CT Detectors: Assembly & Characteristics

Detector Assembly

• Measures radiation transmitted through the body and converts it to an electrical signal proportional to radiation intensity
• CT detectors need high X-ray detection efficiency, fast response, and wide dynamic range
• Detector size is measured in millimeters
• An array includes reference detectors for calibration and artifact reduction

Detector Optimum Characteristics

• High detector efficiency means the ability to capture transmitted photons and convert them to electronic signals
• Low afterglow is a brief scintillation flash needing subtraction before image reconstruction
• High stability allows use without frequent calibration

Detector Efficiency Factors

• Stopping power of detector material
• Scintillator efficiency in solid-state detectors
• Charge collection efficiency in xenon detectors
• Geometric efficiency is defined as the amount of space occupied by detector collimator plates relative to the detector surface area
• Scatter rejection capability of the detector

CT Detector Requirements

Capture Efficiency

• Refers to how well detectors receive photons from the patient related to detector size and distance between detectors

Absorption Efficiency

• Refers to how well detectors convert incoming X-ray photons
• Concerned with the number of photons absorbed depending on physical properties, material, size, and thickness

Conversion Efficiency

• Refers to how well detectors convert absorbed photon information to a digital signal

Desirable CT Detector Qualities

• Stability, maintained by frequent calibration
• Response Time, rapid reaction to incoming photons
• Dynamic Range, a ratio of largest to smallest measurable signal; modern scanners can achieve 1,000,000:1

Scintillation Materials

• Sodium Iodide (NaI) was used in early scanners, but is not suitable for rapid sequential exposures due to phosphorescent afterglow
• Other materials include Bismuth Germanate (BGO), Cesium Iodide (CsI), and Cadmium Tungstate (CdWO4); current choice is Gadolinium/Yttrium Ceramics

Gas-filled Detectors

• Xenon and Xenon-Krypton options exist

Scintillation Detectors

• Solid-state detectors absorb nearly 100% of photons
• They allow 50% detector efficiency

Collimators

• Reduces patient dose by limiting tissue irradiation and improves image contrast by reducing scatter radiation
• Variable from 1-13mm, and is controlled by the software.
• Collimation width determines voxel length or section thickness in conventional CT

Types of Collimators

Pre-patient Collimator

• Limits area of the patient exposed to the useful beam
• It also limits the amount of X-ray emerging from the source
• Determines patient dose and affects the dose distribution across the slice thickness
• Also called source collimator

Pre-detector/Post Patient Collimator

• Restricts the X-ray beam viewed by the detector therefore reduces scatter radiation for better image contrast
• Determines sensitivity profile and slice thickness
• Does not affect patient dose
• Primary functions are to ensure proper beam width and prevent scatter

High Voltage Generators

• All CT scanners use three-phase or high-frequency power for high rotor speeds and power
• Most generators integrate into the gantry's rotating wheel
• CT uses high X-ray tube voltages, hundreds of milliamperes, and scan times between 0.5 - 2 seconds.
• High-frequency power supplies offer stable tube current and voltage
• Large focal spots (1 mm) are used at high settings up to 60kW; small ones (0.6 mm) are used below 25kW
• Heat loading is generally high, requiring high anode heat capacities
• Modern systems have X-ray tube capacities exceeding 3 MJ

Patient Couch Considerations

• Constructed of low atomic number material like carbon fiber
• Designed for smooth, powered, accurate positioning and automatic indexing
• Patient weight limit between 300-600 lbs where exceeding it can cause inaccurate indexing, table motor damage, or breakage

Computer System

• This is a unique subsystem to the CT scanner, linking the technologist and imaging components
• The microprocessor and memory determine reconstruction time.
• Array processors are replacing microprocessors allowing the reconstruction to occur in less than 1 second

Operating Console

• Where the CT technologist controls the scanner
• Many CT scans are equipped with 2-3 consoles for scanner operation, image processing and filming, and physician viewing and manipulation of images
• It controls technique factors, including kVp (above 120), tube current (100mA usual), and scanning time (1-5 seconds)
• Slice thickness is adjustable, with normal settings 1-10 mm, and 0.5 mm for high resolution
• Typically has 2 TV monitors for patient data and scan viewing

Imaging Storage

• Current scanners use magnetic tapes or discs
• A single tape accommodates 150 scans
• CT scans are routinely recorded on film (14x17) using a laser camera

Imaging Characteristics

• X-rays form a stored electronic image displayed as a matrix
• Each cell is assigned a number displayed as an optical density or brightness
• EMI Consisted of 80 x 80 matrix, or 6,400 cells
• Current scans use 512 x 512 matrix giving results to 262,144 cells of information
• Image matrix is the layout of cells in rows and columns

CT Number

• Numerical information within each pixel, otherwise called the Hounsfield Unit
• Pixel value determine the brightness

Image Characteristics

• Image reconstruction diameter is the Field of View (FOV)
• FOV and pixel size are directly proportional
• Matrix size and pixel size are inversely proportional

Image Parameters

• Pixel Size is calculated by Field of View divided by Matrix Size
• Tissue volume known as a Voxel defined by Pixel size squared times slice thickness

CT Numbers

• Pixel size is displayed on the monitor as a level of brightness
• This is related to the X-ray attenuation coefficient of tissue
• It is determined by the average energy of the X-ray beam and the effective atomic number of the absorber

Image Reconstruction

• Acquired projections by each detector are stored in computer memory
• Images reconstructed using a filtered back projection process
• Filters are mathematical functions, not physical components
• Solution of over 250,000 simultaneous calculations are needed

