Computed Tomography (CT) Overview

Computed Tomography (CT)

• CT is a diagnostic medical imaging technique using X-rays
• It creates multiple images (or pictures) of the inside of the body
• It uses special X-ray equipment to assess head injuries, severe headaches, dizziness, and other symptoms of aneurysm, bleeding, stroke, and brain tumors.
• Images of internal organs, bones, soft tissue, and blood vessels are much more detailed than standard X-rays using contrast to show soft tissues and blood vessels well
• CT helps to evaluate the face, sinuses, skull and can plan radiation therapy for brain cancer.
• In emergencies, it reveals internal injuries and bleeding quickly, potentially saving lives.

CT Scanning of the Head

• CT scanning of the head can detect bleeding, brain injuries, and skull fractures.
• It can also pinpoint bleeding from a ruptured aneurysm.
• It is used to identify blood clots or bleeding in the brain after a stroke.
• CT scans can identify brain tumours.
• It is used to evaluate enlarged brain cavities (ventricles) that can indicate hydrocephalus.
• It can be used to diagnose diseases or malformations of the skull.

CT Scanning Uses

• Helps assess the extent of bone and soft tissue damage in facial trauma, planning reconstructive surgery
• Identifying temporal bone diseases which may cause hearing problems.
• Determining if inflammation is present in the paranasal sinuses.
• Planning radiation therapy for brain or other tissue cancers.
• Guiding needles for biopsies of the brain
• Assessing aneurysms, arteriovenous malformations, and blood vessels through a technique called CT angiography

Benefits of CT Scans

• CT scanning is painless, non-invasive and accurate.
• It allows viewing bones, soft tissues, and blood vessels simultaneously.
• CT scans provide highly detailed images of many tissues, including the lungs, bones, and blood vessels.
• CT scans are rapid and can quickly assess internal injury and bleeding in emergency situations.
• CT is shown to be a cost-effective imaging tool for a wide range of clinical problems.
• CT is less sensitive to patient movement than MRI.
• CT can be performed with implanted medical devices unlike MRI.
• Diagnosis determined by CT can remove the need for exploratory and surgical biopsies

Risks of CT Scans

• There is a slight chance of cancer from excessive radiation exposure. However, a precise diagnosis often outweighs the potential risk.
• Effective radiation dose varies; see the relevant procedures page
• Pregnant women should inform their doctor or X-ray/CT technician if they are pregnant.
• IV contrast manufacturers recommend not breastfeeding infants for 24-48 hours after administering contrast material. However, absorbed amounts during breastfeeding are extremely minimal.
• Radiology departments are well-equipped to manage rare potential allergic reactions related to iodine-containing contrast.
• Children's sensitivity to radiation means CT scans should only be performed if essential for diagnosis and not repeated unnecessarily. Low-dose techniques are always preferred with children

Preparation for CT Scan

• Wear comfortable, loose-fitting clothing. A gown may be required.
• Remove metal objects (jewelry, eyeglasses, dentures, hairpins).
• Remove hearing aids and removable dental work.
• Women should remove bras containing metal underwire.
• Remove any piercings if possible.

Patient Positioning for Adult Head CT Scan (Brain)

• Patient should be supine (lying on their back).
• Head first into the gantry and secured in the head-holder whenever possible.
• Centre the table such that the external auditory meatus (EAM) is at the centre of the gantry.
• To minimise eye exposure, the scan angle should be parallel to a line from the supraorbital ridge to the posterior margin of the foramen magnum. This can be achieved by tilting the patient's chin towards the chest or tilting the gantry itself.

Contrast in CT Scans

• Oral contrast is not usually used for brain CT.
• Injected contrast media is used in some situations.
• Administer intravenous contrast as directed by the supervising radiologist following appropriate injection protocols and guidelines (e.g. use of Intravascular Contrast Media).
• A typical amount is 100 cc of 300 mg/cc strength, injected at 1 cc/sec.
• A 4-minute delay between contrast injection and scan start is typical.

Axial Versus Helical Scan Mode

• Both axial and helical scan modes have advantages and disadvantages for routine head CT scans.
• Relevant factors include patient indication, scanner capabilities, and image quality requirements.
• Consult both the manufacturer and a medical physicist to determine the most suitable mode for the individual patient case

CT Scan Protocols (with/without contrast)

• Scan coverage goes from just below the foramen magnum to the vertex, adjusting for trauma below the anterior arch of C1.
• Scan plane is stacked axial, paralleling a line from the orbital roof to the external auditory meatus.
• Slice thickness: 5-3 mm, reconstruction: 1 mm
• kVp, mA, Rotation Time for Non-contrast : [specific values].
• For Contrast: 50 cc Omnipaque 300IV, soft tissue algorithm; soft tissue window: WC:40, WW:80
• Contrast injection protocol: non-contrast first, then delayed contrast aquisition
• Delay time (>5 min) after contrast injection.