primarily uses CT, MRI, and ultrasounds in order to present images of any abnormalities. While ADC values can differentiate between acute and chronic stroke to allow for a prognosis, Dr. Raboi has presented on “Imaging in Acute Stroke”, and published several other works. To learn more about Dr. Raboi and the SteleRAD neuroradiology team, click here.
SteleRAD provides medical imaging services to physicians’ groups, medical imaging centers, and hospitals throughout the South Florida region. If you are interested in finding out more about the services offered by SteleRAD, we urge you to call us at 954-358-5250, or contact us online at any time. We would be more than happy to answer your questions.