Diagnostic Imaging: CT - MRI - PET - Mammography - Bone Scan
Imaging provides a non-invasive and painless way of visualizing tissues and organs in the body so that abnormalities can be identified. There are many different techniques for generating images. Some techniques utilized for detecting or diagnosing cancer including:
- Radiography (x-ray)
- Bone Scan
- Dual Energy X-ray Absortiometry (DEXA) Scanning
- Positron emission tomography (PET) scan
- Magnetic resonance imaging (MRI)
- Computed tomography (CT) scan
- Combined CT/PET scan
Radiography x ray
Radiography involves the use of radiation (x-rays) to create an image of the body. Radiographs are created by passing small, highly controlled amounts of radiation through the human body, capturing the resulting image on a special type of photographic film. Radiation passes through the various structures of the body differently. For example, very little radiation passes through the bones, leaving white “shadows” on the x-ray film. This is why X-rays are very useful for evaluating bones, as in detecting fractures.
X-rays are useful for determining whether cancer has spread (metastasized) to the bones. Because cancer cells are so dense and metabolically active, tumors, or masses of cancer cells, may also appear white on an x-ray, as is the case with lung cancer.
A type of x-ray called a bone scan may be performed to diagnose cancer in the bones or bone metastases. In this test, low level radioactive particles are injected into a vein. They circulate through the body and are selectively picked up by the bones. A high concentration of these radioactive particles indicates the presence of rapidly growing cancer cells in the bones.
Skeletal survey A skeletal survey may be performed to diagnose cancer in the bones that causes extra build-up of bone, called blastic lesions. A skeletal survey is a type of X-ray. Conventional X-rays are used to image small sections of the body that may be of concern, such as the spine; whereas skeletal surveys image all areas of the body.
Dual Energy X ray Absortiometry DEXA Scanning
DEXA scanning is the most widely used method for measuring bone mineral density. Bone density may weaken with bone metastases, cancer that has spread to the bones, or with osteoporosis, a weakening of the bones related to aging. DEXA scanning rapidly directs x-ray energy, alternating from two different sources, through the bone being examined. Once the x-rays have passed through the bone, their strength is recorded. Bone density or bone loss is calculated from the amount of energy that travels through the bone and is picked up by the detector. The minerals in bone, predominantly calcium, weaken the transmission of the x-rays through the bone. The more dense the bone is, the less x-rays get through to the detector. The use of two different x-ray energy sources greatly improves the precision and accuracy of the measurement.
Mammography uses safe, low doses of X-rays to image the inside of the breast. During a mammogram, the breast tissue will be compressed with a smooth plastic shield in order to help produce a highly detailed image. The X-rays pass through the breast and form an image on the X-ray film. Typically, two or three images are made of each breast.
Ultrasound uses high frequency sound waves and their echoes to create an image. The primary advantage of ultrasound is that internal organs and other structures can be observed without using radiation. The ultrasound machine transmits sound pulses into the body using a probe. The sound waves travel through the body until they hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). At the boundary, some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected. The reflected waves are detected by the probe and relayed to the machine, which calculates the distance from the probe to the tissue or organ. The machine displays the distances and intensities of the echoes on the screen, forming a two-dimensional image.
Newer ultrasound machines are capable of creating three-dimensional images. In these machines, several two-dimensional images are acquired by moving the probes across the body surface or rotating inserted probes. The two-dimensional scans are then combined by specialized computer software to form 3D images. 3D imaging allows the physician to see the organ being examined better, and is often used for early detection of cancer in the prostate, colon, rectum, and breast.
Ultrasound that is enhanced with the use of additional technologies appears to be even more effective for detecting cancer. A new development in ultrasound involves the use of color Doppler imaging. Doppler imaging is a technique that can detect differences in velocity (i.e. blood flow versus solid tissue) and transmits these differences in the form of different colors on a screen. This technique allows physicians to better determine the presence and exact location of a mass within the body.
