Imaging of Cancer

Pet scan of a lung mass

PET/CT of the same lung mass

MRI of liver tumors

The whole host of different imaging studies available can cause great confusion for cancer patients. Even most doctors do not have a good understanding of most imaging modalities available. I will give a brief overview of the imaging studies pertaining to cancer. I feel that one of the most important imaging studies when it comes to cancer is the PET/CT. I feel that this study is very important for most cancer patients to have at some point after their diagnosis. I often use PET/CT mapping to help guide the radiofrequency ablation procedure.

Plain Film X-rays

This includes chest x-rays, abdominal x-rays and x-rays of the bony structures. These are obtained when an X-ray beam passes through the body and strikes a film. The image is produced by the difference in absorption of the x-ray beam by the structures within the body. This study has its uses (looking at bones, looking for fluid within the lungs or abdomen) but it is not nearly as sensitive as a CT scan.

Ultrasound

This imaging technique uses sound waves to produce images. The sound waves are transmitted into the tissue and are reflected back to the transducer. The differences in absorption and reflection of the sound waves are used to generate images. This study is mostly used for looking at abdominal structures. It is very good for locating fluid within the chest or abdomen. It is generally not as good as CT for detecting cancer. It is not used to image the lungs (the air in the lungs reflects the soundwaves).

CT Scan (Computed Tomography)

The image from a CT scan is generated from a rotating x-ray tube and a series of detectors. The x-ray tube rotates around the body. The detectors capture the emitted x-rays. A computer is then used to process the information. This study gives good detail of the anatomy. Its strong points are imaging of the chest, abdomen and pelvis. It can also be useful for imaging the bones and brain. CT scans have become the most versatile and useful study in radiology.

PET Scan

This stands for positron emission tomography. This type of imaging has actually been around for about 30 years. It has only been within the last 8-10 years that PET has been making a true impact on cancer imaging. The idea behind PET is that there is an agent (usually FDG) that is taken up in higher amounts by cancerous cells. This agent emits positrons. The positrons collide with an electron and generate a photon. The photon is the imaged by the machine. This can be used to actually visualize activity of the cancer. Other imaging (like CT) can only visualize structural changes. For example a CT may detect an enlarged lymph node, but just because it is enlarge does not necessarily mean that it is cancerous. Also just because a lymph node is normal in size does not mean that it is not cancerous. PET images activity (viable cancer). A normal but large lymph node should appear normal on PET. A normal size, but cancerous lymph node should appear abnormal on PET. Keep in mind, even thought PET is very good for most tumors, there are a few cancer types that are not positive with PET imaging. Fortunately, these tend to be rare.

One important point with PET scans is that they do not give good images of the anatomy. It may be difficult to pin point the exact location of an abnormality on PET. This is where the new imaging device called a PET/CT really shines. A PET/CT give you the activity from a PET but the detailed anatomic views of a CT. This is done at the same time with a single machine. I feel that this has completely replaced PET imaging. I repeat PET scans should not be done. They should be replaced with a PET/CT when at all possible. Please read the section on PET/CT scans.

PET/CT scans

If you have read the other sections you already know that I feel that this is the most important imaging device in cancer. This scan has all the advantages of PET and CT rolled into one. The whole is definitely much greater than the sum of its parts. I feel that this study may be essential for people who are going to have radiofrequency ablation. I like to use the images before the procedure to map out the tumor for ablation. I like to follow up with a PET/CT 2-3 months after the ablation. This is to see what type of results we have obtained. It can also be used to target any additional cancer that may need to be ablated. This helps separate scar (ablated tissue) from viable cancer. The PET/CT mapping technique is key for ablating multiple or larger cancers. The PET portion shows cancer activity and the CT shows the exact location. This is key information when is comes to locating viable cancer for radiofrequency ablation.

MRI (Magnetic Resonance Imaging)

MRI uses radiofrequencies and magnetic forces to generate images. MRI gives excellent detail of certain soft tissue structures. MRI is very valuable in imaging the brain. MRI may also be very helpful when other imaging studies are non-diagnostic.

Bone Scans

Bone scans are used to detect spread of cancer to the bones. It is a very good study. There has been some recent studies that suggest that PET scans may be as sensitive as Bone scans for detecting cancer that has spread to the bones.