Comprehensive Guide to Computed Tomography: Physics, Techniques, and Dose Optimization

What is Computed Tomography?

A technique for displaying a representation of a cross section through a human body or other solid object using X-rays or ultrasound.

What are the two types of filters used in CT?

Beam shaping filter and Reconstruction Filter (Kernel).

What is the purpose of a physical filter in CT?

Reduces beam hardening artifacts, enhances image contrast, reduces image noise, and minimizes overall dose to the patient.

What is pitch in CT scanning?

The measure of overlap of the number of image slices during scanning, calculated as distance couch travels divided by width of slice.

How does pitch affect radiation dose in CT?

A larger pitch reduces dose but also reduces image quality due to less overlap between data acquisitions.

What are Hounsfield Units?

A scale that quantifies a range of radiodensities for different tissues in CT imaging.

What is the significance of windowing in CT?

Windowing allows operator-controlled manipulation of images to display different types of tissues and adjust density relationships.

What are the two effects of windowing?

Changes the type of tissues displayed and changes the density relationship between tissues in the final image.

What is spatial resolution in CT?

Quantifies image blurring or the sharpness of an image, determining the ability to discriminate between objects of varying density.

What factors affect transaxial resolution in CT?

Number of projections, reconstruction filter, and pixel size.

How is pixel size calculated in CT?

Pixel size (d) is given by the equation: d = FOV/n, where FOV is the field of view and n is the image matrix size.

What is z-sensitivity in CT?

The ability to resolve structures along the z-axis, affected by detector slice thickness and overlapping samples.

What is a reconstruction filter?

A filter used to construct a smooth analog signal from a digital input, balancing spatial resolution and noise.

What is the impact of slice thickness on image quality?

Thinner slices improve resolution but increase noise; thicker slices can increase partial volume effects.

What is the effect of rotation time on CT imaging?

Faster rotation reduces scan time and motion artifacts but increases image noise; slower rotation has the opposite effects.

What are scout images used for in CT?

To modulate the patient's attenuation profile from all tissues along the z, y, and x-axis for appropriate mA exposure.

What is the purpose of helical or spiral CT?

To acquire continuous volume data, allowing for faster image acquisition and improved 3D reconstruction capabilities.

What are the advantages of multi-slice CT?

Fast image acquisition, 3D reconstruction, and the ability to window images for better contrast resolution.

What is the relationship between slice thickness and noise?

Thinner slices yield better resolution but worse noise, while thicker slices reduce noise but may compromise resolution.

What is the role of the gantry in CT?

To house the X-ray tube and detectors, facilitating the rotation necessary for image acquisition.

What is the significance of the effective detector width?

A smaller effective detector width results in higher resolution in CT imaging.

What is the purpose of image reconstruction in CT?

To create a usable image from raw data collected during scanning, enabling visualization of internal structures.

What is photon starvation artifact?

An artifact that occurs when insufficient photons reach the detector, leading to image quality degradation.

What is metal artifact reduction (MAR)?

Techniques used to minimize artifacts caused by metal objects in CT images.

What is the importance of voxel volume in CT?

Voxel volume affects image quality and resolution, determined by field of view, matrix size, and slice thickness.

What is the advantage of isotropic scanning?

Allows for consistent pixel sizes in all dimensions, improving multi-planar reformatting and volume rendering.

What is the impact of doubling the number of photons in CT?

It can be achieved by doubling tube current, rotation time, or slice thickness, improving image quality.

What is Automatic Exposure Control (AEC)?

A system that automatically adjusts the radiation dose based on the patient's size and the area being scanned.

What happens if a patient is not centered correctly in the scanner's isocenter?

It can increase dose, affect AEC, increase image noise, and reduce image quality.

What does CT Dose Index (CTDI) measure?

It measures the dose to the detectors from a single gantry rotation, indicating dose per nominal slice width or volume.

What is the formula for Dose Length Product (DLP)?

DLP = CTDI value X length of the body area scanned.

What does SSDE stand for and what does it correct for?

Size-Specific Dose Estimate; it corrects for the patient's size to provide a realistic estimate of the dose delivered.

How does doubling the tube current (mA) affect CTDI and DLP?

It doubles both CTDI and DLP.

What is the effect of doubling the rotation time on CTDI and DLP?

It also doubles both CTDI and DLP.

How does pitch affect CTDI and DLP?

Doubling the pitch halves both CTDI and DLP.

What is the relationship between kVp and radiation dose?

Dose is approximately proportional to the square of kVp; doubling kVp increases the dose by a factor of about 4.

What are motion artifacts in CT imaging?

Artifacts caused by patient motion during a scan, which can mimic pathology and appear as directional shading or streaking.

What is the Partial Volume Effect artifact?

An artifact that occurs when different tissues are averaged within a single voxel, leading to misrepresentation of tissue characteristics.

What is beam hardening artifact?

Low attenuation streaks due to beam hardening, often associated with 'cupping' artifacts.

What is Single Source CT?

A CT system with a single x-ray tube and one detector, capable of acquiring two helical scans at different tube potentials.

What is Dual Energy CT (DECT)?

A CT technology that uses two x-ray tubes and detectors to acquire images at different energy levels, improving material characterization.

What are the applications of DECT in oncology?

Increased lesion conspicuity, better delineation of margins, and more accurate size measurements.

What are the pros of DECT?

Quicker acquisition, improved tissue characterization, material differentiation, and reduced need for additional non-contrast examinations.

What are the cons of DECT?

Increased dataset complexity, additional image reconstructions required, and requires additional training for interpretation.

What is Photon Counting CT?

An emerging detector technology that measures total energy from individual x-ray photons, improving image quality and supporting radiation dose optimization.

What are the advantages of Photon Counting CT?

Higher spatial resolution, improved contrast resolution, reduced image noise, improved dose efficiency, and spectral imaging from a single acquisition.

What is the significance of accurate patient centering in CT scans?

It minimizes dose, optimizes image quality, and ensures effective use of AEC.

What is the impact of over-scanning in CT imaging?

It results in unnecessary radiation exposure beyond the area of interest.

What is the role of image reconstruction algorithms in DECT?

They differentiate materials with similar attenuation by measuring attenuation at each energy level.

What is the effect of thinner slices in CT imaging?

They can lead to increased noise due to fewer photons per voxel.

What is the purpose of Metal Artifact Reduction (MAR)?

To reduce artifacts caused by metal implants in CT images.

How does patient motion affect CT image quality?

It can lead to misregistration of data, mimicking pathological conditions.

What is the importance of CT dose optimization?

To reduce radiation exposure while maintaining diagnostic image quality.

What factors affect the overall dose in CT imaging?

Tube current, rotation time, pitch, and kVp.