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Prepared By:Ala’a Ali Tayem Abed
COMPUTED TOMOGRAPHY II – RAD 473
What is CT?
• CT is a Medical imaging device which reconstructs and generates Cross sectional images on the basis of measurements of attenuation, used to aid in diagnosis of various diseases.
• The cross sectional portion of the body which is scanned for the production of CT image is called a slice.
• The slice has width and therefore volume.
• The width is determined by the width of the x-ray beam.
Computed Tomography Components
Gantry. X-Ray Tube. Detector. Table.
CT SCAN PROTOCOLS OR
PROCEDURES
Contraindications for Computed Tomography
Due to the relatively high radiation dose involved in CT scans, it is important to avoid scanning patients who are pregnant. Radiation exposure to a fetus can cause developmental problems. Thus, CT should only be performed for pregnant patients in critical situations and only after discussion of the potential risks.
Patients who have an allergy to the IV contrast media (IVCM) used in CT scans should not be referred for scans where IVCM is required to attain a diagnosis. These tests include CT angiograms and most abdominal and chest scans.
Renal impairment may also prohibit your patient from having IVCM. You should check the patient’s creatinine and eGFR prior to referral.
All scanners will have weight limits that are specified by the manufacturers. Newer scanners have higher limits than their predecessors with some able to accommodate 220kg. Despite these weight recommendations, there may be limitations to the patient’s size. The gantry of the scanner is a fixed diameter and if the patient cannot fit through the gantry, the scan cannot be performed. A common diameter is about 70cm.
HEAD CT SCAN
PATIENT POSITIONING:
• Patient should be supine, head first into the gantry.• Center the table height such that the external auditory meatus (EAM).• To reduce or avoid ocular lens exposure, the scan angle should be parallel to a line created by the supraorbital ridge and the inner table of the posterior margin of the foramen magnum. This may be accomplished by either tilting the patient’s chin toward the chest or tilting the gantry.
SCAN RANGE: Top of C1 lamina through top of calvarium.CONTRAST: • Oral: None.• Injected: Some indications require injection of intravenous or intrathecally contrast media during imaging of the brain.• Intravenous contrast administration should be performed as directed by the supervising radiologist using appropriate injection protocols. A typical amount would be 100 cc at 300 mg/cc strength, injected at 1- 4 cc/sec, Depending on the cause of the patient's imaging.
INDICATIONS
1. Acute head trauma. 2. Suspected acute intracranial hemorrhage. 3. Vascular occlusive disease or vasculitis (including use of CT angiography and/or venography). 4. Aneurysm evaluation. 5. Detection or evaluation of calcification. 6. Immediate postoperative evaluation following surgical treatment of tumor, intracranial hemorrhage, or hemorrhagic lesions. 7. Suspected shunt malfunctions, or shunt revisions. 8. Mental status change. 9. Increased intracranial pressure. 10. Headache.11. Suspected intracranial infection. 12. Suspected hydrocephalus. 13. Congenital lesions. 14. Suspected mass or tumor. 15. Seizures. 16. When magnetic resonance imaging (MRI) imaging is unavailable or contraindicated, or if the supervising physician deems CT to be appropriate.
C. Contraindications
1. Uncontrolled movement during the scan.2. An obese patient may not be able to fit in the machine.3. For those who have partial renal failure, injection of contrast may not be possible
because of the risk of inducing complete renal failure.
ADULT HEAD – ROUTINE (SEQUENTIAL)
PATIENT POSITIONING: For all head studies, it is very important for image quality to position the patient in the center of the scan field. Use the lateral laser beam to make sure that the patient is positioned in the center. Patient lying in supine position, arms resting along the body, secure head well in the head holder, support lower legs.
GENERAL: Gantry tilt is available for sequence scanning, not for spiral scanning. Gantry tilt is not available for dual source scanners.
TOPOGRAM: Lateral.
Angle of cutsHead CT are performed at an angle parallel to the base of the skull
Thickness of cuts
Slice thickness is generally between 5 and 10 mm
HEAD CT(without and with contrast)• Scan Coverage: Just below foramen magnum through vertex. IF TRAUMA, begin below the anterior C1 arch.• Scan Plane: Stacked axial, parallel to line from orbital roof to external auditory meatus• Scan Slice thickness: 5-10 mm.• Reconstruction Slice Thickness: 5 mm.• kVp, mA, Rotation Time: 120, 250 – 400, 1s.• Non-contrast study; soft tissue algorithm; then repeat with:• Contrast: 100cc Omnipaque 300 IV, Soft tissue algorithm.• Soft Tissue Window: WC : 40, WW : 350 • Bone Window: WC : 200, WW : 2000
Without contrast
With contrast
Normal axial head CT images. Appropriate window selection allows visualization of both intracranial contents and bony calvarium. Note differences in attenuation among gray matter (left thalamus, double black arrows), right internal capsule (single black arrow), cerebrospinal fluid (CSF; frontal horn of the left lateral ventricle, white arrow), and bone (skull, arrowheads).
This is the typical appearance and location of an acute extradural haematoma. Note the high density of the hematoma. Slight midline shift is seen.
A, Large left parenchymal hematoma causing a marked midline shift to the right. B, Severe subarachnoid hemorrhage outlining all the cisterns of the skull base.
CT scan of bilateral acute intraventricular hemorrhages (black arrow). Note the comminuted skull fractures that involve bilateral frontal, temporal, and parietal bones (white arrow).
A, Early subtle infarction. B, Late well-defined cerebral infarction. Subtle loss of the distinction between the gray and the white matter is present on the early CT, in the right middle cerebral artery territory, with good delineation of the affected area on the later CT.
A, Orbital roof fractures; the transverse image on orbit settings shows the bone fragment spearing the right medial rectus muscle and the sagittal image on bone windows shows the rotation of this fragment. B, Comminuted depressed skull fracture with subcutaneous emphysema and fluid in the right frontal sinus.
CT Scan, clearly showing a tumor about the size of a common lemon.
Brain Tumor
A, Multiple ring enhancing metastases in both frontal lobes. B, Enhancing bifrontal (butterfly).
A, Shunted hydrocephalus (enlarged ventricles caused by obstruction to CSF flow) can fail, requiring shunt revision. B, Brain Atrophy can also result in large ventricles, without CSF obstruction.
CTA (Circle of Willis)
• Common Histories : Rule out Aneurysm, AVM, Vascular Occlusion, R/O Vasospasm….• Scan range: Lower margin of C2 to Vertex.• Scan from inferior to superior ( Caudocranial).• Contrast: 120cc Omnipaque 350mgI/ml.• Bolus volume of contrast: 4cc/second.• Slice thickness: 1.0mm x 0.4 mm.• Scan delay: 17 seconds (blind start-Not bolus tracking), or bolus tracking. • IF CONTRAST NOT SEEN OR INADEQUATE, RESCAN WITH SAME PARAMETERS WITHOUT DELAY.• Review increment (interval): 0.3mm.• Reprocess axial images to 3mm x 3mm slice thickness.• Soft Tissue Windows: (800W/ 200L).
circle of Willis on cerebral angiography
circle of Willis - 3D reconstruction
Angiogram shows a narrowing of the middle cerebral artery (red arrows) caused by atherosclerotic plaque.
Cerebral angiogram shows the large aneurysm (1.5×1 cm) affecting the anterior communicating artery and proximal stems of anterior cerebral arteries