Another type of ultrasound is microbubble-enhanced color Doppler, which has been shown to improve the detection of some cancers and reduce unnecessary biopsies compared to color Doppler that is not enhanced. Microbubbles are tiny bubbles of gas that can permeate through small blood vessels without causing harm. Since blood vessels and blood flow are more prevalent in cancerous tissues than regular tissues, microbubbles tend to concentrate in the cancer, which is revealed on the ultrasound image. This allows physicians to more accurately locate where to do the biopsy.
Positron emission tomography PET
Unlike techniques that provide anatomical images, such as X-ray, CT and MRI, PET scans show chemical and physiological changes related to metabolism. This is important because these functional changes often occur before structural changes in tissues. PET images may therefore show abnormalities long before they would be revealed by X-ray, CT, or MRI.
Before a PET scan, a patient will receive an injection of a radiopharmaceutical, which is a drug labeled with a basic element of biological substances, called an isotope. These isotopes distribute in the organs and tissues of the body and mimic natural substances such as sugars, water, proteins, and oxygen. This radioactive substance is then taken up by the cancer cells, thereby allowing the radiologist to visualize areas of increased activity.
After the patient has received the injection, a small amount of radiation is passed through the body, which detects the isotopes and reveals details of cellular-level metabolism. Although the radiation is different from that used in radiography, it’s roughly equivalent to what is administered in two chest x-rays. After the scan is complete the radiation does not stay in the body for very long.
PET is useful for diagnosing lung and breast cancer, and for monitoring response to therapy. Effective therapy leads to rapid reductions in the amount of glucose that is taken up by tumors. PET imaging can easily reveal this drop in metabolic activity and show—sometimes within minutes or hours—whether a patient is responding positively to a particular course of treatment. PET has been shown effective for predicting outcomes, detecting spread of cancer, and/or monitoring therapeutic response in a wide range of cancers, including breast, colon, lung, ovarian, head, neck, and thyroid cancers, as well as melanoma and lymphoma.
Magnetic resonance imaging MRI
MRI uses a strong magnet and radiofrequency waves to produce an image of internal organs and structures. Under the influence of the strong magnet, the hydrogen atoms in the body line up like compass needles. Next, the patient is exposed to radio waves that cause the hydrogen atoms to momentarily change positions. In the process of returning to their orientation under the influence of the magnet, they emit a brief radio signal. The intensity of these radio waves reflects what type of tissue exists in that area of the body. The MRI system goes through the area of the body being imaged, point by point, collecting information from how the radio waves emit. A computer generates an image of organs and structures based on these radio wave recordings.
MRI has proven useful for detecting some types of cancer, and in some cases, may be more effective than biopsy, mammography, or ultrasound.
Computed tomography CT
A CT scan is a detailed radiograph, or X-ray. The CT imaging system is comprised of a motorized table that moves the patient through a circular opening and an X-ray machine that rotates around the patient as they move through. Detectors on the opposite side of the patient from where the X-ray entered record the radiation exiting that section of the patient’s body, creating an X-ray “snapshot” at one position (angle). Many different “snapshots” are collected during one complete rotation of the X-ray machine. A computer then assembles the series of X-ray images into a cross-section, or a picture of one small slice of the body. A CT scan is a series of these cross-sectional images.
PET CT combination scan
Recent research indicates that a combination PET/CT scan may be more effective than whole body MRI for diagnosing the extent of spread for various cancers. Researchers from Germany conducted both combination PET/CT and MRI on 98 patients with various cancers. Overall, PET/CT scanning was 77% accurate for detecting the original cancer, cancer spread to nearby lymph nodes, and cancer spread to distant sites in the body, compared with only 53% accuracy with MRI.(1)
- Antoch G, Vogt FM, Freudenberg LS et al. Whole-Body Dual-Modality PET/CT and Whole-Body MRI for Tumor Staging in Oncology. Journal of the American Medical Association 2003;290:3199-3206.