Performance Physics of Diagnostic and Interventional Radiology with X-Ray - CT

Flashcards testing vocabulary, concepts, formulas, artifacts, and dosimetric indices from the lecture on CT performance physics, image quality, and quality assurance.

CT Internal Alignment Lights

Patient positioning lights inside the CT scanner gantry that explicitly identify the tomographic plane.

CT External Alignment Lights

Laser or tungsten lights that identify the sagittal and coronal planes, plus a transaxial plane positioned at a fixed distance from the tomographic plane.

Scan Projection Radiography (SPR)

A planar scout scan used in CT to define the beginning and end boundaries of the target scanned volume.

Image Noise

The random fluctuation of CT numbers around a mean value when imaging a uniform material, caused primarily by the statistical nature of X-ray photon production and interaction.

Quantum Noise

X-ray photon noise modeled as a Poisson process where signal is proportional to $N$, noise is proportional to \root%1%2{%1}, and signal-to-noise ratio equals \root%1%2{%1}, where $N$ is the number of absorbed photons.

Normalized Standard Deviation ($S$)

A normalized parameter defined in IAEA CT QA guidelines to compare noise across CT scanners with different contrast scales: $S = \frac{\sigma_{water}}{CT_{scale}} \times 100\%$, where $CT_{scale} = CT_{water} - CT_{air}$.

Contrast to Noise Ratio (CNR)

A quantitative parameter representing low-contrast detail detectability, defined as $CNR = \frac{CT_1 - CT_2}{\frac{\sigma_1 + \sigma_2}{2}}$.

Spatial Resolution

The capability of an imaging system to distinguish small, high-contrast objects, evaluated subjectively via visual phantom inspection or objectively via Modulation Transfer Function (MTF).

Modulation Transfer Function (MTF)

An objective measure of spatial resolution derived from the point spread function (PSF) or edge spread function (ESF), typically evaluated at $50\%$ and $10\%$ frequency values.

Streak Artifact

Single or multiple black or white line patterns that partially or completely obscure a CT image.

Windmill Artifact

A rotating star-like streak artifact in multislice helical CT that appears around high-density objects due to helical interpolation and reconstruction processes.

Shading Artifact

A CT artifact appearing as a dark band between dense bone structures, such as in the base of the skull, across the shoulders, or between femoral heads.

Cupping Artifact

An image artifact characterized by artificially reduced CT voxel values near the centre of a homogenous object.

Ring Artifact

A hardware-related artifact displaying single or multiple concentric rings centered on the rotation axis due to miscalibration or failure of CT detector elements.

Motion Artifact

A patient-based artifact resulting from voluntary or involuntary movement during scanning, manifesting as blurring, streaking, or shading.

Metal Artifact

Severe streaking artifacts caused by metal objects (e.g., dental fillings, prosthetics) whose extreme density exceeds the attenuation handling range of the computer algorithm.

Beam Hardening

The shift of a polychromatic X-ray beam toward higher average energy as lower-energy photons are preferentially attenuated when passing through matter.

Bowtie Filter

A specialized beam filter that pre-hardens the beam edges to match the thinner peripheral paths through a patient's body.

Compton Scatter

X-ray photon scattering proportional to $\frac{1}{E}$ that causes photons to change direction and impart excess unscattered signal to detectors, producing dark bands and cupping artifacts.

Partial Volume Artifact

Shading or inconsistent pixel values occurring when an anatomical detail is only partially in the beam path or when materials of widely different absorption occupy the same voxel.

Photon Starvation

An artifact occurring when an X-ray beam is severely attenuated (such as laterally through the shoulders), causing detector noise to be magnified into horizontal streaks.

Ray Aliasing

Under-sampling within a projection that occurs when the effective detector size is large compared to the anatomical detail.

View Aliasing

Angular undersampling in frequency space resulting from an excessively large projection interval during fast gantry rotation.

Zebra Artifact

Faint stripe artifacts appearing on multiplanar and 3D reformatted images due to z-axis noise inhomogeneity produced by helical interpolation.

Stairstep Artifact

Distortions along structural edges in sagittal and coronal reformatted images caused by wide collimation and non-overlapping reconstruction intervals.

