CT Scan Flashcards
What are the 3 major elements of a CT scanner?
- The gantry
- Operators console
- The computer
How many detectors are arranged around a patient in a CT scan?
- Up to 1000
- 4 to 16 rows of detectors
What is the data acquisition system of a CT scanner?
- Amplifies signal from amplifies signal from detectors
- Converts from analog to digital
- Sends to computer
What is a scout image?
- 2-d digital radiograph produced by CT
- Determines localized structures to be scanned.
What is a CT scanner’s pitch?
The amount the table advances during each subsequent scan. Modern CT scanners move continuously, and do not have a pitch.
What is the mondern spiral technology of CT scanners called?
multislice CT (MSCT) volumetric scanning.
What type of slice does a MSCT scanner make?
A helical.
What does a pixel represent?
- The radiodensity in the cell of a matrix formed from converting digital signals of the computer that resulted from the scan.
How is a pixel represented in the image?
A shade of grey.
What is three-dimensional CT?
3-D images created by CT data that can be rotated in space.
What is CT myelography?
Injection of a contrast agent into the subarachnoid space to explore the space in the spinal column.
Why is CT myelography more accurate than MRI?
- Better distinguishes osteophytes, ligamental infolding, and annular material.
What is a voxel?
A slice created by a CT scan ranging from 0.1mm to 10mm thick.
What is volume averaging?
The averaging of all the tissues in a voxel to one radiodensity.
How is an axial image viewed?
As if the patient were supine, and the viewer were looking up from below.
How are sagittal images viewed?
From left to right.
What is windowing?
- Selecting range of specific radiodensities to be displayed in the image.
What 3 factors improve spatial resolution?
- Large matrix size
- Small field of view
- Thin slices
What 3 factors degrade image quality?
- Hardening
- Artifacts
- Motion artifacts
When must thinner slices be used? What is the effect on the image?
- Areas with finer anatomical detail
- More radiation
- More noise in the image due to less radiodensity
What does CT image best?
- Subtle fractures/ complex fractures
- Degenerative changes
- Multiple injuries to osseous and soft tissue
- Spinal stenosis
- Intervertebral disks
- Loose bodies in a joint
What advantages does CT have over MRI?
- Less expensive
- Accurate measurement of bone in any plane
- Less problematic for patients with claustrophobia
What are 2 disadvantages of CT?
- Not sensitive to histological properties
- Huge radiation exposure
In what type of settings is CT the modality of choice for neuroimaging?
- Acute settings
- Trauma
How is ischemic stroke imaged on CT?
Thrombis in vessel causes vessel itself to be more radiodense than the surrounding brain tissue.
How is ischemic stroke imaged on MRI?
- T2 MRI picks up on edema associated with brain death.
How is cerebral hemmorage/ hematoma imaged on CT in the acute phase?
Acute: Parenchyma is bright; blood in CSF is bright
How is cerebral hemmorage/ hematoma imaged on CT in the subacute phase?
- Hemorrhagic area less radiodense than surrounding tissue,.
How is acute cerebral hemorrhage/ hematoma imaged on MRI?
- T1: high signal intensity
- T2: Low
Pronounced edema –> high in T2
How is chronic hemorrhage/ hematoma imaged on MRI?
T1: Low intensity
T2: High intensity
