CT scans Flashcards
What are the different CT planes? (x3)
Coronal, sagittal, axial.
! How does CT work? Density of different tissues?
Records attenuation of tissues to ionising radiation and calculates a mean value for a given volume of tissue (a ‘voxel’) which is represented in greyscale as a single point. The greyscale of the pixel is measured on the Hounsfield scale relative to the attenuation of water (0 units; air is -1000 units; soft tissue is whiter than water and between 20-70 units). Greyscale can be described as hyperdense, hypodense and isodense.
How can contrast mediums be administered in CT scans? (x3)
- IV: to image vascular anatomy and vascular structures such as highly perfused tumours. Images acquired at different times after injection will show the agent in anterior or venous structures, or during ‘washout’ (clearing).
- PO: 1-12 hour before imaging bowel
- PR: examining distal colonic lumen
What are the disadvantages of contrast medium in CT? (x2)
Nephrotoxic and increases radiation exposure.
What are the relative advantages of CT compared to MRI? (x6)
- Faster than MRI better for acute situations e.g., stroke, haematoma
- Less sensitive to patient motion
- Costs less
- Detailed evaluation of cortical bone
- Can be performed in patients with implantable medical devices
- Less noise
What are the relative advantages of MRI compared to CT? (x3)
- No ionising radiation (better used in young, pregnant and patients with chronic disease likely to undergo multiple scans)
- Greater detail of soft tissue
- Contrast agents have a smaller risk of causing potentially lethal allergic reaction
What is a streak artefact?
High-attenuation items such as metal fillings and even bone are so hyper-dense, they can cause interference.
What should you assess when interpreting head CT?
‘Blood can be very bad’: BLOOD, cisterns (gaps in brain), brain, ventricles, bones.
What fissures can be identified on CT head?
Fissures are large CSF-filled clefts which separate structure of the brain. Interhemispheric fissure separates the cerebral hemispheres. Sylvian fissures separate the frontal and temporal lobes.
How does density of blood differ in haematomas in CT head?
- Fresh blood is hyperdense
- In old haematomas, Hb breaks down and loses attenuation, so subacute haematoma at 2 weeks is isodose
- A chronic subdural haematoma will be hypodense (relatively low attenuation)
How do you identify fractures on head CT?
Cortical, hypodense region of bone is consistent with the fracture margin. In sutures, the margins are hyperdense. The bone also widens as it approaches a suture, and the margin is more zig-zagged than a fracture.
How do tumours/abscesses look like on head CT? Grey vs. white matter?
Ring-enhancing mass: mass with hypodense core, surrounded by vasogenic oedema. Vasogenic oedema occurs from leaky capillaries and spreads through the white matter (grey matter is spared). Mass effect also causes compression of the sulci, ipsilateral ventricles, and may also cause herniation.
How do meningiomas look like on head CT?
They are not ring enhanced. The whole mass is hyperdense.
How does arterial occlusion show on head CT?
Artery is hyperdense with little/no brain parenchyma changes (though becomes hypodense eventually).