Acceptance Testing

Testing carried out jointly by the vendor and medical physicist upon installation to verify that equipment fulfills purchase contract specifications.

Commissioning Tests

Baseline performance testing conducted by a medical physicist before putting newly installed radiological equipment into clinical service.

Constancy Tests

Routine quality control tests conducted at regular intervals (e.g., monthly or annually) to confirm ongoing satisfactory performance of equipment.

Catphan 600

A standard modular performance phantom used for objective CT quality assurance evaluations including spatial resolution, CT number linearity, and slice thickness.

Hounsfield Unit (HU)

The standard metric for CT numbers defined as $HU = \frac{\mu_{material} - \mu_{water}}{\mu_{water}} \times 1000$, where $\mu$ is the linear attenuation coefficient.

CT Number Uniformity

The maximum difference between the mean CT number of a central ROI and peripheral ROIs across an image of a homogenous phantom.

Z-Sensitivity

The effective imaged slice thickness along the z-axis, measured as the full width at half maximum (FWHM) of CT numbers across an inclined phantom test ramp.

Irradiated Slice Thickness

The full width at half maximum (FWHM) of the radiation dose profile along the z-axis from a single scan, which is always equal to or greater than the imaged slice thickness.

Mean Absorbed Dose ($D_T$)

The mean energy imparted by ionizing radiation per unit mass in a specific organ or tissue, measured in Gray ($1\,Gy = 1\,J/kg$).

Equivalent Dose ($H_T$)

The absorbed dose to a tissue multiplied by the radiation weighting factor $w_R$ ($H_T = \sum w_R D_{T,R}$), measured in Sievert ($Sv$).

Effective Dose ($E$)

The sum of tissue-weighted equivalent doses across all body organs ($E = \sum w_T H_T$), measured in Sievert ($Sv$), representing total stochastic biological risk.

Computed Tomography Dose Index (CTDI)

The integral of the single-scan radiation dose profile along the z-axis normalized to nominal slice thickness $T$: $CTDI = \frac{1}{T} \int_{-\infty}^{\infty} D(z)\,dz$.

CTDI100, air

A CTDI measurement performed free-in-air at the gantry center of rotation using a 100-mm pencil ionization chamber.

Weighted CTDI ($CTDI_w$)

An estimate of average single-slice cross-sectional dose in standard 16-cm or 32-cm PMMA phantoms: $CTDI_w = \frac{1}{3} CTDI_{100,c} + \frac{2}{3} CTDI_{100,p}$.

Volume CTDI ($CTDI_{vol}$)

A CT radiation dose index representing average dose across a scanned volume in helical CT, calculated as $CTDI_{vol} = \frac{CTDI_w}{pitch}$.

Multiple Scan Average Dose (MSAD)

The average radiation dose along the z-direction over the central scan in a series of contiguous axial CT exposures.

Planar Average Equilibrium Dose ($D_{eq}$)

The dose metric proposed in AAPM Report 111 to replace $CTDI_{vol}$ using direct measurement in wide phantoms with small ion chambers.

Overbeaming

The extension of the physical X-ray beam along the z-axis beyond the active detector array width in multidetector CT to deliver uniform radiation intensity across all active detector rows.

Overscanning

The additional gantry rotation required at the start and end of a helical scan volume to provide sufficient data for tomographic reconstruction.

Z-Axis Geometric Efficiency

The ratio of the dose profile area integrated over the imaged slice range to the total z-axis dose profile area, expressed as a percentage ($GE = \frac{A}{A_{tot}} \times 100\%$).

Dose-Length Product (DLP)

A measure of total radiation exposure for a CT examination, defined as $DLP = CTDI_{vol} \times \text{scan length}$ and expressed in units of $mGy \cdot cm$.

Size-Specific Dose Estimate (SSDE)

A patient dose metric introduced in AAPM Reports 204 and 220 that tailors $CTDI_{vol}$ using patient anatomical width or water equivalent diameter conversion factors.

Diagnostic Reference Level (DRL)

An investigation level tool used in medical imaging to support dose optimization without functioning as a legal upper safety limit or optimum dose standard.

DRL Value

A benchmark standard established at the 75th percentile of distribution medians for a measured DRL quantity collected across broad regional or national facility surveys.