Chapter 10 - Computed Tomography

What does CT stand for and what does it primarily image?

Computed Tomography — it images cross-sectional anatomy by measuring x-ray attenuation through the patient from many angles.

What are the two main factors that drove the dramatic increase in CT clinical use over the past three decades?

Significant advances in image quality and a ~1000-fold reduction in acquisition time.

How has CT reconstruction evolved over time?

"From filtered backprojection (FBP) → iterative reconstruction → deep learning (DL)-based reconstruction

What is the principal reconstruction plane of most CT scanners?

The axial (transverse) plane. Coronal and sagittal images are typically synthesized from thin reconstructed axial slices.

What is a voxel in CT imaging?

A volume element (3D) in the patient that corresponds to a pixel (2D picture element) in the CT image.

What are typical voxel dimensions for a normal resolution (NR) CT scanner?

"~0.60 mm × 0.60 mm (Δx

Why is dual energy CT useful in radiation therapy treatment planning?

It allows accurate assessment of electron density for treatment planning and enables 3D bone/soft tissue segmentation.

What is the routine tube voltage (kV) used for CT scanning?

120 kV for routine scanning; 80

What is the effective energy range of typical CT x-ray spectra?

Approximately 43–70 keV across the 80–140 kV spectra commonly used in CT.

Which x-ray interaction dominates in soft tissue at CT energies?

Compton scattering — it is approximately 10× more likely than the photoelectric effect or Rayleigh scattering in soft tissue at CT energies.

What physical property does the CT Hounsfield Unit (HU) primarily reflect in soft tissue?

Electron density — because Compton scattering (which dominates at CT energies) is proportional to electron density.

What is the Hounsfield Unit (HU)?

A scaled measure of the linear attenuation coefficient of tissue relative to water: HU = [(μK − μw) / (μw − μair)] × 1000.

What is the HU value of pure water?

HU = 0 (by definition).

What is the HU value of air?

HU = −1000 (by definition); in practice bowel air reads −800 to −900 due to scattered radiation contamination.

Why does adipose tissue appear darker (lower HU) than soft tissue like liver?

Adipose has lower mass density (~0.94 g/cm³) than soft tissue (~1.0 g/cm³)

Why do HU calibrations differ slightly between 80 kV and 120 kV acquisitions?

The linear attenuation coefficient is energy-dependent; the effective attenuation of water (μw-eff) changes with tube potential. Water is always HU = 0 but other tissues shift slightly.

What is the geometry of modern clinical CT scanners?

Rotate-rotate (third generation): both the x-ray tube and detector arrays are fixed to the rotating gantry and rotate together around the patient.

What is the isocenter of a CT scanner?

The center of rotation of the CT gantry; typically corresponds to pixel (256

How is the magnification factor (M) calculated in CT?

M = B/A

What is a "ray" in CT?

A line integral between the x-ray source and a single detector element — the fundamental measurement in CT.

What is a "view" (or projection) in CT?

A collection of all rays acquired at a single gantry angle — one fan-beam projection.

What is the typical fan angle of modern whole-body CT systems?

Approximately 50°–60°.

What does MDCT stand for and what is its key advantage?

Multi-Detector array CT — uses 16

For a 64-slice MDCT with 0.625 mm detector width

what is the total z-axis coverage per rotation?

What is detector binning in CT?

Combining signals from adjacent detector arrays to create effectively wider detectors — reduces electronic noise and data volume at the cost of z-axis resolution.

How did MDCT decouple slice thickness from collimated beam width?

In MDCT

What is a cone beam CT system?

A CT system where the x-ray beam diverges significantly in the z-axis (cone angle)

What are the trade-offs of wide cone-beam (e.g.

16 cm coverage) CT systems?

What is the purpose of the slip ring in CT?

Allows continuous gantry rotation in one direction by transferring electrical power and digital signals between the stationary and rotating frames without cables — essential for helical scanning.

How did slip rings improve CT gantry rotation times?

Previous cable-based gantries: 2–3 s rotation. Modern sliprings: 0.28–0.35 s rotation (approaching 0.20 s with air bearings).

Why must x-ray tubes be oriented with the anode disk plane parallel to the plane of gantry rotation?

To minimize gyroscopic forces from the spinning anode in the rapidly rotating gantry.

In which direction does the heel effect run on a CT scanner?

Along the z-axis (cranial-caudal direction)

How many projections are typically acquired per CT rotation?

"Between 1

What is focal spot dithering (steering) in CT and why is it used?

