MRI Sets the Stage for Cancer Imaging
HIIT070801The advantages of 1.5T MRI over other modalities for staging cancer are twofold: No radiation and decreased nephrotoxicity. In particular, radiologists focused on oncology say the modality is particularly useful for imaging brain tumors, noting that it is valuable for problem solving and treatment planning because of its superior capability to provide soft-tissue contrast.

MRI is used in the initial staging scan and when radiologists see something shown on other imaging but want more precise information. It is considered the most important “second step” imaging tool if x-ray or ultrasound findings are suspicious for a bone or soft-tissue tumor. In addition to assessing a lesion’s malignancy, MRI can reveal its extension and guide the biopsy.

Priya Bhosale, MD, assistant professor in the department of diagnostic radiology at the University of Texas, M.D. Anderson Cancer Center in Houston says 1.5T MRI is beneficial for diagnosing and treating cancer patients because it yields information based on the molecular and metabolic profiles of tissue, while providing better visualization of the entire body.

“We use it on a day-to-day basis, on all kinds of cancer, from head to toe,” Bhosale notes.

The modality is particularly suited as a problem-solving tool in body imaging where patients cannot tolerate iodine-based IV contrast agents used in CT. However, contrast agents commonly used in MRI contain gadolinium, which has been linked to nephrogenic systemic fibrosis—particularly in patients who have compromised renal function.

Bhosale says if a patient’s glomerular filtration rate (GFR) is below 30, the MRI exam is performed without a contrast agent.

“The contrast capabilities of 1.5T MRI are what make it such an attractive modality for cancer imaging,” says Dan Chernoff, MD, PhD, director of radiology services at Adirondack Radiology Associates at the Glens Falls Hospital in Glens Falls, N.Y.
“As long as it [tissue] has water in it, MRI has an almost unlimited ability to alter the contrast between different tissues, depending on the type of sequences you run, to bring out the difference between diseased tissue and normal tissue,” Chernoff says.

He notes that it provides better contrast than CT and better spatial resolution than PET.

Although Chernoff says his facility uses 1.5T MR as the primary modality for imaging brain tumors, he adds that it has an important role in muscoskeletal oncology and plays an increasingly larger role in breast cancer imaging and abdominal malignancies, including renal cancers, hepatic metastases and other lesions in the liver.

One of the most distinctive uses of 1.5T MRI at MD Anderson’s Cancer Center is its role in treating brain tumors—the facility has a brain suite with an MRI scanner in the operating room. Patients have radiology markers placed on their forehead the day before surgery and undergo a pre-operative MRI scan. The operating table has a pivot, allowing it to be moved into the MRI scanner, which enables surgeons to scan the patient during surgery.

“As they do surgery, they can scan the patient and make sure that the appropriate part of the brain is removed, and those areas which are needed—like the basal ganglia [motor control and learning]—are not touched so when the patient comes out of surgery, he has all his functions retained,” Bhosale says.

Using MRI to examine brain tumors is “more accurate and feasible,” says Bhosale, because it is faster than CT and provides superior delineation of the organ’s structures.

It also is a better modality for treatment planning and surgical purposes, she says, especially in cases involving cardiac tumors that are intimately associated with the heart and adjacent vessels.

When prostate cancer cases involve neurovascular bundles, surgeons find MRI useful in their attempts to spare nerves and avoid impotence for the patient. It is the most commonly used modality to guide catheter needle placement in the prostate, while providing better visualization of the surrounding anatomy.

Chernoff says it also is useful in evaluating superior sulcus tumors in the chest or lung. “CT is great for seeing what’s normal, but if you’re not sure if it’s invading the chest wall, MRI is better,” he notes.

It is often used for treatment planning in rectal cancer cases when the tumor involvement is classified as P3, the stage when tumors extend backward to involve the sacrum, Bhosale says. Surgeons use it to determine how much of the sacrum is involved and if a sacrectomy is needed. 1.5T MRI’s accurate image guidance is particularly useful because surgeons try to avoid removing the sacral plexus when possible—namely because if the imbedded sciatic nerve is damaged, it can result in lose of leg function, Bhosale points out.

When patients have a localized disease—as in the pelvis, rectum or bladder—surgeons will do an MRI study to see which organs are involved to determine exactly what they need to remove and whether small arteries or nerves are involved, she says.
In cervical cancer cases, especially involving young patients, surgeons want to know if the tumor extends into the uterus and whether a hysterectomy is required. But if it is confined only to the cervix, it can be very well seen on the MRI. The patient then undergoes a photolytic preserving procedure, a trachelectomy (removal of the cervix), but the uterus is left intact, preserving the ability to bear children.

“[MRI is] definitely better than CT and probably better than ultrasound for the staging of cervical cancer,” observes Chernoff.

Some researchers now suggest that MRI should be used to estimate a tumor’s biological activity rather than its status as benign or malignant. This is achieved using contrast-enhanced or perfusion MRI to evaluate typical enhancement patterns in relation to their time-activity curve.

Dynamic contrast imaging is particularly useful in determining therapy effectiveness in patients receiving anti-angiogenesis drugs (which target blood vessels that cancer cells need to survive and grow).

“In the liver, for instance, sometimes you can only detect a tumor on a specific phase of the contrast enhancement so you have to image multiple times after you inject,” says Chernoff. “Also, even if you can see the tumor on more than one sequence, you may be able to do an even better differential diagnosis of what this tumor is—based on the way it enhances.”

Dynamic contrast-enhanced MRI also can be used to differentiate slowly enhancing lesions; for example, hematoma and hemorrhagic sarcoma, as well as cystic and aneurysmatic lesions.

When patients come in with cystic lesions of the pancreas, and they also have a family history of pancreatic cancer, 1.5T MRI exams are useful, says Bhosale. If a CT scan and biopsy with endoscopic ultrasound proves indeterminate, an MRI study can provide the radiologist with molecular and metabolic information not available with the other modalities.

Another application of MR-based cancer imaging include proton spectroscopy, which can examine the biochemistry of brain tumors.

MRI-guided biopsies of the liver and muscles are particularly useful because of their superior capability to reveal tumors that are not seen via other modalities, Bhosale notes.

“[MRI] helps in many, many different situations,” Bhosale observes. “Especially for the surgeons: It helps them in planning the treatment, and how they’re going to do radiation therapy.”