Diagnosing Mesothelioma: MRIs
Magnetic resonance imaging (MRI) is one of several imaging techniques that doctors use to detect, stage and evaluate the progression of mesothelioma.
These non-invasive scans use magnets and radio waves to help doctors visualize a patient’s organs, tissues, bones and tumors. Many radiologists consider MRIs ideal for viewing the anatomical structures of the chest and abdomen – including the pleura and peritoneum, where mesothelioma tumors most commonly develop.
The first commercial MRI units emerged in the 1980s. Since then, the technology has advanced considerably. Modern MRI units use superconductor magnets and coils to produce a constant magnetic field, as well as radiofrequency energy to measure signals from the nuclei of hydrogen atoms inside the body. Computers inside the scanner register these signals and turn them into images. Most modern units also include a shield, which prevents the scanner from picking up interference from outside signals sources like televisions and radio stations.
MRIs work by aligning the water molecules in your body. Radio waves then cause these aligned particles to emit signals, which register on the scanner. The images reflect the amount of activity that is occurring in each internal structure. Each MRI-generated image shows a thin slice of the body.
The MRI Process
The entire MRI process takes about an hour or two to complete. The scan itself takes between 30 and 60 minutes, but the appointment also includes pre-scan positioning and other preparation activities.
Once patients arrive at the MRI center and fill out their paperwork, they must remove all metal objects from their bodies. The strong magnets in an MRI scanner can attract zippered clothing, jewelry, watches, belts, keys and credit cards. Implanted medical devices that contain metal may also cause complications with the scan. Next, patients will put on a hospital gown and ear plugs.
Fast FactSome MRI scans use a contrast dye to improve image detail. The most common dye is gadolinium, which is safe and effective for most patients with properly functioning kidneys. This magnetic metal ion can visually enhance lesions on MRI images, indicating growths that may be mesothelioma tumors.
For contrast-enhanced MRIs, patients will receive an injection of a contrast dye. This makes certain areas show up more clearly on the test. After the injection, patients lie down on the imaging table.
The technologist then arranges a coil around the part of the body that is being imaged. For pleural mesothelioma, this will be the chest. For peritoneal mesothelioma, it will be the abdomen. Once the technician positions the patient’s body correctly for the exam, the patient and table are slid into a tube-like opening in the MRI machine.
During the scan, the machine makes repetitive knocking sounds as the magnetic field gradients turn on and off. The test itself is painless. Patients should try not to move during the scan, but they can communicate with their technician via microphone if they feel scared or claustrophobic. Patients can leave the MRI center immediately after the scan.
A post-processing technologist will then highlight abnormal areas on the images. Once the final images are ready for review, a radiologist interprets the results and provides the patient’s primary doctor with a report. From there, the physician can examine the scan on a computer monitor, send them electronically to the rest of the treatment team or print them out for the patient’s medical records.
MRI Side Effects
Patients occasionally experience minor side effects after an MRI scan.
|Magnetophosphenes (brief flashes of light across the retina)||Vertigo/Dizziness||Metallic taste in the mouth||Nausea||Physical burns or burning sensations (extremely rare)|
MRIs do not place patients at risk for radiation-induced damage. Because MRIs do not use ionizing radiation, most doctors prefer MRIs for patients who need routine imaging scans. The U.S. Food and Drug Administration (FDA) concludes that as long as the field strengths are kept below 2.0 Tesla, MRIs are safe for repeated use.
MRIs for Diagnosing Mesothelioma Tumors
Magnetic resonance imaging currently plays a limited role in diagnosing mesothelioma. When doctors do prescribe MRI scans to diagnose the disease, they often use them to complement CT scan results. MRI-generated images can help differentiate between normal tissue and tumor tissue, which cannot be determined with a CT scan alone.
MRI scans produce a visual representation of differences is signal intensity between cancerous and noncancerous tissues. Because cancerous tissues emit more intense signals than surrounding healthy tissue, malignant mesothelioma tissues appear as white spots on the scan results with varying brightness. The difference between malignant and noncancerous tissue is even more pronounced in contrast-enhanced MRIs.
