This set of Clinical Science MCQs focuses on “Computerized Axial Tomography”.
1. CAT scan is essentially _________
a) a finer X – ray of bones
b) a detailed x – ray
c) x – ray of soft tissues
d) x – ray of brain
Explanation: The X – rays pass through the body completely. The bones are able to provide resistance to them to a limit by absorbing them. This resistance causes the image to appear. However, for soft tissues, the single shot of X- Rays cannot give an image as they do not pose any obstruction to them. To image soft tissues, multiple X – Rays of high energy need to be passed. Since it can negate out the effect of the bone and image in slices, CT is good for imaging soft tissues.
2. CAT scan can be performed even without a dye.
Explanation: A CAT scan is basically an X-ray of soft tissues. It will not necessarily need a dye or a contrast medium. A contrast is used when some portion needs to be enhanced or better details of the organ need to be seen.
3. CAT scan is often combined with _____
Explanation: PET scan is used to look into the metabolic activities while the CT scan is used to look into the anatomical structure of the organ. Thus, while a CT scan may hint the presence of a tumour, a PET scan can confirm if the tumour is benign or malignant.
4. Why is contrast used in CT scan?
a) To suppress particular tissues
b) To enhance a particular tissue
c) To ensure correct tissue is being imaged
d) To reduce bone interference
Explanation: The contrasts are usually of iodine based and get absorbed by the body. They weaken the radiation passing through the area of interest making white patches turn up. These white patched actually represent a tumour that was not seen previously or a patch of diseased cells. Basically, they enhance the image by weakening the amount of radiation passing through.
5. The current generation CT scanner use __________ for scanning
a) pencil beam and stationary detectors
b) pencil beam and rotating detectors
c) fan beam and detectors
d) electron beam and detectors
Explanation: In the olden generation of CT, the beam of X – Rays and detectors used to move together to scan and take the image. However, in the latest CT scanners, an electron gun fires an electron beam at the target rings, the focus of which is decided by a tungsten anode. This target rings give of X – Rays which are used to image the desired organ. The X – Ray produced can move along a certain diameter. This system can take a better image of the heart.
6. In order to visualize the flow of blood in the renal arteries, what process is done with CT?
a) CT Angioplasty
b) CT Angiography
c) Simple CT
d) CT Cardiography
Explanation: Angiography is a process in which the blood vessels are visualized usually with the help of a contrast. Angioplasty is a process in which a catheter is inserted to remove or open a blocked blood vessel. CT Angiography has an advantage of being 3D as compared to normal fluoroscopic angiography. Thus, the image formed is more detailed.
7. The radiodensity or the radiopacity is measured in _________
Explanation: HU stands for Hounsfield Unit. It represents the opacity to the X – rays. Higher the opacity, the brighter the image is formed on the screen. A normal X – Ray has the bones and tissues blocking its path so the opacity is really low but with CT Scan, with its 3D imaging technique, the opacity for the tissue is increased and the image is taken better. The CT is able to negate out the resistance provided by the bone and other tissues.
8. Given is a table of values derived from a CT image. Find the Hounsfield scale Unit (HU) for CT and X – Ray.
a) ΔCT = 28, X – Ray = 571.4285HU
b) CT = 571.4285HU, X – Ray = 2.304HU
c) CT = 2.304HU, ΔX-Ray = 2.304
d) CT = 2.304HU, X – Ray = 571.4285HU
Explanation: To solve the above,
ΔCT = l1 – l2
= 63 – 35
HU for CT = (l1 – l2)X 1000/(l1 + l2)/2
= (63 – 35)X 1000/(63 + 35)/2
For X – Ray
ΔX-Ray = Σl1 – Σl2
= 1738 – 1734
HU for X – Ray = (Σl1 – Σl2)X1000/(Σl1 + Σl2)/2
= (1738 – 1734)X1000/(1738 + 1734)/2
Sanfoundry Global Education & Learning Series – Clinical Science.
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