Week 5 Physics

What is cone beam CT (CBCT) acquisition?

-Uses a large cone beam and large array of detectors

-Table movement usually not needed

-Disadvantage: Increased x-ray scatter and increased cone-beam artefacts

-Common in radiotherapy

What is prospective cardiac gating (Step-and-shoot)

-ECG signal used to trigger x-ray exposure

-Data only acquired during end diastole

-Suited for coronary artery assessment, suspected CAD, calcium scoring

-Lower radiation dose

-Does not capture full cardiac cycle, sensitive to irregular heart beat

-Used for anatomy

What is retrospective cardiac gating (helical+ ECG tagging)

-Continuous data acquisition throughout cardiac cycle

-Allows full cardiac cycle functional assessment -physiological info

-Results in higher radiation dose

What is CT angiography (CTA)?

-Contrast enhanced CT technique used to visualise blood vessels

-Increasingly used as a non-invasive alternative to fluroscopy-based angiography

-Excellent contrast resolution and excellent 3D visualisation of vascular anatomy

-Requires only venous access unlike conventional aniography

-Iodine-based contrast used

-Automatic injector at controlled rate and timing

-Accurate timing (bolus timing) essential for optimal vessel opacification

What is CT perfusion imaging?

-Used to assess vascular perfusion and blood flow in specific organ

-Commonly used in stroke and vasospasm evaluation

-Acquires baseline and after contrast agent

-Used to generate perfusion maps

-CTP is high-dose procedure due to repeated imaging

How does CT image transmitted to computer?

-Detectors measure intensity from different angles

-Each voxel is calculated

-Voxel of each pixel converted to CT number

-Corresponding grayscale assigned

What is filtered back projection?

-Projection data acquired back-projected onto an image matrix

-All projections are summed at each pixel location to form the image

-Simple back projection produces a blurred image with characteristic “star” artefacts

-Filtering process used to remove star artefact

-More sensitive to noise and artefacts compared to iterative reconstruction

What is iterative reconstruction?

-Series of iterations during reconstruction

-First start with a guess image and compare it with the projection data

-Then goes through iterations to produce a final accurate depiction of object scanned

-Advantages: improved image quality, reduced noise and artefacts, dose reduction while maintain image quality

-Significant computing power

What is hybrid iterative reconsturction?

-Combines filtered back projection with itertive projection

-Fatser than full IR, less noise compared to FBP

What is model-based iterative reconstruction?

-Advanced form of IR using a detailed system and noise modelling

-Excellent noise reduction + image quality, significant dose reduction but computing is slow

What is cone-beam reconstruction?

-Used in cone-beam CT

-Based on filtered back projection

-Used in radiotherapy

What is deep learning reconstruction?

-CT reconstruction using AI

-Learns how to convert noisy/low quality images to high quality images

-Very low noise, excellent spatial resolution + contrast resolution, dose reduction, reduced artifacts

-Computationally intensive

What is helical CT reconstruction?

-In helical CT, data is acquired continously as table moves through gantry (unlike other methods for axial plane)

-Interpolation is required

-After interpolation, images are reconstructed using standard methods

What is isotropic reconstruction?

-Modern CT can produce isotropic voxels (equal dimensions in x, y and z

-Allos high-quality multiplanar (coronal, sagittal) and 3D reconstructions

-No loss of spatial resolution

-Essentail for CTA, radiotherapy

What is MIP -maximum intesnsity projection?

-Displays the highest attentuation value (maximum hu) alond each viewing ray

-Highlights high density structures

-Used for contrast-material filled structures

What is minimum intensity projection?

-Displays the lowest attenuation value (minimum HU) along each viewing ray

-Highlights low-density structures (air-filled regions)

-Can be used to generate images of central airways

What is AIP- average intensity projection?

-Displays average attenuation value along each viewing ray

-Provides smoothed representation of structures

-Can be useful for characterising internal structures of a solid organ or walls of hollow structures

-Suitable for contouring and dose calculation in radiotherapy

What is the dose like from CT?

