By: Dan Sperling, MD

Magnetic resonance imaging (MRI) and color Doppler ultrasound (CD-US) are both methods of gaining information about internal bodily conditions. They use different energy technologies, and each of them may be more advantageous than the other in certain areas. For instance, MRI registers changes in the body’s magnetic field and generates clear, crisp pictures of specific anatomical structures. On the other hand, CD-US bounces sound waves off red blood cells as they circulate; it is typically used to gather information about blood flow in the heart (echocardiogram), and show areas of blockage in the neck, arms and legs.

Both MRI and CD-US can contribute information about the possible presence of prostate cancer. If a patient’s PSA is elevated, and his doctor recommends a biopsy, before making that decision patients should consider specialized imaging. Each of these imaging technologies can play an important role in detecting the presence of unusual blood flow that is a hallmark of cancerous tumors. As cancer begins to grow, like all cells it requires oxygen and nourishment. Cancer cells use biological signals called angiogenic growth factors to manipulate the development of their own blood supply. This process is called angiogenesis, meaning the beginning of new blood vessels. Therefore, one way to identify a suspicious growth in the prostate gland is to assess the presence of an abnormal blood supply. This article explains how both MRI and CD-US can be a logical first step before having a biopsy.

Today’s prostate MRI has evolved beyond simple imaging. State-of-the-art software allows for computerized image “highlighting” along several different parameters. When performed under a very powerful (3 Tesla) magnet, the term for this imaging suite is 3T multiparametric MRI (3T mpMRI for short). One parameter in particular, called Dynamic Contrast Enhancement (DCE-MRI) involves the injection of a contrast agent which helps show organs and blood vessels more clearly. In the prostate, DCE-MRI is tracked by a set of fast images revealing the uptake rate of the agent in prostate tissue, tumors and blood vessels as concentrations move from high to low over time—in other words, the dynamics of the contrast uptake. If angiogenesis has occurred, rapid arterial enhancement as shown by the contrast agent will be a telltale sign of a tumor. According to an excellent review of prostate imaging, “Early contrast enhancement and high (relative) peak enhancement are the most accurate predictors of PC (prostate cancer) of the PZ (peripheral zone), while fast washout of contrast agent and high permeability of the blood vessels are most sensitive for central gland PC.”[i]

There are three disadvantages to DCE-MRI:

1. Not every patient will have geographic access to 3T mpMRI. Even though the number of radiology facilities that are equipped with such powerful equipment is increasing, it may not be feasible for a patient in a more remote or rural area to travel to a center that has the type of magnet and software for this diagnostic test.
2. Patients with certain kidney disorders may not be candidates for the contrast agent. Healthy kidney function is necessary to wash the agent from the patient’s body.
3. Patients with metallic implants, such as pacemakers, may not be candidates for MRI due to the interaction of the magnetic field with the metal in their bodies.

In these circumstances, CD-US is an excellent alternative. The equipment is more widely available because it is less expensive than MRI technology and does not require a special type of building. Thus, many doctors, including radiologists and urologists, can place it in their own offices. However, the most accurate interpretations require a high level of training and experience in using it.

Conventional ultrasound imaging, such as that used to show a developing fetus in the womb, is called grayscale. This type of imaging, which is done transrectally, shows the anatomy of the gland. However, it can’t reliably show prostate cancer tumors because it does not distinguish all the tissue in all areas of the gland. On the other hand, CD-US can detect the abnormal blood flow that is a hallmark of a tumor’s angiogenesis. This will show up on a monitor as brightly colored areas overlaid on the grayscale images of the prostate. Generally speaking, the greater the blood flow, the more it correlates with tumor aggression (higher Gleason grades). However, CD-US is a more limited detection method than DCE-MRI because it is not sensitive enough to pick up blood flow in microvessels (very small vessels) associated with tumor growth. In addition, it is not always specific for cancer; a condition like prostatitis may also have increased blood flow that shows up on CD-US.

According to the article cited earlier, “In a study of 96 patients with lower urinary tract symptoms and PSA levels over 4 ng/ml … the degree of Doppler signal correlated with the microvessel density and Gleason score of a lesion. One study achieved Doppler imaging-based detection rates of 40% … However, a drawback of Doppler imaging is the high inter-observer variability … reflected in the widely spread sensitivity and specificity figures in the literature (27 – 98% and 46 – 84%, respectively).”¹ What this means is that interpretation of CD-US findings varies a great deal from one reader to the next.

To sum up, CDE-MRI performed using a powerful 3T magnet is a preferable diagnostic tool over CD-US for detecting the abnormal blood flow of a tumor’s angiogenesis. Still, in cases where a patient cannot have access to this test, or is not a candidate for it, undergoing a CD-US of the prostate as done by a highly experienced reader can provide valuable insight as to whether a biopsy is indicated.