Magnetic steering of the electron beam counterclockwise to oppose gantry rotation — effectively freezes focal spot motion during each detector dwell interval to preserve spatial resolution.

What type of detector is used in nearly all modern MDCT scanners?

Indirect solid-state detectors: sintered ceramic scintillators (e.g.

What is sintering in the context of CT detectors?

"Heating phosphor crystals just below their melting point with repeated compression to increase density

What is a photon counting detector (PCD) and why is it notable for CT?

A direct solid-state detector (no scintillator) that counts individual x-ray photons and assigns them to energy bins — enabling spectral/dual-energy imaging without multiple scans. High photon flux is a key technical challenge.

What is over beaming in CT?

Intentionally placing the x-ray beam penumbra just outside the active detector array edges so all detector arrays have a uniform rectangular slice sensitivity profile (SSP).

Why does over beaming reduce dose efficiency?

The penumbra region irradiates the patient but is not detected. Early 4–8 slice MDCT lost up to 30% dose efficiency; modern 64+ slice systems approach 95% dose efficiency.

What is the slice sensitivity profile (SSP)?

The distribution of detected signal along the z-axis for a given detector array — ideally rectangular for central detectors in MDCT.

What is the purpose of a bow tie filter in CT?

A beam-shaping filter that attenuates the x-ray beam more at the periphery of the fan (where patient tissue is thinner)

What body parts determine the two primary bow tie filter sizes in CT?

Head (~17 cm diameter) and body (24–45 cm diameter). The head filter is also used for pediatric body imaging on some scanners.

How does noise propagate from projections into a CT image?

As the sum of squared noise variances from all projections intersecting a pixel (adding in quadrature): σ²_image = σ²_p1 + σ²_p2 + … + σ²_pN.

What is the scanned projection radiograph (SPR) and what are its other names?

"A digital radiographic image acquired with gantry stationary and patient translating — also called scout

What is axial (sequential) CT scanning?

"Step-and-shoot acquisition: gantry rotates 360° with table stationary

What is helical (spiral) CT scanning?

Continuous gantry rotation while the table translates at constant speed — the x-ray source traces a helical path around the patient. Requires slip ring technology.

What is the pitch in helical CT scanning?

Pitch = F_table / nT

How does pitch affect radiation dose in helical CT?

Dose ∝ 1/pitch (all else equal). Pitch < 1 → overscanning → higher dose. Pitch > 1 → underscanning → lower dose.

When is low pitch used in CT and when is high pitch used?

"Low pitch (~0.75): cardiac imaging

What is over-ranging in helical CT?

Extra patient irradiation at the start and end of a helical scan needed to acquire sufficient angular data for reconstruction (approximately ½nT beyond planned scan length at each end).

What is adaptive beam collimation and what problem does it solve?

Motorized collimators that narrow the beam at the start and end of helical scans — reduces over-ranging dose while maintaining full coverage of the desired scan range.

What is the relationship between reconstructed slice thickness and image noise in CT?

Thicker slices → more photons → lower noise. SNR is proportional to √(slice thickness). A 5 mm slice is ~2.8× better SNR than a 0.625 mm slice at the same dose.

What is x-ray tube current modulation (TCM) in CT?

Automatically varying mA during rotation (to account for patient shape) and along z (to account for anatomy changes) — equivalent to AEC in radiography. Reduces dose with little image quality loss.

Why must mA increase exponentially to maintain constant detector signal in TCM?

Because x-ray attenuation is exponential with tissue path length — the same principle as Beer-Lambert law (Equation 10-6).

What is CT angiography (CTA)?

"CT acquisition timed to the arterial or venous phase of IV iodinated contrast injection

What is CT perfusion imaging?

Repeated CT scanning of an organ over time during contrast injection to quantify blood volume

What is retrospective cardiac gating in CT?

"Continuous CT acquisition throughout multiple cardiac cycles with simultaneous ECG recording; desired cardiac phases are selected after acquisition. Allows cine/beating heart reconstruction but delivers HIGH radiation dose

What is prospective cardiac gating in CT?

"ECG-triggered x-ray pulsing to acquire data only during end-diastole (the heart's most stationary phase). Significantly lower dose than retrospective gating but provides only a single temporal snapshot."

What gantry rotation time is required to achieve <100 ms temporal resolution for cardiac CT without dual-source technology?

A single-source CT needs about 0.20 s (200 ms) gantry rotation; even 0.25–0.35 s systems struggle with cardiac motion without gating or dual-source design.

