Computed Tomography (CT) Basics Flashcards

In computed tomography, the Hounsfield unit (HU) value for water is:

Answer: 0 HU

By definition in the Hounsfield scale, water is assigned a value of 0 HU, serving as the reference point for the CT density scale.

The CT scan parameter that most directly controls the radiation dose to the patient is:

Answer: mAs (tube current × time)

The mAs (product of tube current in mA and scan time in seconds) is the primary determinant of radiation dose in CT, as it controls the number of x-ray photons produced.

Which CT reconstruction algorithm produces images with the sharpest edges and highest spatial resolution, typically used for lung and bone imaging?

Answer: High-frequency (edge-enhancing) kernel

High-frequency or edge-enhancing reconstruction kernels amplify high spatial frequencies, producing sharper edges at the cost of increased image noise — ideal for bone and lung detail.

In helical (spiral) CT, pitch is defined as:

Answer: Table travel per rotation divided by the total collimation width

Pitch = table distance traveled per 360° rotation ÷ total x-ray beam collimation width. A pitch of 1 means table travel equals beam width per rotation.

The CTDI (CT Dose Index) metric most commonly used to represent dose for a specific scanner and protocol is:

Answer: CTDIvol (volumetric CTDI)

CTDIvol is the most widely used single-number representation of CT dose for a given protocol, accounting for pitch and providing a volume-averaged dose estimate.

Window width (WW) and window level (WL) in CT imaging refer to:

Answer: The range and center of HU values displayed as the visible gray scale

Window width sets the range of Hounsfield units mapped to the visible gray scale; window level sets the center HU of that range — together they optimize tissue contrast display.