By: Dan Sperling, MD
Advancing magnetic resonance (MR) imaging methods to assist in the noninvasive identification of prostate cancer is of key importance in diagnosis, treatment, and post-treatment monitoring of tissue changes. Progress in this direction includes two imaging parameters that give information about fluid content and flow in tissue. The two parameters explained here are largely taken from the work of a Dutch team, published in the journal Magnetic Resonance Imaging Clinics of North America.[i] The first is called diffusion weighted MR imaging (DWI); the second is dynamic contrast-enhanced MR imaging (DCE, also referred to as perfusion-weighted MR imaging).
DWI with Apparent Diffusion Coefficient (ADC) mapping
Diffusion Weighted Imaging (DWI) is a method of magnetic resonance imaging that reflects thermal molecular motion of water molecules in tissues. Unlike random molecular motion in unrestricted states, molecular diffusion in human tissue is affected by its structural nature and other biochemical factors, e.g. motion is limited by cell membranes. Hydrogen is one of the two atomic components of the water molecule, and the DWI pulse sequence labels hydrogen nuclei in space and thus determines the distance that a water molecule travels on its path during a very short time. Free motion of water molecules is more restricted in tissues with a high cellular density. These effects measurably impact the magnetic resonance signal. Although the information is indirectly quantified, it clearly differentiates between healthy and diseased tissues noninvasively. Thus, DWI magnetic resonance images can provide important indicators of possible prostate cancer.
Diffusion imaging is performed optimally on a high-field (?1.5 T) echo-planar system. The intensity of each image element (voxel) represents an estimate of the rate of water diffusion at that location. Due to the interplay of factors in tissues, the actual diffusion coefficient of water cannot be measured directly by MRI. Therefore the diffusion coefficient from orthogonal DWI in all three planes is obtained as the apparent diffusion coefficient (ADC). Diffusion data can then be presented as an image map of the ADC or signal intensity. Calculation of the ADC requires 2 or more acquisitions with different diffusion weightings. ADC and signal intensity are inversely related: a low ADC corresponds to high signal intensity (restricted diffusion), and a high ADC to low signal intensity on DWI.
The prostate is primarily composed of glandular tissue but is structured in differing anatomical zones. According to the Dutch article, “A prerequisite for the correct interpretation of diffusion and ADC images relies on good knowledge of the diffusion characteristics of the different anatomic zones of the prostate and of benign prostatic conditions compared with prostate cancer.” Such knowledge helps an experienced reader determine conditions such as benign prostatic hyperplasia (BPH, a normal enlargement phenomenon that occurs as men age), acute prostatitis, and chronic prostatitis (both inflammations).
Prostate cancer is distinctive from benign conditions and inflammation, and this shows up with DWI. The cancer is characterized by higher cellular density, and takes the place of normal gland tissue. This leads to a decrease in ADC values, and as stated above, the lower the ADC the greater the restricted diffusion. However, DWI lacks precise special resolution, so it is combined with T2-weighted imaging, which shows anatomic details very well and in high resolution. It helps to distinguish between the peripheral zone and the central gland. More importantly, it is “necessary for a correct interpretation of ADC mapping” and helps in differentiating BPH from cancer on DWI, since both present with decreased ADC.
Dynamic Contrast Enhanced Imaging
Cancerous tumors cannot grow beyond 1-2 mm in solid tissue without an established blood supply to provide oxygen and nutrients. Through a series of complex biochemical signals, the tumor signals and promotes the development of its own blood supply. This process is called angiogenesis. A tumor’s network of blood vessels has properties that set it apart from normal blood flow, and thus is amenable to detection by Dynamic Contrast Enhanced (DCE) MR imaging, as well as by DWI. Suffice it to say there is, overall, increased blood flow to the timorous area. By intravenously administering a contrast agent that shows up clearly on MRI, DCE can be used to track the more rapid arrival time of the agent to the tumor than to the rest of the gland. “Data reflecting the tissue perfusion (blood flow, blood volume, and mean transit time), the microvessel permeability, and the extracellular leakage space can be obtained.”
When combining DWI, T2-weighted and DCE, a multiparametric data set can be built to assist in detection, localization, assessment of prostate cancer aggressiveness and tumor staging. In addition to the services of an experienced reader, special software to process and translate the data coming in from the imaging is also necessary. With the increasing availability of powerful (3T) magnets, the benefits of multiparametric MRI include early diagnosis of prostate cancer metastasis to the lymph nodes or bone, following any procedure. In addition, detecting cancer spread beyond the prostate capsule prior to treatment can help avoid putting a patient through an invasive procedure, only to have the cancer recur. It is safe to say that important roles for DWI and DCE in the field of prostate cancer diagnosis and treatment will continue to expand.
[i] Somford D, Futterer J, Hambrock T, Barentsz J. Diffusion and perfusion MR imaging of the prostate. Magn Reson Imaging Clin N Am. 2008;16:685–695