Unlike MRI, CT uses ionizing radiation to image the body. MRI uses powerful magnets and radio frequency. The MRI unit looks similar to a CT unit. For both, the patient is placed on a table that slides through the middle of the machine to obtain images of specific anatomy. The difference is the CT tube, where the patient is placed, is much shorter and the scans take much less time than in MRI.
Magnetic Resonance Imaging (MRI )
MRI uses a powerful magnet and pulsing radio waves (Radio Frequency or RF). The collected data is reconstructed into a two dimensional illustration through any axis of the body. Bone is virtually void of water and therefore does not generate any image data. This leaves a black area in the images. MRI scanners are best suited for imaging soft tissue.
Computed Tomography (CT)
CT, Computerized Axial Tomography, uses rotating x-rays to generate images of the body, including bone. The x-ray tube rotates around the patient lying on the table. On the opposite side of the patient is the x-ray detector. This detector receives the beam that makes it through the patient. The beam is sampled via 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 piece of scan data is compared to calibration scan data of air, water and polyethylene (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 obtained, the better the image.
Advantages of CT:
- CT works well for imaging bone structures.
- Some patients who have received certain types of surgical clips, metallic fragments, cardiac monitors or pacemakers cannot receive an MRI but can have a CT scan.
- The time taken for total testing is shorter in CT than for MRI.
- CT can be more comfortable for patients who are claustrophobic.
Advantages of MRI:
- A CAT scan uses x-rays to construct an image. MRI uses a magnetic field to do the same and has no known side effects related to radiation exposure.
- MRI has much higher detail for soft tissue structures.
- With MRI, we have the ability to adjust the sharpness of the black, white and gray of the images by making small changes in the radio waves and the magnetic fields. Different settings will highlight different types of tissue.
- MRI offers the ability to change the imaging plane without moving the patient.
- Contrast agents are used in both MRI and CT, however MRI contrast does not contain iodine.