Download Lecture 1(4)- Sources in diagnostic Rad. – Computed Tomography

Radiation Sources in medicine
diagnostic Radiology
Computed Tomography
IAEA
International Atomic Energy Agency
Day 7 – Lecture 1(4)
Objective
• To become familiar with the technology of Computed
Tomography (CT) scanners.
• To become familiar with specific radiation risks associated
with this equipment.
IAEA
2
Contents
• Scanner technology, main specifications;
• Technical and clinical developments in computed
tomography;
• Management of patient doses by optimizing scan
protocols;
• Equipment malfunction affecting radiation protection;
• Quality control.
IAEA
3
Computed Tomography Scanners
CT scanners use one or more narrow,
fan-shaped x-ray beams that rotate
around the patient to produce a series
of thin, transverse images of the patient.
• The x-ray beam, attenuated by the patient, reaches a detector
array consisting of several hundred separate detectors which
record the intensity of the x-ray beam at each point.
• Gas detectors (usually Xenon) convert x-ray photons into
electrical signals. Solid state detectors perform an indirect
conversion, using photo-diodes coupled to scintillator systems.
IAEA
4
Computed Tomography (Cont.)
• The CT image is mathematically reconstructed from the
measured data.
IAEA
5
Computed Tomography (cont)
• CT operates with an x-ray tube potential in the range of
perhaps 90-140 kV peak.
• The geometry of the x-ray beam is primarily determined by
the size of the x-ray tube focal spot (0.5 - 2 mm) and prepatient collimation.
• Shaped attenuation filters and / or detector collimation are
often used to minimize scattered radiation reaching the
detectors and hence degrading the image.
IAEA
6
Computed Tomography (cont)
The introduction of slip rings in the gantry structure enabled the
development of helical scanners and more recently multi-slice
(multi detector-array) CT, where the x-ray tube rotates
continuously while the patient couch moves through the gantry.
The main advantages of helical (and multi-slice) CT are:
•
faster scanning which allows, for example, an
examination in a single held breath;
•
the possibility to choose, after
scanning, the position and spacing
of reconstructed images.
IAEA
7
Computed Tomography (cont)
Multislice CT
scanner
IAEA
8
Computed Tomography (cont)
•
These technological developments have led to an increasing
use of CT in routine patient management and an increasing
variety of examinations.
•
Patient doses from CT examinations are relatively high (10 -100
mSv). Substantial dose variations for similar examinations can
arise from different imaging protocols or between scanner types.
•
To minimize unnecessary radiation exposure, prior clinical
justification of all CT examinations should be encouraged.
Imaging protocols must be optimized to provide the required
clinical information with the minimum dose to the patient.
IAEA
9
Scan Parameters influencing Patient Dose
Patient dose depends on the intrinsic qualities of the CT scanner
(scanner geometry, beam geometry, collimation and filtration, etc).
However, changes in a range of selectable operating parameters
can also significantly affect patient radiation dose. i.e.
•
x-ray tube potential, tube current, exposure time.
•
slice thickness, number of slices (or helical rotations).
•
slice interval (incremental mode) or pitch factor (helical
mode).
•
window width, matrix size and field of view.
IAEA
10
Malfunctions affecting radiation protection
•
x-ray tube voltage / current inaccuracy and inconsistency;
•
x-ray tube output inconsistency;
•
discrepancies between measured exposure and image quality
parameters and the manufacturer’s data: e.g. image noise,
resolution, slice thickness, CT number values and uniformity,
Computed Tomography Dose Index (CTDI).
Radiation protection issues are generally affected more by poor
understanding and management of the selected parameters than
by poor equipment performance.
IAEA
11