Week 2/3 - CT Instrumentation

Explain centrifugal (g) force

Acceleration of a rotating object relative to gravity, 9.81m/s². Therefore an object rotating at accelerations of 10g means that the force is 10 x the force of gravity.

Explain the CT gantry

The large cylindrical machine that houses the x-ray tube, detectors and generators.

State these components of the x-ray tube

• A - Cathode

• B - Focussing Cup

• C - Filament

• D - Rotating Anode

Explain the role of the filament

The filament is heated and releases electrons via thermoionic emission, creating an electrode cloud that accelerates electrons towards the anode.

Explain why the anode is rotating

The rotation of the anode disperses heat

Explain the role of the anode

The electrons hit the anode made of tungsten metal which produces x-rays.

Explain the orientation of the x-ray tube in relation to the patient and the x-ray beam produces

The anode-cathode axis is orientated parallel to the long-axis of the patient and is perpendicular to the direction of the x-ray beam produced.

State the 2 requirements of an x-ray tube for CT

• Produce high intensity x-rays

• Ensure rapid heat dissipation

Differentiate between x-ray intensity and quality

• Intensity - the number of x-ray photons

• Quality - the energy of the x-ray photons

Explain the anode heel effect

Radiation intensity is greater at the cathode than at the anode due to the absorption of x-rays at the anode heel.

Explain how the x-ray target angle alters the anode heel effect

Small x-ray target angles will increase the amount of x-rays absorbed at the anode heel.

Explain how high x-ray intensity is achieved with the x-ray tube

1. High x-ray tube current (200-800 mA) increases number of electrons produced

2. Large focal spot size to prevent anode melting

3. High voltage (>120 kV) applied across the x-ray tube from the generator

State the formula to calculate power of the x-ray tube with units

Power (kW) = voltage (kV) x current (A)

State the range of power that can be produced from the latest CT scanners

20-100 kW

Explain focal spot size

The specific area of the anode bombarded by electrodes

Differentiate between the advantages and disadvantages of small vs large focal spot sizes

• Smaller - better resolution however greater and more concentrated heat deposition that reduces the amount of x-ray power that can be used

• Larger - poorer resolution however lower and less concentrated heat deposition that allows for higher x-ray power use

Explain why the x-ray beam intensity should not vary by more than 1%

It will form beam hardening artefacts and falsely change the value of the linear attenuation coefficient as the transmitted intensity will be higher than it should due to the absorption of lower energy photons, increasing average beam energy.

Explain how rapid heat dissipation is achieved with the x-ray tube

The anode has a large diameter and thickness that rotates at 10,000 rpm, switching the position of the focal spot as the anode rotates.

State how the use of a rotating anode alters detector width and number of projections

• Halves detector width

• Doubles no. of projections

Explain how older (first and second-generation) x-ray tubes achieved heat dissipation

Stationary anodes that were oil-cooled.

Explain heat units (HU)

A value that represents the amount of thermal energy the CT’s x-ray tube can safely endure.

State the range of heat capacity of a CT machine in HU

3-5 MHU (mega/million heat units)

State the formula for HU

HU = voltage (kVp) x current (mA) x time (sec)

Explain why voltage, current and time cannot be decreased to reduce heat produced

This will create a noisy image

Explain the modifications that can be made for pediatric imaging

Increased current (mA) will reduce the time required for the child to sit still during the scan

State the type of generator within the CT gantry

High-voltage, high-frequency, 3 phase generator

State and explain the components of the generator

• Rectifier - converts AC to DC

• Step-up transformer - amplifies voltage to be applied across the x-ray tube

• Step-down transformer - lowers voltage to heat the filament

Explain why high kV generators are used in CT

To minimise photoelectric absorption and maximise Compton scattering to reduce noise, improve image quality and reduce patient dose.

State normal frequency (pulses/sec) and high frequency in CT (pulses/sec)

• Normal - 50 pulses/sec

• High - 30,000 pulses/sec

State whether CT scanners emit x-rays continuously or in pulses

• Majority - continuously

• Some, eg. Dual energy - in pulses

Explain reasons for energy loss in CT

• Heat loss

• Filtration

• Collimation

State and explain the type of filter used in CT

A bowtie filter that removes low-energy x-rays to harden the beam, make it as mono-energetic as possible and reduce patient dose. Additionally its shape equalises the x-ray beams intensity in which it is thinner in the centre as there is more attenuation by the patient whereas it is thicker on the edges as there is less attenuation by the patient.

State and explain the types of metal that make up the filter

• Copper - located towards x-ray tube in which it filters medium-energy photons and produces characteristic x-rays

• Aluminium - locates towards the patient and filters low-energy photons as well as the characteristic x-rays to reduce patient dose

Explain filtration with reference to this graph

Filtration reduces radiation beam intensity (no. Of photons), specifically filtering lower-energy photons in which the average photon energy is shifted to the right, therefore greater.