What is a dual source CT (DSCT) scanner?

A CT system with two complete x-ray tube + detector chains mounted ~90° apart on the same gantry — enables ~83 ms temporal resolution for cardiac imaging (quarter rotation).

What clinical advantage does CT angiography offer over coronary angiography?

"CT requires only IV (intravenous) contrast injection

What is dual energy CT (DECT) and what was Hounsfield's original prediction?

Acquiring CT data at two different effective x-ray energies to separate material composition from physical density. Hounsfield predicted this capability in his original CT notes.

What are the four main methods to acquire dual energy CT data?

"(1) Dual source (two tubes at different kV)

What does a dual energy CT HU scatter plot show?

The relationship between HU at 80 kV vs. 140 kV for different materials — high atomic number materials fall below the line of identity; lighter materials fall above.

What clinical application uses dual energy CT to identify uric acid crystals?

Diagnosis of gout — uric acid crystals are color-coded (typically purple) in 3D dual energy reconstructions of affected joints.

What is filtered backprojection (FBP)?

A traditional CT reconstruction algorithm that applies a mathematical filter (kernel) to projection data then backprojects filtered rays into image space — fast but noise-limited.

What are iterative reconstruction methods in CT?

Reconstruction algorithms that repeatedly compare measured and modeled projections and update the image estimate — allow lower noise or lower dose compared to FBP.

What is deep learning (DL) reconstruction in CT?

AI-based (convolutional neural network) post-processing or reconstruction that further reduces noise and improves image quality — can enable dose reduction below iterative reconstruction levels.

What is a CT reconstruction kernel and how does it affect image quality?

A mathematical filter applied during FBP reconstruction. Smooth kernels → lower noise

What parameters are typically included in a CT protocol?

"kV

What preprocessing steps are applied to CT raw data before reconstruction?

"Air calibration correction

What is the reference detector (Sr) in CT preprocessing?

A detector near the x-ray tube (outside the patient FOV) that continuously measures x-ray output — used to normalize each projection measurement for tube output fluctuations.

What is the mathematical relationship between preprocessed CT projections and tissue attenuation?

After preprocessing: Pj = μ₁Δl₁ + μ₂Δl₂ + … (a line integral of linear attenuation coefficients along each ray) — this allows linear reconstruction algorithms.

What is view sampling in CT and why does it affect spatial resolution at the periphery?

The angular interval between consecutively acquired projections. As sampling angle = dr

What is the field of view (FOV) geometry in CT?

The acquisition FOV is a cylinder (circle in x-y plane × z-extent). The display FOV is chosen by the technologist and can be smaller than the acquisition FOV.

What are the major impediments to high spatial resolution in whole-body CT?

Large detector element size and large focal spots on high-power CT x-ray tubes.

What image matrix sizes can high-resolution CT (HRCT) scanners reconstruct?

"Standard 512²

What is the anti-scatter grid in CT and why is it 2D?

A 2D grid of septa aligned with detector dead spaces — suppresses scattered x-rays detected in the multi-row detector array. Needed because modern wide beams (40+ mm) produce far more scatter than early narrow-beam CT.

Why do flat panel cone-beam CT systems need flat-fielding correction?

The flat (non-curved) detector creates non-uniform x-ray fluence (inverse square law + heel effect) across the detector surface. Flat-fielding corrects for these spatial differences in signal.

What g-forces does a modern CT gantry generate at its periphery?

Greater than 20 g's — requiring special hardware design for all components mounted on the rotating gantry.

Why is the CT x-ray tube more powerful than tubes used in radiography?

"CT tubes: 5–7 MJ (megajoule) rating. Radiographic tubes: 0.3–0.5 MJ. CT requires continuous high-output acquisition through many angles; tubes are replaced every 9–12 months in busy departments."

How does opening the beam collimation increase CT tube output efficiency?

Opening from 10 mm to 40 mm beam width provides a 4× (400%) increase in usable x-ray output — making MDCT more efficient with each tube rotation.

What was the rotation time of cable-connected CT gantries vs. slip-ring gantries?

"Cable-connected: 2.0–3.0 s per 360°. Slip-ring: 0.28–0.35 s per 360° (prototypes approaching 0.20 s)."

What does the CT table material need to satisfy for head imaging?

Head holders are made from carbon fiber — highly transparent to x-rays and conforming to the head shape for motion reduction.

Why is table positioning accuracy critical in CT?

CT images are used for radiation therapy targeting