ARRT CT Physics 3

Question 1

In CT, 'effective mAs' (mAs/pitch) is important because:

Accepted Answer

It remains constant regardless of pitch, maintaining image noise consistency

Answer

Effective mAs = mAs ÷ pitch; keeping this constant as pitch changes maintains consistent image noise regardless of table speed.

Question 2

What is the effect of decreasing detector collimation (thinner slices) on image noise, assuming all other factors remain constant?

Accepted Answer

Noise increases

Answer

Thinner collimation means fewer photons contribute to each voxel, increasing statistical noise in the image.

Question 3

The Hounsfield unit (HU) value of pure water is defined as:

Accepted Answer

0 HU

Answer

The CT number scale defines water as 0 HU and air as −1000 HU, with other tissues placed relative to these references.

Question 4

Which reconstruction kernel (filter) should be selected to best visualize lung parenchyma and fine pulmonary detail?

Accepted Answer

High-resolution (sharp/edge-enhancing) kernel

Answer

A high-resolution or edge-enhancing kernel accentuates fine structural detail needed for lung parenchyma evaluation, at the cost of increased noise.

Question 5

In CT dose reporting, CTDI_vol represents:

Accepted Answer

Volume-averaged radiation dose within the scan volume

Answer

CTDI_vol is the volume CT dose index, representing average dose within the irradiated volume; it accounts for pitch in helical scanning.

(ARRT) American Registry of Radiologic Technologists Computed Tomography Certification Practice Test