CT and PET Scan vs MRI

Accurate diagnosis can depend upon the equipment used.

By Kevin RR Williams

HEALTH What's the difference between a CT scan and an MRI? Each resembles a donut hole that encircles a patent. But which is better suited for locating cancers within the body? This can depend upon where tumors may be located. Multiple Myeloma, for example is a bone cancer that shows up better in a CT (or CAT) scan since, much like a high-resolution x‑ray, it is best suited for bone injuries, lung or chest imaging, and detecting cancers. CT scans are widely used in emergency rooms because the procedure takes less than 5 minutes.

An MRI, on the other hand, can take up to 30 minutes. It excels in distinguishing soft tissues (i.e. ligament and tendon injury, spinal cord injury, brain tumors etc.). One advantage of an MRI is that it does not use radiation while CT scans do. This radiation is harmful if there is repeated exposure, so MRIs may be used to evaluate progress during a course of cancer treatment.

A PET scan uses nuclear medicine imaging to produce a three-dimensional picture of functional processes in the body. PET scans provide metabolic information and are increasingly read alongside CT or MRI (magnetic resonance imaging) scans, which provide anatomic information.

Radiology Comparison Chart

CT (CAT) Scan
CT Scan
PET Scan
PET Scan
Cost: MRI costs range from $1200 to $4000 (with contrast); which is usually more than CT scans and X-rays, and most examining methods. CT scan costs range from $1,200 to $3,200; they usually cost less than MRIs (about half the price of MRI). PET scans cost $3,000 to $6,000; much higher than regular CT scans.
Time taken for complete scan: Scanning typically runs about 30 minutes. Usually completed within 5 minutes. Actual scan time usually less than 30 seconds. Therefore, CT is less sensitive to patient movement than MRI. Usually takes 2 to 4 hours.
Radiation exposure: None. MRI machine control/limit energy deposition in patient The effective radiation dose from CT ranges from 2 to 10 mSv, which is about the same as the average person receives from background radiation in 3 to 5 years. Usually, CT is not recommended for pregnant women or children unless absolutely necessary. Moderate to high radiation.
Effects on the body: No biological hazards have been reported with the use of the MRI. Despite being small, CT can pose the risk of irradiation. Painless, noninvasive. Radiation risk from the injection of a radioactive tracer is about the same as an X-ray.
Scope of application: MRI is more versatile than the X-ray and is used to examine a large variety of medical conditions. CT can outline bone inside the body very accurately. PET scans can image biological processes within the body.
Acronym for: Magnetic Resonance Imaging Computed (Axial) Tomography Positron Emission Tomography
History: First commercial MRI in 1981, with significant increase in MRI resolution and choice of imaging sequences over time. The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom; the first patient brain-scan was done on 1 October 1971. The compound was first administered to two normal human volunteers by Abass Alavi in August 1976 at the University of Pennsylvania.
Principle used for imaging: Body tissue that contains hydrogen atoms (e.g. in water) is made to emit a radio signal which is detected by the scanner. Search for "magnetic resonance" for physics details. Uses X-rays for imaging Radioactive tracers that emit positrons are used. The positrons are tracked by the system to generate a 3D image over time.
Limitation for Scanning patients: Patients with cardiac pacemakers, tattoos and metal implants are contraindicated due to possible injury to patient or image distortion (artifact). Patient over 350 lb maybe over table weight limit. Any ferromagnetic object may cause trauma/burns. Patients with any metal implants can get CT scan. A person who is very large (e.g. over 450 lb) may not fit into the opening of a conventional CT scanner or may be over the weight limit for the moving table.
Intravenous Contrast Agent: Very rare allergic reaction. Risk of nephrogenic systemic fibrosis with free Gadolinium in the blood and severe renal failure. It is contraindicated in patients with GFR under 60 and especially under 30 ml/min. Non-ionic iodinated agent covalently binds with fewer side effects. Allergic reaction is rare but more common than MRI contrast. Risk of contrast induced nephropathy (especially in renal insufficiency (GFR<60), diabetes & dehydration).

Not all equipment is created equally. CT scanners range from 4‑slice to 16- or 64‑slice units. MRI machines are available in 1.5‑T (Tesla) and 3‑T with higher image quality and shorter scanning times. An experienced oncologist is more likely to be familiar with and have access to better equipment. A diagnosing factor that is equally as important as the equipment is the skill of the one interpreting the images. For critical decisions, a second radiologist reading may be warranted.

Related: Benefits of Dynamic MRI

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