PHYS C9

Radiologists

doctors who help make a diagnosis from a medical image and use interventional radiology to guide minimally invasive procedures and/or provide treatment

Radiographers

use x-rays and MRI to produce images of internal body structures

Fluoroscopy

real-time, continuous x-ray image allows visualisation of organ movement

radiographer operates imaging equipment while radiologist, surgeon, or other specialist performs invasive part of procedure

Radiography Imaging Systems

different shapes and sizes

25 to 150 kVp

10 to 1200 mA

Radiography Imaging Systems Components

X-ray generator

Coolidge x-ray tube (cathode/anode)

Digital detector that captures non-attenuated radiation

Modern Radiography Imaging System Examples

Digital Radiography (DR)

Computed Radiography (CR) - older

Portable/Mobile Units

Dental

Mammography

Fluoroscopy

Computed Tomography (CT) and Cone Beam Computed Tomography (CBCT)

Medical Imaging:

MRI

PET/SPECT

Ultrasound

Supporting the Imaging System

X-ray tubes can be suspended from ceiling or on a stand attached to floor

detector (bucky) beneath table or behind bucky stand (chest x-rays)

Portable C-arms rotate freely around patient without being attached to table in surgical or interventional settings (e.g. fluoroscopy)

Tables floating, easily unlocked, and are moved manually or via motor

Operating Console

Anatomically Programmed Radiography (APR)

Automatic Exposure Control (AEC)

Safety features: twexposure switches, emergency stop buttons, and indicators for tube status and radiation emission

Integrated displays show thumbnail images and allow image annotation

Anatomically Programmed Radiography (APR)

pre-set protocols (e.g. Spine Cervical AP/Lateral) load recommended factors (kVp, mA, time) but can be modified by radiographers via touchscreen controls

Automatic Exposure Control (AEC)

console detectors measure radiation reaching the receptor and terminates exposure time (mAs) once appropriate dose is receive

DAP vs AEC

DAP (Dose Area Product): ionisation chamber mounted on collimator, indicates radiation dose

AEC: ionisation chamber positioned after patient, before image receptor, indicates image quality

keV Radiation Therapy

Low energy radiation used when cancer is close to/on skin surface

E.g nasal cavity tumours

Superficial RT: 50 - 200 keV used to treat cancer up to 5mm from skin surface

Orthovoltage RT: 200-500 keV used to treat cancer 4-6cm deep

X-ray Generator Circuit

autotransformer adjusts input mains voltage

step-up transformer increases voltage

Rectifier converts high voltage AC to DC

step-down transformer reduces voltage to tube filament

Autotransformer

portions of the same winding act as both the primary winding and secondary winding sides rather than separate primary and secondary windings with no conducting connective path

Autotransformer Self-Inductance

changing current in a coil induces a voltage in that same coil

AC current flows through coil, producing a changing magnetic field. This induces a voltage along same coil

Depends on number of turns (higher = more) and material conductivity

Bottom = low V

Top = high V

Autotransformer Tap point

movable or fixed connection that controls the output voltage

Step-Up vs Step-Down Autotransformer

Step-Down:

N₂ < N₁

V₂ < V₁

V₁ applied across entire winding, output taken from a potion of it

Step-Up:

N₂ > N₁

V₂ > V₁

V₁ applied across portion of winding, output taken across full winding

Autotransformers in an X-ray Circuit

One autotransformer simultaneously serves 3 functions:

power supply for the high-voltage (kV) circuit that selects tube kVp

power supply for the filament (mA) circuit, which uses tap points to control filament temp, thus tube current

Compensates for incoming mains voltage fluctuations

Autotransformer’s kVp selector

varies kVp to tube by controlling input to step-up transformer

major (increments of 10) or minor (increments of 1)

Mutual Induction

effect of one coil's magnetic field on another coil's magnetic field

Magnetic field produced by circuit 1 induces a current in circuit 2. This induced current has its own magentic field that interacts with the magnetic field of circuit 1

dependent on geometrical arrangement - the closer the two circuits, the larger the magnetic flux through circuit 2 (larger mutual induction)

X-ray Generator Circuit Sections/Sub-Circuits

Low-voltage circuit

Filament circuit

High-voltage circuit

Low-voltage circuit

Power supply (Mains AC)

Main switch - ensures mains voltage ts within 5% either way of expected voltage

Autotransformer

Autotransformer’s kVp selector

Exposure switch - closes circuit so current flows to primary side of step-up transformer

Primary side of Step-up transformer

Filament circuit

Autotransformer

mA selector (sets filament amperage, controlling filament heat and number of electrons at filament - thus tube current)

Step-down transformer

X-ray tube filament

High-Voltage Circuit

Current only flows during exposure

Danegrous because of high voltage so cables are thick to meet insulation requirements

Secondary side of step-up transformer (minimum ratio 500:1)

X-ray tube

High-Frequency Generator (rectifier)

High-Frequency Generator

Rectifier unit (diode) - changes AC to DC so current only flows from filament to target

DC voltage fed into inverter circuits (high-speed solid-state switches e.g MOSFETs or IGBTs) that switches on and off quickly to convert DC into high frequency AC (kHz to MHz)

high frequency AC boosted by Step-Up transformer

transformer output converted from AC into smooth, high-voltage DC via rectifier circuit (voltage multiplier or bridge)

Purposes of different voltages

high frequency AC allows for smaller and lighter transformers and capacitors

smooth (low ripple), high-voltage DC improves image quality, speeds up response times, reduces patient dose and energy wasted as heat

Computed Radiography (CR)

CR plate phosphor layer traps electrons when exposed to x-rays

Reader uses laser to scan plate causing trapped electrons to release stored energy as light

