- Ultrasound
- X-rays
- CT scans (conventional and high resolution)
- PET scans
- MRI
Ultrasounds
Ultrasounds use high frequency sound waves to measure tissue density. Many types of cancer will show up with an ultrasound as an area of uneven density (often as denser tissue). Ultrasounds do not use any form of ionizing radiation, and so will not cause any harm to tissues. Ultrasounds are most often used to generate images of an unborn child and so are widely available.
The main limitations of ultrasounds include the following:
- An inability to view the interior of bony structures. Ultrasound can also be blocked by bone, and so is not useful in imaging past the ribs of the chest.
- An inability to pass through air. Ultrasound imaging will not be useful for evaluating the digestive system (stomach, intestines, pancreas, aorta).
- The clarity of an ultrasound attenuates as it passes through tissue, and so will not be useful for imaging deep within the body. As such, ultrasounds will be less effective with obese patients.
- The resolution of an ultrasound is fairly limited, and so will not be able to detect microcalcifications within breast tissue.
- The accuracy of an ultrasound depends to a large degree on the skill of the technician and radiologist.
Ultrasound imaging is fast an inexpensive.
X-Rays
X-rays use a form of ionizing radiation to generate two-dimensional images. They are very effective in imaging bone and dense tissues, but are not effective at imaging soft tissues. In some cases the patient may be given oral or IV contrast to make it easier to discern soft tissues. A mammogram is a form of x-ray used to look for signs of breast cancer.
A key limitation of x-rays is the two dimensional nature of the images. This can lead to false positives when an area of diffuse or stacked densities causes the appearance of "heaviness" in one section of the x-ray image. This is because all of the densities are projected into the plane of the 2-dimensional image in an additive fashion, causing phantom anomalies. For this reason technicians often take two images, one from the front and one from the side. (Even this isn't perfect, and it might be better to also take images at a 45-degree angle.)
Another limitation is that X-rays are not good at distinguishing anomalies in dense tissue. This is why mammograms are less effective in younger women, who have denser breast tissue.
The resolution of an x-ray is not very high.
X-ray imaging is fast and inexpensive.
X-rays are not recommended for pregnant women.
CT Scans
CT Scans are like x-rays, but take many more images in a precise and controlled fashion that permits the reconstruction of a 3-dimensional image. CT Scans are often taken in 'slices'. The patient may be given oral constrast and IV contrast to enhance the imaging of soft tissues.
Occluding factors aren't as much of an issue with CT scans as they are with x-rays, because the CT takes multiple fan-shaped images at each point as it rotates around the body, which is enough for a computer to reconstruct both the position and density of the objects in the body that absorb the x-rays.
Conventional CT scans are slow and require the patient to be immobilized during the scan. If the chest is being imaged, the patient may be told to hold his or her breath. Alignment problems between slices can sometimes miss a small mass that lies between slices. A conventional CT scan has a slice thickness of 8 mm to 10 mm.
Higher resolution CT scans, known as HRCT, helical CT and spiral CT, are much quicker. Instead of scanning each slice with the patient bed stationary, the bed is moving continuously as the CT scanner scans around the body (i.e., in a spiral pattern). This is fast enough that the patient doesn't need to hold his/her breath. It is also much higher resolution than conventional CT, so in many cases contrast dye will not be needed. A HRCT scan has a slice thickness of 1 mm to 1.3 mm.
A key limitation of HRCT is the higher resolution can lead to false positives. At higher resolutions more anomalies can be seen, but a much greater number of them turn out to be benign.
Another limitation of CT scanning in general is an inability to view very fine details in soft tissues such as muscles or ligaments. An MRI might be more appropriate in such situations.
Patient motion during a CT scan can cause the images to be "blurry". Also, any metal artifacts in the body can cause 'streaks' in the image. Flat tumors may also be harder to image with a CT scan. Scar tissue may also show up on a CT scan.
CT scanning is slower and more expensive than x-rays and ultrasounds. The radiation exposure from a whole-body CT scan is approximately 100 times that of a chest x-ray.
CT scans are not recommended for pregnant women. The intravenous contrast dye may not be recommended for patients who are allergic to iodine or shellfish.
PET Scans
While a CT scan uses an external source of radiation to image the body, a PET scan uses an internal source of radiation. Otherwise the imaging equipment is quite similar. About 45 minutes to an hour before the scan, the patient will be injected with a form of radioactive sugar. (In some cases the patient will be given the radioactive sugar two hours before the scan, to give more time for the sugar to be absorbed. This yields a more detailed image.) After the sugar has had a chance to be absorbed by the body, the patient will be scanned in 5-7 sections. Since the amount of radiation is very low, the PET scanner needs to focus on each section of the body for 20-30 minutes. The patient must not move during this time period, and often is strapped in very tightly. (PET scans are not fun for claustrophobic patients or for patients who develop a maddening itch on the nose.)
Hot spots on the PET scan show areas of metabollic activity. This is very useful in distinguishing active cancer from fibrotic (scar) tissue. False positives, however, can occur due to inflammatory lesions due to recent surgery or chemotherapy. One generally waits at least six weeks after the end of chemotherapy before conducting a PET scan. With some cancers the wait may be as much as 12 weeks, in order to allow enough time for a response to become evident.
Since the PET scan involves the use of radioactive sugar, it is important that the patient not eat before the scan. The quality of a PET scan can also be adversely affected by diabetes. Patients should not take any diabetic or glucose control medication before a PET scan.
Since PET scans and CT scans use similar equipment, it is possible to integrate them into a single PET-CT scanner. A PET-CT scan can yield more information than either scan on its own.
Since a PET scan measures metabollic activity, it may not be as effective for identifying tumors that have a low metabollic rate, such as carcinoid tumors, mucinous cancers (e.g., ovarian cancer), and low grade tumors (e.g., bronchioaveolar cell carcinoma fo the lung). PET scans are particularly useful in imaging the lungs and in measuring response to treatment for cancers that exhibit high FDG uptake.
Because of the length of time required to generate the image, patient movement and breathing can generate a blurry image.
Although the false positive rate for PET scans is low for many cancers, the false negative rate isn't zero. So a negative PET scan result does not mean that there is no cancer.
The radiation from a PET scan lasts for only a short period of time and isn't very strong. The maximum resolution of a PET scan is 5 mm to 7 mm. As such, a PET scan is usually not used to identify tumors smaller than 1 cm in size.
PET scans are not recommended for pregnant women.
MRI
An MRI uses strong magnetic fields to cause water molecules to resonate. It is very good at imaging soft tissues, such as the heart, lungs, liver and other organs. An MRI cannot image calcifications and bone and cannot always distinguish between cancerous and non-cancerous anomalies. An MRI can image anomalies that are obscured by bone. Patients should notify the technician about any metal implants in their bodies.
MRI does not involve radiation. MRI is not recommended during the first 12 weeks of pregnancy.
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