FAQ: Imaging Modalities

FAQ: Imaging Modalities

What is the difference between X-ray and CT scan?

X-ray creates a two-dimensional (flat) image using a single beam of radiation. It is quick, inexpensive, and good for evaluating bones and lungs. CT scan takes multiple X-ray images from different angles and uses a computer to create detailed three-dimensional cross-sectional images. CT is more sensitive and can detect smaller abnormalities, but uses higher radiation and costs more. A doctor orders CT when X-ray is unclear or when more detail is needed.

Is ultrasound safe? Can it be used during pregnancy?

Yes, ultrasound is safe and is the preferred imaging modality during pregnancy. Unlike X-ray or CT, ultrasound does not use ionizing radiation. It is non-invasive and has no known harmful effects to the fetus when used appropriately. Ultrasound is used routinely in obstetrics to monitor fetal growth and detect birth defects.

Why does MRI take longer than CT?

MRI acquires data by using strong magnetic fields and radio waves, which is inherently slower than the rapid rotation of a CT scanner. A typical MRI may take 20–60 minutes depending on the sequences needed and anatomy being imaged. CT usually takes only a few minutes. The longer time is worth it for MRI because it provides excellent soft-tissue contrast and avoids ionizing radiation.

What is the radiation dose from a CT scan?

CT uses ionizing radiation, so exposure is a consideration. However, the radiation dose from a single CT scan is generally considered acceptable for diagnostic purposes because the clinical benefit usually outweighs the risk. Multiple repeated CT scans or CT in young patients should be justified and minimized when possible. If you have concerns about radiation exposure, discuss them with your doctor.

Can patients with metal implants have an MRI?

Some metal implants are MRI-safe, while others are not. Ferromagnetic implants (such as older pacemakers, certain aneurysm clips, or metallic prostheses) can be unsafe in an MRI because the strong magnetic field can move or heat the metal. Before any MRI, a safety screening questionnaire must be completed. If a patient has an implant, the MRI technologist will verify its safety with the device manufacturer or medical records. Never force an MRI on a patient with a non-safe implant.

What is the difference between a routine EEG and a sleep EEG?

A routine EEG is performed while the patient is awake and typically lasts 20–40 minutes. A sleep EEG is performed while the patient is asleep and may last 1–2 hours. Sleep EEG is more sensitive for detecting certain abnormalities (such as seizure activity during sleep) because abnormal brain rhythms are sometimes only visible during sleep. Sleep deprivation before a routine EEG sometimes activates hidden abnormalities.

Why might an endoscopy be therapeutic, not just diagnostic?

Endoscopy can both visualize internal structures and treat problems. For example, during a colonoscopy, a polyp can be removed (polypectomy), or bleeding can be stopped (hemostasis). During an upper endoscopy, a stricture can be dilated or a foreign body removed. The therapy is performed with tools passed through the scope. The doctor decides during the procedure whether therapy is possible and appropriate.

How should I prepare for different imaging studies?

Preparation varies by modality and region:

• X-ray: minimal preparation; remove jewelry and metal objects.
• Ultrasound: may require fasting for abdominal studies; for gynecologic ultrasound, a full bladder may be needed.
• CT: fasting required if contrast is used; metal objects must be removed.
• MRI: no fasting required; metal objects must be removed; safety screening for implants is mandatory.
• Colonoscopy: bowel cleansing prep is required (fasting and laxative); fasting usually 6+ hours.
• Upper endoscopy: fasting 6+ hours; sometimes a medication to thin secretions is given.
• EEG: no special preparation; hair should be clean for better electrode contact.

Always follow your doctor's or imaging center's specific instructions for your study.

What should I expect during imaging? Will it hurt?

Most imaging is painless:

• X-ray, CT, ultrasound, MRI: non-invasive; you may feel cold gel or hear loud noise, but no pain.
• EEG: painless; you may feel mild itching from electrode paste, but electrodes do not deliver electrical shock.
• Endoscopy: may involve mild discomfort or a gagging sensation, but sedation usually makes the patient comfortable and amnestic (not remembering the procedure).

What if I am claustrophobic and need an MRI?

MRI can feel confining because you lie inside a tube. If you are very claustrophobic, tell your doctor or MRI technologist. Options include:

• Using an "open" MRI machine (if available).
• Mild sedation to help you relax.
• Mental preparation and focusing on the technologist's voice through the intercom. Talk to your healthcare team about your concerns; they can help make the experience tolerable.