Answers
16. Sir Godfrey Hounsfield first introduces CT scanners for brain study. later it underwent many improvements in the number of detectors and scan time.
First generation
detectors: one
type of beam: pencil-like X-ray beam
tube-detector movements: translate-rotate
duration of scan (average): 25-30 mins
Second generation
detectors: multiple (up to 30)
type of beam: fan-shaped x-ray beam
tube-detector movements: translate-rotate
duration of scan (average): less than 90 sec
Third generation
detectors: multiple, originally 288; newer ones use over 700 arranged in an arc
type of beam: fan-shaped x-ray beam
tube-detector movements: rotate-rotate
duration of scan (average): approximately 5 sec
Fourth generation
detectors: multiple (more than 2000) arranged in an outer ring which is fixed
type of beam: fan-shaped x-ray beam
tube-detector movements: rotate-fixed
duration of scan (average): few seconds
Fifth gen CT scanners:
they were developed to use in cardiac CT imaging
detectors: fixed
type of beam: large xray tube inside which the patient lies during the scan
Behind the patient, there is electron beam, which ejects electrons.
scan time: 50 milliseconds
sixth generation CT scanners:
This is called helical CT and use slip ring technology
detectors: rotating
scan time: 30 seconds to scan entire abdomen
The data is acquired helically, so planar sections will not be produced . This can be done by reconstruction process.
Seventh generation CT scanner:
This is the latest generation CT scanner and they consist of multiple detector array and cone shaped x ray beam.
It is highly sophisticated to give a very large amount of information in a very short span.
17. The quality of SPECT or PET scan image is affected by many factors.
a)
Nuclear imaging depends on the estimation of the radioactivity circulation of a tracer which was injected to a patient. This circulation is controlled by the attributes of the tracer just as the tissue under scrutiny.
If the dose of the tracer is not appropriate, it may cause poor quality of image due to ineffective uptake of the tracer by the organs.
How to control the quality of image:
Choice of the ideal tracer is a fundamental advance to upgrade both affectability and particularity. The dose of the tracer must be sufficient enough to produce high quality images.
b) If the injection time and the scan time is not as per the schedule, it may cause low quality images. Usually the patient is injected with the tracer and after 60 minutes, the imaging is done. This timing is very crucial as the decay time of the tracer is very short.
How to control the image quality?
The injection time and scan time must be scheduled and followed properly, to get the high quality images.
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