X-Ray
X-ray exams use a stationary X-ray machine to focus beams of radiation on a specific area of the body to produce 2-D images. X-rays are particularly useful in identifying pathology of the skeletal system, but they can also be used for some soft-tissue imaging. For example, chest x-rays are often used in the diagnosis of lung cancer, pneumonia and pulmonary edema. Major disadvantages of X-rays include their production of overlapping images that can be difficult to interpret, their limitations on soft tissue imaging, and their use of ionizing radiation, which in large doses is carcinogenic. X-ray is the least expensive imaging option.

Computed Tomography (CT)
A CT scan utilizes an X-ray tube, which rotates axially or helically around the body and a diametrically opposed array of detectors that detect residual radiation traversing the body, to produce cross-sectional images of the inside of the body. CT is superior to conventional X-ray due to its increased contrast resolution (Conventional X-rays can detect differences in physical density of approximately 5%, whereas CT can detect differences of less than 0.5%), and its elimination of the superimposition of structures outside the area of interest. However, CT scans, like X-rays do expose patients to ionizing radiation. It is estimated that CT scan use may cause up to 1 in 50 future cases of cancer and thus, CT is considered a moderate to high radiation diagnostic technique and is not recommended for pregnant women.
CT scans are the recommended form of imaging for the skull base, chest, abdomen and pelvis, especially in the following instances:


• Examination of the larynx and thyroid
• Evaluation of acute intracranial hemorrhage
• Diagnosis of sinusitis, otitis, conductive hearing loss and erosion
• Chest examination following abnormal chest x-ray
• Cancer staging in the chest, abdomen and pelvis
• Evaluation of abdominal or pelvic pain, mass or other dysfunction


CT scans are more widely available, less expensive, and much quicker than MRIs. Because CT scans usually take less than a minute of scan time (vs. 20 minutes for an MRI) during which the patient must remain still, they are often the method of choice for young children and claustrophobic patients in order to avoid the need to sedate patients. Also, CT scans can safely be used for patients with pacemakers and other metallic implants, who cannot undergo MR imaging.

Magnetic Resonance Imaging (MRI)
MRI uses a combination of magnetism and radio waves to create detailed cross sectional images of the body. When a patient lies in an MRI scanner, a powerful magnetic field aligns the abundance of hydrogen atoms in the water molecules of the bodily tissues with this strong magnetic field. Then, an electromagnetic field, perpendicular to the main field, oscillates at varying radiofrequencies to disalign a portion of the hydrogen atoms from the main field. As these atoms come back into alignment with the main field, they emit radiofrequency signals, which can be recorded. Hydrogen atoms in different bodily tissues realign at varying speeds and thus, create measurably different signals, which can be converted into a clear image of the soft tissues of the body.

Unlike CT, MRI does not use x-rays and thus, does not expose patients to ionizing radiation. MRI also produces more detailed images than CT by providing much greater contrast between soft tissues of the body. However, CT is preferred for solid tumors of the abdomen and chest due to less motion artifact. MRI is also the most expensive type of imaging and it cannot be used in patients with metallic implants or occasionally, patients who are claustrophobic or obese.

MRI is preferred for brain, skeletal and spine imaging, especially in the following instances:
• Examination of the brain for tumors, masses, strokes, etc.
• Evaluation of orbit, sensoryneural hearing loss, pituitary gland and cranial nerves
• Evaluation of the spinal cord
• Evaluation of all joints, including the soft tissues, such as tendons, ligaments, and menisci
• Cancer staging in the soft cervical tissue
• Determination of presence, location and size of uterine fibroids
• Provision of additional information if CT findings are nondiagnostic
• Necessary examination of pregnant women (unlike X-ray and CT, MRI does not use ionizing radiation, which can be particularly harmful to the developing fetus)
• Diagnosis and monitoring of congenital defects

There are two main types of MRI:
1. T1-weighted scans, in which fat containing tissues are bright and water or fluid containing tissues are dark

2. T2-weighted scans, in which fat containing tissues are dark and water or fluid containing tissues are bright. T2-weighted scans are especially sensitive for pathology, because damaged tissue tends to develop edema and thus, appears bright.

Resources
Chernoff, D., Stark, P., Muller, N. & Yeon, S. B. Principles of Magnetic Resonance Imaging (MRI). Up-to-date. Retrieved on July 16, 2008.

Stark, P., Muller, N. & Wilson, K.C. Principles of conventional and helical CT scanning. Up-to-date. Retrieved on July 16, 2008.

ThumbMRI. Retrieved on July 16, 2008 at http://www.thumbmri.com/mrivscat.asp.

Website created by: Craig Johnson & Kendra Lesiak with assistance from Joel Michalski, Alex Lesiak, Gordon L. Todd, Ph.D., Paul Larsen M.D., and Robert Binhammer, Ph.D.