State and explain the 2 types of collimators

• Pre-patient - defines the slice thickness

• Post-patient - reduces scatter reaching the detector

State the 2 types of CT detectors currently used

• Solid state scintillation detectors (most common)

• Gas-filled ionisation chambers (older and less common)

Explain dynamic range

The range of photon energies that the detector can pick-up/differentiate. High dynamic range means the detector can differentiate between very small differences in photon energies, therefore in attenuation.

Explain why traditional scintillation/PMT detectors are no longer used in CT

Geometric efficiency is limited as PMT’s cannot be closely packed together due to their bulkiness.

State the types of scintillation materials that were used

• Na(TI)

• BGO

• CaF2

Briefly explain how gas-filled detectors work

Long narrow chambers filled with xenon gas and separated by metal electrodes. X-rays interact with gas and cause ionisation reactions in which ions are attracted to their respective electrodes, forming an electric pulse.

Explain why xenon gas is used in gas-filled detectors

Gas is low density which decreases the chance of interaction, therefore xenon gas with a high atomic number increases the chance of interaction.

Why are gas-filled detectors also considered intrinsically directional

The metal electrodes converge towards the x-ray tube, increases scatter rejection.

Explain why solid-state scintillation detectors are preferred over traditional scintillation detectors

Scintillation crystal is couples to thin photodiodes that convert light into electric signal rather than bulky PMTs, increasing geometric efficiency.

State the type of scintillation materials typically used in solid-state detectors

• CsI

• CdWO4

• Yttrium

• Gadolinium

Explain cross-talk between detectors

When visible light is scattered and detected by neighbouring detectors.

Explain how cross-talk is reduced

By placing a light absorbing material between detectors.

Briefly explain how photon counting detectors work

Contain a semiconductor material that directly converts photons in electric signals rather than light, then electric signals. Therefore the detectors counts each individual photon.

Briefly explain first-generation CT scanners

A rotate-translate type systems in which a pencil x-ray beam was used with one detector that were fixed in a relative position. To acquire a projection, the tube and detector were translated laterally to acquire raysums over a series of positions. The gantry then rotates by a small angle and another projection is acquired.

Explain the trade-offs of first-generation CT scanners

• Very time consuming (4-5 mins per projection) therefore could only image static areas of the body, eg. Brain

• Low efficiency due to only a very small fraction of the generated x-ray power being used after filtration and collimation to produce a pencil beam

• Excellent scatter rejection due to use of single-detector

Briefly explain second-generation CT scanners

This involves the same rotate-translate system as first-generation scanners however a transition to the use of a small fan beam and an arc of 30 detectors.

Explain the trade-offs of second-generation CT scanners

• Fan beam means x-ray tube output is more efficient

• Reduced scan time due to fan beam and multiple detectors reducing the number of rotations required

• Greater number of detectors increases susceptibility to scatter, artefacts and reduced image quality.

Briefly explain third-generation CT scanners

Transition to rotate-rotate type systems in which the fan beam was widened to cover the entire cross-section of the patient with an arc of 500-900 detectors. Therefore translation no longer required and only rotation required to acquire each projection.

Explain the trade-offs of third-generation CT scanners

• Reference detection in which the detectors on the ends of the arc detect rays that haven’t passed through the patient, therefore continuously measure I₀ for calibration.

• The elimination of translational motion has dramatically reduced scan times

• X-ray tube and detector are still fixed in a relative position, therefore an anti-scatter grid can be used to reduce side scatter

• Due to increased number of detectors, complexities with the number of cables required and their requirement to extend and retract during gantry rotation.

Briefly explain fourth-generation CT scanners

Transition to rotate-stationary type system in which the x-ray tube is mounted and rotates between the patient and a ring of thousands of stationary detectors that completely encircle the patient.

Explain the trade-offs of fourth-generation CT scanners

• Stationary detectors eliminates cabling issues

• Each detector can measure I_0, not a few of them.

• Ring of detectors means that anti-scatter grids cannot be used

• Nutation occurs in which the detectors behind the x-ray tube must wobble/tilt out of the way for the rotating source.

• Image reconstruction can only occur after entire scan is complete, not after each projection as detectors are continuously being exposed.

Briefly explain fifth-generation CT scanners

Transition to stationary-stationary type systems in which all motion is eliminated. An electron beam is magnetically steered around the tungsten anode ring below the patient, producing fan beams that are detected by an arc of detectors above the patient.

Explain the trade-offs of fifth-generation CT scanners

• Extremely fast acquisition times that allow for cardiac imaging without motion blurring

• No rotation within the gantry

• Extremely expensive

State what sixth and seventh-generation CT scanners are also named

• 6th - helical CT

• 7th - multi-slice CT