Monday, August 26, 2013

CT Scan vs MRI

CT Scan (or CAT Scan) and an MRI operate differently and are better suited for different types of diagnoses. An MRI suited for examining soft tissue, (e.g. ligament and tendon injury, spinal cord injury, brain tumors etc.) while a CT scan is better suited for bone injuries, lung and 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.
An MRI typically costs more than a CT scan. One advantage of an MRI is that it does not use radiation while CAT scans do. This radiation is harmful if there is repeated exposure.

Comparison chart</> EMBED THIS CHART

Improve this chartCT ScanCT ScanMRIMRI
Cost:CT Scan costs range from $1,200 to $3,200; they usually cost less than MRIs (about half the price of MRI).MRI costs range from $1200 to $4000 (with contrast); which is usually more than CT scans and X-rays, and most examining methods.
Time taken for complete scan:Usually completed within 5 minutes. Actual scan time usually less than 30 seconds. Therefore, CT is less sensitive to patient movement than MRI.Scanning typically run for about 30 minutes.
Radiation exposure:The effective radiation dose from CT ranges from 2 to 10 mSv, which is about the same as the average person receives from backgroundradiation in 3 to 5 years. Usually, CT is not recommended for pregnant women or children unless absolutely necessary.None. MRI machine control/limit energy deposition in patient
Ability to change the imaging plane without movingthe patient:With capability of MDCT, isotropic imaging is possible. After helical scan with Multiplanar Reformation function, an operator can construct any plane.MRI machines can produce images in any plane. Plus, 3D isotropic imaging also can also produce Multiplanar Reformation.
Effects on the body:Despite being small, CT can pose the risk of irradiation. Painless, noninvasive.No biological hazards have been reported with the use of the MRI.
Scope of application:CT can outline bone inside the body very accurately.MRI is more versatile than the X-Ray and is used to examine a large variety of medical conditions.
Application:Suited for bone injuries, Lung and Chest imaging, cancer detection. Widely used in Emergency Room patient.Suited for Soft tissue evaluation, e.g. ligament and tendon injury, spinal cord injury, brain tumors etc.
Details of bony structures:Provides good details about bony structuresLess detailed compared to X-ray
Acronym for:Computed (Axial) TomographyMagnetic Resonance Imaging
Details of soft tissues:A major advantage of CT is that it is able to image bone, soft tissue and blood vessels all at the same time.Much higher soft tissue detail as compare to CT scan.
Principle used for imaging:Uses X-rays for imagingUses large external field, RF pulse and 3 different gradient fields
History: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.First commercial MRI in 1981, with significant increase in MRI resolution and choice of imaging sequences over time.
Principle:X-ray attenuation was detected by detector & DAS system, follow by math. model (back projection model) to calculate the value of pixelism then become a image.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.
Image specifics:Good soft tissue differentiation especially with intravenous contrast. Higher imaging resolution and less motion artifact due to fast imaging speed.Demonstrates subtle differences between the different kinds of soft tissues.
Limitation for Scanning patients: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 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.
Intravenous Contrast Agent:Non-ionic iodinated agent is covalently bind the iodine and have fewer side effects. Allergic reaction is rare but more common than MRI contrast. Risk of contrast induced nephropathy (especially in renal insufficiency(GFR<60 class="IL_AD" id="IL_AD8" nbsp="" span="" style="background-attachment: scroll !important; background-color: transparent !important; background-image: none !important; background-position: 0% 50%; background-repeat: repeat repeat !important; border-bottom-color: rgb(0, 153, 0) !important; border-bottom-style: dotted !important; border-bottom-width: 1px !important; color: rgb(0, 153, 0) !important; cursor: pointer !important; display: inline !important; float: none !important; padding: 0px 0px 1px !important; position: static;">diabetes
 & dehydratio
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.


How the scans work

EDITHow MRIs work

Using a very powerful magnet and pulsing radio waves the detection coils in the MRI scanner read the energyproduced by water molecules as they re-align themselves after each RF alignment pulse. The collected data is reconstructed into a two-dimensional illustration through any axis of the body. Bones are virtually void of water and therefore do not generate any image data. This leaves a black area in the images. MRI scanners are best suited for imaging soft tissue.
The CT scan of a person's torso.
The CT scan of a person's torso.

EDITHow a CT Scan works

CT, Computerized Axial Tomography, uses xrays to generate images of the body, including bone. In the CT scanner the x-ray tube, (source) rotates around the patient laying on the table. On the opposite side of the patient from the tube is the x-ray detector. This detector recieves the beam that makes it through the patient. The beam is sampled via some 764 channels, (approximate number of channels). The signal received by each channel is digitized to a 16 bit value and sent to the reconstruction processor. Measurements are taken about 1000 times per second. Scan rotations are usually 1 to 2 seconds long. Each view/channel chunk of scan data is compared to calibration scan data of air, water and polyethelyne, (soft plastic) previously acquired in the exact same relative location. The comparisons allow the image pixels to have a known value for a particular substance in the body regardless of differences in patient size and exposure factors. The more samples, or views, the better the picture.
The following video explains how the different types of scans work — Ultrasound, CT scan, MRI and PET scan.

EDITPros and Cons

In the following ABC News video, Paul Christo, M.D. at Johns Hopkins explains the use of MRI or CT scans for diagnosing problems related to the spine.

EDITAdvantages of MRI over CAT Scan

  • A CAT scan uses X rays to build up a picture. MRI uses a magnetic field to do the same and has no known side effects related to radiation exposure.
  • MRI has much higher detail in the soft tissues.
  • One of the greatest advantages of MRI is the ability to change the contrast of the images. Small changes in the radio waves and the magnetic fields can completely change the contrast of the image. Different contrast settings will highlight different types of tissue
  • Another advantage of MRI is the ability to change the imaging plane without moving the patient. Most MRI machines can produce images in any plane.
  • Contrast agents are also used in MRI but they are not made of iodine. There are fewer documented cases of reactions to MRI contrast and it is considered to be safer than X-ray dye.
  • For purposes of tumor detection and identification, MRI is generally superior. However, CT usually is more widely available, faster, much less expensive, and may be less likely to require the person to be sedated or anesthetized.
  • CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh (iodine, barium). Contrast agents for MRI are those which have paramagnetic properties. One example is gadolinium. Iodine use may be associated with allergic reactions.

EDITAdvantages of CT Scan over MRI

  • CT is very good for imaging bone structures.
  • Some patients who have received certain types of surgical clips, metallic fragments, cardiac monitors or pacemakers cannot receive an MRI.
  • The time taken for total testing is shorter than taken by MRI
  • MRI cannot be done in patients who are claustrophobic as the patient has to remain inside the noisy machine for about 20-45 minutes
  • It is cheaper than an MRI. A CT scan costs $1,200 to $3,200 while an MRI can cost up to $4,000.

EDITCost of Machines

Not surprisingly, there are various CT scanners available and there is a large variation in price depending upon the features and brand. This is a good pricing guide for CT scan machines. A vanilla 4-slice CT scanner costs $85,000 to $150,000. A 16-slice scanner costs $145,000 to $225,000 and the top-of-the-line 64-slice CTs can cost up to $450,000. The machines also typically need annual maintenance, which can also cost tens of thousands of dollars.
MRI machines are available in 1.5 T and 3 T (T stands for Tesla) models. 3T models are more expensive but offer higher image quality and shorter scanning times. 1.5 T MRI scanners start at around $1 million and 3T models are 50% more expensive. Manufacturers may include accessories in their quote, which include a workstation to view images, contrast injectors, This is a good guide for MRI scanners.

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