To arrive at a mesothelioma diagnosis, radiologists usually inspect MRI-generated images for a mass on the pleura, which encases the lungs. These masses often emit signals of intermediate intensity. The fluid located between the lungs and pleura can also indicate mesothelioma, as areas of pleural fluid with very intense signals sometimes surround pleural masses. MRI scans are generally superior to CT scans for characterizing pleural fluid as benign or malignant.
|Chest wall infiltration||Mediastinal pleural involvement||Circumferential pleural thickening||Nodularity||Other irregular changes in pleural tissue|
Features such as bilateral pleural involvement, pleural shrinkage, pleural effusions and pleural calcifications may also show up on MRI-generated images. These features may suggest mesothelioma, but cannot be used to make a definitive diagnosis.
MRIs for Staging Mesothelioma Tumors
Most studies indicate that MRIs and CT scans are equally effective for accurately staging malignant mesothelioma tumors. While MRIs are less effective at detecting lymph node involvement, they are generally superior at detecting the extent of a tumor’s invasion of other local structures – one of the key steps in staging a mesothelioma tumor.
When radiologists use MRIs to stage a mesothelioma tumor, they look for the following features:
- Loss of normal fat planes
- Extension into mediastinal fat
- Tumor growth that encases more than half the circumference of an organ or mediastinal structure
Radiologists can exclude patients as good surgical candidates if an MRI scan shows mediastinal or full-thickness pericardial involvement, diffuse or multifocal chest wall disease or involvement of the diaphragm or spine.
By revealing the stage of a mesothelioma tumor, MRI images can help doctors determine whether or not the patient is a good candidate for invasive surgery. MRIs are especially useful for detecting two primary features of patients who are unlikely to benefit from an aggressive operation: chest wall invasion and involvement of the diaphragm.
In one study, MRIs detected diaphragmatic spread with 82 percent accuracy, while CT scans detected the same condition with only 55 percent accuracy.
MRIs are useful for staging mesothelioma with the TNM system. Some studies suggest that MRI scans can differentiate between T3 and T4 disease, but not earlier stages like T1 and T2. One study found that MRIs understaged half of the mesothelioma tumors by failing to detect pericardial invasion, which advances tumors from stage T2 to stage T3. However, the same MRIs were effective at detecting involvement of the internal pericardium, which also advances tumors from stage T3 to T4. The study correctly identified all of the tumors that were stage T3 or lower (while excluding the T4 tumors) with a positive predictive value of 100 percent.
MRIs for Evaluating Response to Treatment
Oncologists consider the MRI an accurate and reproducible technique for evaluating patient response to mesothelioma treatment. When evaluating the MRI scan results of mesothelioma patients undergoing treatment, radiologists often measure the tumor from several separate sites. This helps account for the rind-like growth pattern of the cancer. The primary measurement that the doctors look for is an increase or decrease in pleural thickness.
Fast FactIn one study of 50 mesothelioma patients, MRI scans correctly categorized the tumor response in 92 percent of patients.
If there is no visible disease on the post-treatment imaging scan, doctors call this complete response. If there is a 30 percent decrease in the sum of linear tumor measurements, they generally refer to that as a partial response to treatment. If the MRI indicates a size increase of at least 20 percent (or shows a newly developed lesion), the disease is considered progressive.
Doctors may prescribe lung spirometry tests when using MRIs as another way to evaluate treatment response. Patients whose MRIs indicate a partial or complete response to mesothelioma treatment often display simultaneous improvements in lung function, which can be measured with a spirometer.
When doctors study MRI results to determine treatment response, they can adjust their patient’s prognosis accordingly. In one study, patients whose MRIs indicated a response to therapy had a median survival of 15.1 months, while patients whose MRIs indicated no response had a median survival of only 8.9 months.