CHAPTER 38 - COMPUTED TOMOGRAPHY PATIENT RADIATION DOSE

Computer Tomography

___________ is a projection x-ray imaging modality unlike that of radiography or fluoroscopy

image contrast

The principle purpose for tomography is to increase________

superior contrast resolution.

Conventional tomography has largely disappeared with the introduction of digital imaging. The first digital imaging modality was CT, which has ______________.

more scatter radiation

The cone beam results in ____________ rejection than an area beam and that results in better image contrast.

CT

________ results in a higher dose than other diagnostic x-ray procedures.

high-dose procedure

Because of increasing use of multislice helical CT, CT must be considered a __________

average fluoroscopic dose

The CT tissue dose is approximately equal to the ____________

uniform

The CT dose is nearly __________ throughout the imaging volume for a head examination

50%

The CT central axis radiation dose is approximately _______ of the ESE for body CT.

Scatter radiation

___________ increases patient dose and reduces image contrast.

proper adjustment

It is essential that CT collimators be monitored periodically for ____________.

penumbra

Multislice helical CT results in lower patient radiation dose than step-and-shoot CT because fewer tails are seen on the dose profile for a given volume of tissue. The dose profile tail is called ________

higher

The ______ the multislice value, the lower the patient radiation dose will be.

the same

At a pitch of 1.0:1, the patient radiation dose is approximately _______

increased

At a lower pitch, patient dose is ________

noise

Sigma (σ) is _______.

Sigma (σ)

This is equivalent to quantum mottle in screen-film radiography and represents random statistical variations in CT numbers.

increases

A reduction in the noise or beam width, while other factors remain constant, _______ patient dose.

pencil dosimeter

The radiation measurements are made with an ionization chamber called a __________

nominal beam width

The penumbra beyond the ______________ contributes to patient radiation dose during multislice helical CT.

computed tomography dose index (CTDI)

The average of the multiple rotation imaging is the ___________ and this is the value measured with the pencil ionization chamber.

CTDI_vol

__________ is computed by averaging the peripheral pencil dose measurements and comparing them with the central axis dose as follows

Dose Length Product

A new metric is required to properly describe patient radiation dose in CT. With multislice helical CT, more than a single rotation is usually engaged. Suppose the x-ray beam width is 16 cm, the widest now available.

overranging

If the CT pitch is 1, the patient will move 16 cm during each rotation. Actually something more than 32 cm of tissue will have been irradiated because of the x-ray beam dose tail and because of __________.

overranging

Patient movement is somewhat more than the tissue imaged because of __________

dose length product (DLP)

_________ is an attempt to better address the extent of patient radiation dose.

dose length product (DLP)

This combination of output radiation intensity, CTDIvol, and volume of tissue imaged is called ____________ and it has units of mGyt-cm.

Effective Dose

It is essentially the whole body equivalent of the actual partial body radiation dose received during medical imaging.

Effective Dose

This dose metric is that which is used to evaluate risk in populations exposed to radiation.

Effective Dose

It is that dose which is used to describe stochastic radiation responses.

anatomical region

CT patient radiation dose is not delivered to the whole body, just an __________

• various tissues irradiated

• age of the patient.

Effective dose in CT depends on the __________ and ___________

• part of the body examined

• age of the patient

Consequently CT effective dose is dependent on the ____________ and ____________.

30 to 50 mGyt (3000 to 5000 mrad)

Typical CT doses range from __________ during head imaging and from 20 to 40 mGyt (2000 to 4000 mrad) during body imaging.

20 to 40 mGyt (2000 to 4000 mrad)

Typical CT doses range from 30 to 50 mGyt (3000 to 5000 mrad) during head imaging and from ________ during body imaging.