Topic 6. Quality Assurance & Dose Reduction for CT Flashcards
list two reasons why determination of doses is of importance in CT
- CT is the largest medical contributor of collective dose to the patient population
- Irradiation of the image receptor must be sufficient to obtain a usable image
CT invariably results in very ____-________ irradiation of a variety of organs with different radio sensitivities
non-uniform
overall dose in CT known as the
effective dose
Overall risk from CT exposure may be estimated from
CT machines display a crude estimate of the effective dose, this is found using a 3-part computation of?
- Calculate the CT Dose Index (CTDI) for the given technique factors chosen for the current patient
CTDI: is a patient independent measure of machine output for the given set of technique factors - Multiply the CTDI vol. by the length of the region irradiated and obtain a dose-length product (DLP)
- Another conversion factor (obtained from Monte Carlo) transforms DLP into Effective Dose
CTDI: is a patient independent measure of machine output for the given set of technique factors. what is it?
An index of the x-ray output of a CT machine, Invented to give an average dose for a single axial slice.
CTDI phantom is an acrylic cylinder 15 cm long.
True or false? Clinically adequate images can be obtained with lower exposure levels.
TRUE. This is driving the development of improved technologies and the re-examination of long accepted protocols.
Distribution of dose being deposited in the phantom at any instant depends on the
beam angle
CT Dose index is a measure of dose in a ____ full rotation of gantry
single
as many as 30% of CT’s done on children are
- medically unnecessary
- could be carried out with safer modalities (US, MRI)
TRUE OR FALSE? Dose reductions of up to 50%, still with adequate image quality, have been reported.
TRUE, the dose does not need to be high for acceptable image quality. This can be achieved from a simple change of kVp
CT profiles have a bell-shaped graph due to
- The finite size of the focal spot causes a small amount of shadow blurring at the edges of the primary beam
- Compton scatter within the phantom or patient
CTDI 100?
defined as the integral dose recorded by the chamber over its entire 100 mm length
measured with the ion chamber in the central hole and in one or more of the peripheral one, what is being measured?
CTDI 100
CTDIw?
the weighted average of CTDI100 readings