CT and PET Scan vs MRI

Accurate diagnosis can depend upon the equip­ment used.

Unable to interpret the results them­selves, for some physi­cians, an MRI is a figurative black hole into which a patient may be inserted to see what comes out. This often occurs without a definitive diagnosis. The hope is that the radiologist, or more importantly the examiner of the images, will rule out major illnesses. But using the wrong equip­ment can lead to inaccurate and costly conclusions.

Sometimes the terms CT and MRI are used interchangeably. What’s the difference between a CT scan and an MRI? Each machine may resemble 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 3D picture of functional processes in the body. PET scans provide metabolic informa­tion and are increas­ingly read alongside CT or MRI (magnetic re­so-nance imaging) scans, which provide anatomic infor­ma­tion.

Patient out-of-pocket costs vary considerably depend­ing upon health plans. Pricing has declined drama­tically over the years. View mone­tary values mere as relative comparisons between various technologies.

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.
Update: Outdated prices are for relative comparison only. An MRI can now be obtained for $300 to $800.
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: No radi­a­tion. 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 re­so-nance" 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. 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 and dehydration). 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 and 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.

Human images from world’s first quick total-body scanner —UC Davis

How Accurate Is An MRI Report?

True or false? A false negative is when a test reveals favor­able news that contra­dicts empirical evi­dence. Conversely, a false posi­tive occurs when an incorrect unfavor­able condi­tion is reported. One might assume that results from a million-dollar piece of equip­ment would represent an authori­ta­tive conclusion. Quality of the imaging coils put around the body part being scanned and the computer programs used to control the imaging and to analyze the images are important. But perfectly tuned equip­ment is only as reliable as the person who views the images and prepares the reports.

Statistics From Clinical Studies

Radiologist Emmanuelle Bouic Pagès, MD, and colleagues at CHU Lapeyronie, Montpellier, France, reported in the magazine Radiology that potential observer error existed in 47% of breast MRI studies performed from January 2005 to December 2010. Medscape clarifies that the many critics who came to the defense of the equip­ment may not have paid attention to the report content.

“Machines don’t make diagnoses; people do,” Herbert Y. Kressel, MD, editor of Radiology, explained to Medscape. “This is a study about errors made by people using the results of MRI exams.” Mis­inter­pre­ta­tion produced most of the false-positive results, according to the study. With false-positive readings, a patient may undergo an unnecessary mastec­tomy or amputation.

It is hoped that the error rate is much lower wherever we might have tests performed. To determine if better resolu­tion (higher Tesla) improves evalua­tion, a study by Grossman JW, et al. compared accuracy rates of 3-T and 1.5-T MRI in diag­nosing medial and lateral menis­cal tears in 200 patients. Fifteen of the 26 missed menis­cal tears were not seen in retrospect even with know­ledge of the tear type and location. The study concluded compar­able accuracy of 3-T and 1.5-T MRI.

A study by Akiko Shimauchi, et al. of 220 sequen­tially diag­nosed breast cancer lesions found seven (3.2%) false negative results—consi­der­ably fewer than other published studies. Although the overall sensi­ti­vity of cancer detec­tion was high (96.8%), it should be empha­sized that a nega­tive MRI should not influence the manage­ment of a lesion that appears to be of concern on physical examina­tion or seen by other imaging methods.

A negative MRI should not influence the care of a lesion.

Living up to his name, an ortho­pedic surgeon who blogs under the nom de plume Angry Othopod vents:

“Did you know that a growing number of doctors don’t even read the tests themselves? MRI is unnecessarily overused. In a study of 221 patients who had MRIs, the results showed that only 5.9% actually needed to have an MRI done.… If you suspect your doctor is just being quick or using MRI to reach that ‘aha’ moment, then you’re in a bad scenario.”

“When I order an MRI, I am 90% certain about what the results are going to show. Doctors need to have a clear-cut idea on what they can expect to see from the results. Next time you’re told to get an MRI, and your doctor has little clue to your diagnosis, you may want to get a second opinion. Also, be sure to ask the physician if they read the MRI themselves.”

Unless it is of considerable volume, a mass detected on an x-ray, MRI, or CT scan is often a dot or speck requiring a judgment call. Determining whether a dot is an anomaly depends upon the skill, experience and alertness of the technician. Add these variables to the need for properly tuned equip­ment and you can see there is a margin for error. ”Advances in AI algorithms have narrowed the gap between computers and human experts in detecting breast cancer in digital mammograms,” Ioannis Sechopoulos, PhD, of Radboud University Medical Centre in Nijmegen, the Netherlands, and colleagues wrote. If a radio­lo­gist reports back with positive results, get a second opinion. When results are negative, a second opinion is still advised.

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Kevin said:

Rheumatology Network. Getting the Most Out of 3 Tesla MRI of the Spine. By Filippo Del Grande, MD, MBA, Avneesh Chhabra, MD, and John A. Carrino, MD, MPH. March 2, 2012 — The main advantage of 3 Tesla MRI scans can be summarized in 1 phrase: signal to noise ratio (SNR). This, the ratio between the intensity of the image’s signal and the background noise, is roughly double that of 1.5 Tesla MRI. In the spine, high spatial and high contrast resolution play a special role in detecting nerve root, spinal cord, and neuroforaminal pathologies. The higher SNR also allows for higher spectral resolution; this is a substantial advantage in proton MRS and other advanced techniques. In addition, the higher contrast resolution may be useful in decreasing the dose of paramagnetic contrast needed for enhanced imaging.

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