-CT examination considerably higher compared to projection radiography

-Abdominal CT approx 8.15 mSv

-Important to minimise patient dose while maintaining diagnostic image quality

-Generally, kVp is fixed

-mAs primary parameter adjusted to control dose

-Lower mAs -lower dose but increased image noise

What is dose modulation?

-Modern CT scanners use mA modulation, automatic exposure control

-mA automatically adjusted during the scan

-Twos types angular and longitudinal

What is angular modulation (x-y plane) ?

-mA varies as the tube rotates around the patient

-Adjusts for difference in patient thickness

What is longitudinal modulation (z-axis)?

-mA varies along length of patient

-Adjusts for different anatomical regions

-Maintain consistent image quality while optimising dose

-Scout image used to determine the mA modulation profile before the scan

What is CT dose index?

-CTDI-based metrics are the standard method for estimating dose

-Dose indices (not direct patient dose measurment)

What is CTDI100?

-Represents dose distribution along z-axis

-not used clinically on own

What is weighted CTDI?

-Accounts for dose variation between centre and periphery

-Better approximation of human dose

What is volume CTDI?

-Adjusts CTDIw for helical pitch , pitch inversely proportional to dose

-CTDIvol =CTDIw /pitch

What is DLP (dose length product)?

-DLP= CTDIvol x scan length

-Represents total scan dose output, displayed on scanner console

-Approx proportional to effective dose

What are limitations of CTDI?

-CTDI is a dose index, not actual patient dose

-Measured in standard PMMA phantom, not real patients

-Dose not fully account for patient size or organ sensitivity

What is nationl diagnostic reference level service?

-Must regularly compare patient ose to national diagnostic level

-Not a regulatory limiy, it is a benchmark that when exceeded triggers a review

What is beam hardening?

-As an x-ray beam passes through dense materials, low energy photons are absorbed

-Increases average energy of beam

-Leads to beam hardening artefacts

-Cupping artefact: centre appears less attenuating than edges

-Dark streaks/ bands between dense strucutres

What is streak artefact?

Occurs when the attenuation levels of a region in the patient are excessive, which excees the dynamic range of the detector system.

-e.g metal filling in the teeth

What is partial volume?

-CT artefact when the voxels are large enough to encompass several types of tissue-such as bone and tissue

-High-resolution images reduces this artifact considerably

What is ring artifact?

-Occur due to a dead detector or miscalibrated detector

-Appear as a ring on multiple slices at exact location

What is dual-energy CT?

• Uses two X-ray spectra (e.g. 80 and 140 kVp)

• Attenuation depends on material composition, density and photon energy

• Materials with similar HU at single energy can be differentiated

• Enables material characterisation

What are some DECT Acquisition Techniques?

• Rapid kVp switching: single tube alternates between low/high kVp

• Dual-source CT: two tubes and detectors at different kVp

• Twin-beam CT: split filter produces high and low energy simultaneously

DECT Material Differentiation

• Separates materials based on energy-dependent attenuation

• Bone removal improves vessel visualisation

• Iodine mapping for blood pool imaging

• Virtual non-calcium imaging shows bone marrow oedema

DECT Clinical Applications

• Metal artefact reduction

• Renal stone characterisation (uric acid vs non-uric acid)

• Gout detection (monosodium urate crystals)

• Detection of silicone leakage

Photon-Counting Detector CT (PCD-CT)

• Uses semiconductors to directly convert X-rays to electrical signals

• Counts individual photons and measures their energy

• Produces energy-resolved (spectral) data

PCD-CT vs Conventional CT

• Conventional CT: energy-integrating detectors (measure total energy)

• PCD-CT: counts individual photons and sorts into energy bins

• Conventional CT loses low-energy photon information

PCD-CT Advantages

• Higher spatial resolution (smaller detector pixels)

• Reduced electronic noise (energy thresholding)

• Improved dose efficiency → dose reduction

• Spectral (multi-energy) imaging capability

• Improved iodine signal

DECT vs PCD-CT

• DECT: two energy spectra

• PCD-CT: multiple energy bins (true spectral imaging)

• PCD-CT provides improved contrast, resolution and dose efficiency