Feature article

Diffusion imaging demystified

Diffusion imaging is a form of magnetic resonance imaging (MRI) that uses the random motion of water molecules (Brownian motion) to detail tissue structure in the body. There are two types of imaging tests that take advantage of diffusion: diffusion tensor imaging (DTI) and diffusion-weighted imaging (DWI). MR scanners can manipulate the Brownian motion through the use of gradients (variations in the magnetic field). The changes caused by these gradients are measured by the apparent diffusion coefficient (ADC). DTI and DWI are used in many fields within medicine, especially for diagnosing and monitoring breast and prostate cancers.

Diffusion-weighted imaging has become a promising aspect of breast imaging.¹ DWI shows the three-dimensional Brownian motion within the body, allowing for indirect assessment of breast tissue. Through the use of ADC values, DWI can differentiate between benign and malignant breast tumors. It also has the potential to monitor the body's response to cancer treatment. The ADC varies between people, partially due to differences in tissue density. DWI determines tumor malignancy based upon the ADC value of breast tissue. The more restricted diffusion is in the breast, the higher the signal intensity diffusion weighted images and the lower the ADC. Tumor cellularity is increased in malignant tumors, causing restriction of diffusion. This means that malignant tumors are easily identified both on the images and through the ADC value.

Dynamic contrast enhanced MRI is highly sensitive to breast cancers, but most women don't have access to this exam  due to the high cost.¹ Additionally, DCE-MRI requires the use of a contrast agent. DWI breast imaging would be possible at a lower cost and without the contrast agent. It also can be more accurate in identifying malignancy in the breast, because there is overlap between the appearance of benign and malignant tumors on DCE imaging.

Prostate cancer can be detected using conventional T2-weighted imaging alone.² However, the accuracy of cancer detection is increased with the addition of DWI prostate imaging. Like breast cancer malignancies, prostate cancer has been shown to have lower ADC values. Conventional MRI provides localization information for the cancer, but DWI provides more accurate measurement of tumors due to the ADC value. DWI can be used to detect clinically significant cancer, meaning cancer with a Gleason score at or above 6 and a diameter larger than 4 mm.

Diffusion MRI is extremely useful in the detection of cancer in the breast and prostate. It has increased accuracy compared to T2-weighted images alone. This is due to the apparent diffusion coefficient and the Brownian motion of water molecules that is observed with MRI gradients. The detailed pictures diffusion presents let radiologists see tumors that may have been missed on other tests.

For more information on diffusion imaging, see "Diffusion imaging demystified."

1. Savannah C. Partridge and Elizabeth S. McDonald. "Diffusion weighted MRI of the breast: Protocol optimization, guidelines for interpretation, and potential clinical applications." Magn Reson Imaging Clin N Am. August 2013; 21(3): 601-624. Web. 11 December 2018. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3740446/>.

2. Masoom A. Haider, et al. "Combined T2-Weighted and Diffusion-Weighted MRI for Localization of Prostate Cancer." AJR. 2007; 189: 323-328. Web. 11 December 2018. <https://www.ajronline.org/doi/full/10.2214/AJR.07.2211>.