Image Quality Standards

• Conventional Radiographs: Spatial Resolution, Contrast Resolution, Noise
• CT Image Attributes: Spatial Resolution, Contrast Resolution, Noise, Linearity, Uniformity

Spatial Resolution

• Defined as the ability to image small objects with high subject contrast, thereby discriminate adjacent objects where good spatial resolution is function of pixel size
• CT spatial resolution is worse than X-ray radiography
• Matrix and FOV size determine spatial resolution
• The degree of blurring is a measure of spatial resolution
• A larger pixel size leads to lower subject contrast and poorer spatial resolution

Edge and Modulation Specifications

• Edge-Response Function (ERF) to reproduce a high contrast edge accurately is expressed mathematically
• Modulation Transfer Function (MTF) measures spatial resolution and equals the image ratio to the object

Spatial Frequencies

• Line Pair (lp) is defined as 1 bar and its equal width Spatial Frequency measures lines per unit length and is expressed in line pairs per centimeter (lp/cm)
• CT Scanners are measured by spatial frequency at an MTF of 0.1
• The best spatial resolution can be determined by the pixel size
• In addition, can be described by high-contrast resolution, blur, and modulation transfer function (MTF)

Resolution Improvement

• Achieved with a Smaller focal spot
• Systems with smaller detector
• Typical resolution in CT scanning is 0.5 to 1.5 lp/mm
• Detail (bone) reconstruction filters needed

Resolution Factors

• Section thickness (collimation) affects resolution in longitudinal plane critical to sagittal and coronal reconstructions
• Slice-sensitivity profile measures effective section thickness
• Factor that affects slice-sensitivity profile is the Pitch ratio

Low Frequency

• Large Objects, better contrast resolution

High Frequency

• Small Objects, better spatial resolution

Spatial Resolution Factors

• Detector Pitch is the center-to-center spacing of detectors along the array
• Better spatial resolution is achieved via Smaller detectors
• Views influence CT image resolution ability

Image Artifacts

• Higher spatial frequencies without artifacts
• More geometric unsharpness is experienced from larger focal spots therefore reduced spatial resolution
• Amplifies magnification blurring
• Larger slice thickness reduces spatial resolution
• Greater helical pitch reduces spatial resolution

Motion Artifacts

• Involuntary (e.g. Heart) and voluntary blurring is proportional to motion distance during scans

Field of View

• Defines physical dimensions of each pixel
• Smaller FOV provide high resolution and is a larger image of a smaller area

Contrast Resolution Defined

• Ability to distinguish soft tissues without regard for dimension and form
• CT excels in this and is superior to projection radiography, which is detecting low-contrast differences, and in the HU tissue values

Contrast Factors

• The capacity to scan objects is limited by size & uniformity of the object and the amount of system noise
• Absorption in the tissue is characterized by the linear attenuation coefficient which is a function X-ray energy and atomic number of the tissue
• CT resolution depends on the X-ray

Contrast Influences

• Voltage decreases increases contrast
• Tube current or scan times have no affect
• Medium like iodine aids resolution artificially
• Settings influence emphasis allowing operators to distinguish density

Factors Affecting Contrast Resolution

• mA, which linearly influences the number of X-ray photons
• Dose increase linearly with mA per scan
• Pixel Size (FOV)

Resolution Influences

• Thickness increase provides more photons
• Bone filters lower
• Soft tissues improve
• Larger patients show fewer X-rays

Noise and Graininess

• Can be defined as a uniform signal produced by x-rays
• In CT it is the variation in CT numbers above or below averages
• Appears as graininess that determines image quality which occurs from photon detection

Noise Considerations

• Increasing tube voltage, current, and scan time reduces noise
• Also reduces it through a larger voxel size, FOV or section thickness

Linearity Defined

• refers to the relationship between CT numbers and attenuation values
• it must be calibrated
• five=pin performance ensure calibration that scans air
• a deviation determines errors

Uniformity Defined

• a constant pixel value for regions that are reconstructed.
• The test can performed through an internal software for a spatial test

Level Mapping Functions

• CT commonly possesses 12 gray shades that range to create more images
• Attenuation can be described as linear and optimized through CT scans
• Window adjustments determine how densities are displayed

Scan Features

Scanogram

• Displayed through the examination via highly collimated scans

Air Calibration

• Performing accurate scan when air is set around 1000 for scanner functions

Algorithms

• Functions that are complex to perform for various edges

Aperture

• CT hole where the patient lays during exams

Artifacts

• Errors/Distortions from a subject

Attenuation Coefficient

• CT reminant energy from tissues

Bolus

• Radiopaque amount for high flow that performed by injectors

CRT

• Imaging monitors that use lines for protocols

Reverse Display

• Reversing an image and the vice versa

Histogram Functions

• Diagnostics through graphs

Helical Fields

• Calibrations to determine data cell amount, and inaccuracies from images (called out-of-field)

Interpolation

• Performs reconstruction for z-axis with value estimates

Interpolation Algorithms

• Aides linearity within older scans

Pitch

• Pitch movements during relation to collimations
• Defining acceleration through the slice

Slip Rings

• Helps with rotation that aid conductivity

Spiral

• Energetic rays allow 45-60 seconds
• The axes include collimation, FOV, Matrix

Multiplaner

• Helps to show several axis to create axial views

MIP functions

• Reconstructs pixels rapidly

SSD Shading

• Software to include values that belong to all objects within the region
• VR- Images along the CT

DAQ

• Electrified amplifier signal for multi-slice

3D functions

• Partial objects with volume to reconstruct images

Resolution Factors

• Table speed for long ways travels to the beam

Patient Factors

• Noise from levels for clear scans with good levels