PMT collects released light and converts it to electrical signal

High-intensity light washes over plate to erase latent image, enabling reuse

Computed Radiography (CR) - Pros and Cons

Pros: low inital cost, flexibility

Cons: processing time, reader, cost of replacement plates

Digital Radiography (DR): Direct Conversion

Photoconductor/semiconductor captures x‐ray energy and converts it to electron-hole pairs proportional to intensity of incident energy

Electron-hole pairs collected under high voltage to prevent recombination and lateral spreading

Resultant charge stored at local capacitor in TFT array

E.g: CdTe, CZT, Si (mammography)

Digital Radiography (DR): Indirect Conversion

scintillator’s thin needle-shaped crystals converts x‐ray energy into visible light proportional to intensity of incident energy

photodiodes convert light to proportional charge

Resultant charge stored at local capacitor in TFT array

E.g: CsI/GOS/LYSO with SiPM/Silicon PIN photodiode

DR Pros and Cons

Pros: Image acquisition speed, excellent image quality

Cons: High initial and detector replacement cost, redundancy concerns, downtime possibilities (systems stop working for a period of time)

Cons of Indirect Conversion Only: lower spatial resolution (as light lost) and increased noise

DR and CR Similarities

Use a medium to capture x-ray energy

Produce an enhanceable digital image within seconds

How to distinguish between DR and CR

CR uses cassette to house imaging plate whereas DR just uses flat panel detector

Transistor

semiconductor electronic switch

controls the flow of electricity in circuits

Thin Film Transistor (TFT) Array

Millions of individual detector elements (DELs) arranged in a grid, each controlled by a switch, capacitor, and active area

deposited onto a glass substrate in multiple layers

Lowest level = readout electronics, Highest level = charge collector arrays
Charge collection electrodes (direct) or light sensing elements (indirect) deposited at top layer

DEL has either semiconductor (photoconductor) (direct) or scintillator and photodiode (indirect)

charge collected at each storage capacitor is amplified and quantified to a digital code value for the corresponding pixel

Amplifier boosts the weak data signals

ADC (Analogue-to-Digital Converter) converts signals into digital values

timing of the readout (charge from DELs) is controlled, enabling high-speed image acquisition

TFT Switch

Keeps DEL "off" during exposure, allowing storage capacitor to collect and store the charge generated from x-ray photons or visible light

Closes to release stored charge to readout electronics row-by-row onto data lines

Capacitor

Storage device that holds the electrical charge (signal) generated by X-ray interactions within each DEL from exposure until TFT switch closes to create image

Active/Sensing Area

collects charge generated by x-ray absorption (photodiode or photoconductor)

ADC (Analogue-to-Digital Converter)

continuous analogue electrical signals, proportional to x-ray photon intensity at DELs, translated into discrete digital binary data

analogue electrical signals measured at specific, regular intervals

Specific grey level (binary number) assigned to each measurement and mapped to a pixel in image

Measurements to define receptor’s accuracy and efficiency

Fill Factor

Point Spread Function (PSF)

Modulator Transfer Function (MTF)

Detective Quantum Efficiency (DQE)

Exposure Index (EI)

Sensitivity number (S‐number)

Radiographic contrast

Fill Factor

The size of the active area compared to total area on each DEL in TFT array

Higher = more efficient = fewer x-rays needed (lower pateint dose) as more x-rays contribute to signal rather than being lost

Point Spread Function (PSF)

how a single, ideal point of x-ray radiation is blurred across neighbouring pixels in final image

measures spatial resolution: narrow PSF = high spatial resolution

Larger pixels, larger focal spots, and thicker scintillators (spread light before detection) create wider PSF which reduces image sharpness

Modulator Transfer Function (MTF)

measures accuracy of detector in transferring objects’ contrast to an image at specific spatial frequencies (resolutions)

High MTF = better spatial resolution (good at maintaining sharpness)

Ranges from 0 (no structures are visible) to 1 (all structures are visible)

MTF of 0.5 means system reproduces 50% of object contrast at that spatial frequency

Detective Quantum Efficiency (DQE)

Measures how efficiently a digital receptor captures x‐rays and converts them into acceptable radiographic density

High DQE = less x-rays needed (lower patient dose) and better radiographic detail

Low DQE = increased noise

DQE of 0.4 means receptor can utilise 40% of received x‐rays

0.3 - 0.5 = acceptable/moderate performance

Exposure Index (EI)

Aka: DEI, REX, EXI

numerical value representing the radiation level incident on the image detector

used to compare systems’ sensitivities

linear estimate of detector radiation exposure, values proportional to dose, so can determine whether exposure used was appropriate and limit dose creep

Deviation Index (DI)

compares the actual Exposure Index (EI) to the protocol Target Exposure Index (EIT) for an image

Tells radiographer how far they are from ideal exposure

0 is perfect

+1 is 25% over, +2 is 50% over…

-1 is 20% under, -2 is 40% under…

-2 to +2 usually acceptable

Radiographic Contrast

determined by number/size of pixels in a given field of view, which is directly proportional to number of DELs on TFT array

Many/small pixels = increased image contrast

If a DEL is dead, the ouput from surrounding DELs is averaged to form that pixel

DR - Quality Assurance Regular Tasks

Daily: warm-up, visual check

Monthly: detector calibration

Annually: full system evaluation

DR - Quality Assurance Specific Activities

tracking image rejects using software

image receptor cleaning

spatial resolution, contrast, and uniformity tests

ensure detector responses are reproducible at fixed exposures

DR - Quality Assurance Purpose

Ensure consistent image quality

Maintain detector performance over time

Support dose optimisation (avoid dose creep)

Early detection of faults and artefacts