Radiography Exam Computed Tomography (CT) Basics 1 — Questions and Answers

Question 1: In computed tomography, the Hounsfield unit (HU) value for water is:

0 HU (Correct answer)
-1000 HU
+1000 HU
+40 HU

Correct 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 Hounsfield scale assigns water = 0 HU, air = -1000 HU, and dense cortical bone = +1000 HU. Typical tissue HU values: fat (-100 to -50), blood (+35 to +45), soft tissue (+20 to +80), liver (+45 to +65). Understanding HU values is essential for identifying normal anatomy and pathology. The scale is calibrated daily in quality control procedures using a water phantom.

Question 2: The CT scan parameter that most directly controls the radiation dose to the patient is:

mAs (tube current × time) (Correct answer)
Pitch
Slice thickness
Field of view (FOV)

Correct 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.

In CT, mAs directly controls photon fluence (number of x-ray photons). Higher mAs = more photons = lower image noise = higher dose. Modern CT scanners use automatic tube current modulation (ATCM) to automatically adjust mAs based on patient size and anatomy, reducing dose while maintaining image quality. kVp also affects dose, but mAs is the most direct control. Pitch affects the effective mAs in helical CT.

Question 3: Which CT reconstruction algorithm produces images with the sharpest edges and highest spatial resolution, typically used for lung and bone imaging?

High-frequency (edge-enhancing) kernel (Correct answer)
Soft-tissue (smoothing) kernel
Standard kernel
Noise-reduction (iterative) algorithm

Correct 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.

CT reconstruction kernels (filters) determine the balance between spatial resolution and image noise. High-frequency (sharp/bone) kernels amplify edge information, providing excellent spatial resolution for cortical bone and lung parenchyma. Soft-tissue kernels smooth out noise for better low-contrast detectability in abdominal and brain imaging. The choice of kernel is a critical factor in CT image quality and must match the clinical indication.

Question 4: In helical (spiral) CT, pitch is defined as:

Table travel per rotation divided by the total collimation width (Correct answer)
Slice thickness divided by table speed
Number of detector rows multiplied by rotation time
Field of view divided by matrix size

Correct 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.

For single-detector CT: pitch = table travel per rotation / slice collimation. For multi-detector CT: pitch = table travel per rotation / (number of detector rows × detector row width). Pitch >1 means faster scanning (reduced dose but possible data gaps). Pitch <1 means overlapping data (higher dose but better image quality and MPR capability). Typical diagnostic pitch ranges from 0.8–1.5.

Question 5: The CTDI (CT Dose Index) metric most commonly used to represent dose for a specific scanner and protocol is:

CTDIvol (volumetric CTDI) (Correct answer)
CTDIw (weighted CTDI)
CTDI100
DLP (dose-length product)

Correct 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.

CTDIvol = CTDIw / pitch. It represents the average dose delivered to a standardized phantom volume during a CT scan. CTDIw is measured in standard PMMA phantoms (16 cm for head, 32 cm for body). DLP = CTDIvol × scan length and represents the total energy delivered. CTDIvol is displayed on the scanner console before each scan to help operators assess dose relative to diagnostic reference levels (DRLs).

Question 6: Window width (WW) and window level (WL) in CT imaging refer to:

The range and center of HU values displayed as the visible gray scale (Correct answer)
The slice thickness and reconstruction interval
The tube voltage and tube current used for the scan
The matrix size and pixel size of the image

Correct 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.

CT scanners acquire 12-bit data (4096 HU values), but monitors display only 256 shades of gray. Window width (WW) determines how many HU values are compressed into those 256 shades. Window level (WL, or window center) sets the midpoint HU. A narrow WW provides high contrast for small density differences (e.g., brain WW=80/WL=40); a wide WW displays a broad range of densities (e.g., lung WW=1500/WL=-600).

Radiography Exam Computed Tomography (CT) Basics 1

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