Radiography: Principles of Exposure

Fundamental Principles of Exposure

• Optimal receptor exposure is achieving the ideal radiation amount at the radiographic receptor during examination
• High number of photons relates to higher receptor exposure
• Low number of photons equates to lower receptor exposure

Factors Influencing Optimal Exposure

• Milliampere-Second (mAs) quantifies radiation produced over time via an x-ray tube
• Kilovoltage peak (kVp) measures the electrical force driving electrons
• Collimation reduces the beam area reaching the patient using lead shutters
• A grid, placed between the patient and receptor, absorbs scatter radiation with thin lead strips
• Source-to-image distance (SID) is the distance between the x-ray source and image receptor
• Speed class refers to the imaging system's sensitivity to radiation
• Fog refers to unwanted exposure on an image receptor

Types of Grids

• Linear grids, commonly used in radiology, feature lead strips in a focused, parallel direction
• Parallel grids have parallel lead strips that do not converge
• Focused grids have lead strips angled to match the x-ray beam's divergence
• Cross-hatched (crisscross) grids utilize two sets of perpendicular lead strips
• Moving (Bucky) grids are used in the Bucky mechanism

Effect of Grids

• Grids reduce scatter, improving image contrast
• Grids provide sharper images and better resolution
• Grids require higher radiation doses to compensate for X-ray absorption
• Risk of grid cutoff causes uneven exposure if misaligned
• Grid lines may be visible but can be minimized with moving grids

Speed Class and Exposure

• Higher speed class systems need less radiation for a diagnostic image
• Lower speed class systems need more radiation for proper image quality

Fog

• Fog is unwanted exposure on an image receptor, resulting in a gray background

Causes of Fog

• Scatter and light exposure (film radiography)
• Over development is a cause of fog

Digital Radiography

• Digital radiography revolutionized medical imaging, offering improved image quality, efficiency, and reduced patient dose
• Digital systems do not eliminate all radiation
• Factors like developer sensitivity, exposure parameters, and algorithms affect dose level
• Safety and proper protocol are essential to minimize exposure

Control Patient Exposure

• Patient exposure should be controlled to minimize radiation risk while maintaining diagnostic image quality

Strategies for Controlling Patient Exposure

• Optimize exposure parameters
• Employ digital image processing
• Use dose reduction techniques
• Train and educate personnel
• Regularly maintain and calibrate equipment

Higher kVp Levels

• Higher kVp levels improve image quality, reduce scatter radiation, and increase detector sensitivity

Minimizing Dose with Higher kVp

• Radiologic technologists should optimize exposure parameters and use digital image processing

Additional Methods to Reduce Exposure

• Monitor patient dose to stay within safe limits
• Implement additional filtration

Additional Filtration

• Additional filtration involves using materials like aluminum to absorb low-energy photons

Filtration Purpose

• Filtration improves x-ray beam quality and reduces scatter, enhancing image quality

Regulatory Standards for Filtration

• Radiology departments must adhere to specific filtration standards for patient safety and optimal image quality

Benefits of Filtration

• Filtration enhances the overall effectiveness of x-ray examinations and improves diagnostic quality

AEC

• AECs, or "Photo Timers," achieve consistent densities while reducing patient dose and repeat radiographs

Types of AEC Detectors

• Photo multiplier tubes are common in AECs, made up of lucite
• Ion chambers consist of two thin parallel sheets of aluminum or lead foils

Thyratron

• Both devices operate on similar mechanisms and allow for rapid control of electric signals

Time With AEC

• AEC controls the exposure time and thus mAs only
• Optimum kVp must be set or preprogrammed

Minimum Response Time (MRT)

• MRT is a concern when high-speed receptor systems, high-power generators, high mA stations, or small anatomical parts are used

Optimum mA

• Optimum mA is that high enough at a given focal spot size (FSS) to minimize motion, but not so high that the needed exposure times are shorter than the AEC can handle

Back-up Time

• Back-up time prevents patient overexposure if the AEC unit fails

Repeating Radiographs

• Whenever there is a technique analysis problem, the radiographer should always utilize a manual technique for the repeated exposure

ALARA

• ALARA, As Low As Reasonably Achievable, minimizes radiation doses

Three Major Principles for Radiation Safety

• Minimize time, maximize distance, and use shielding

Time, Distance, and Shielding

• Reducing time decreases radiation dose
• Increasing distance reduces exposure by the square of the distance
• Doubling the distance reduces radiation by a factor of 4
• Shielding uses impenetrable barriers.

Monitor Patient Exposure

• Monitoring patient exposure involves tracking and recording radiation a patient receives

Quality Assurance Program

• A QA program ensures equipment and staff perform their best
• QA includes equipment maintenance, patient radiation exposure monitoring, and image quality

QA Program Function

• QA includes Equipment maintenance, Patient Radiation Exposure, Image quality, Personnel
• Equipment maintenance includes visual checks and routine testing
• Patient Radiation Exposure monitors and reports patient doses
• Image quality analyzes results and takes corrective action
• Personnel QA ensures radiographers perform tasks correctly

Vendor-Supplied Software

• Vendor supplied software is the specialized software provided by the medical imaging equipment manufacturer

Evidence of Appropriate Exposure

• The exposure indicator (EI) or sensitivity (S) number indicates whether the image was taken at an appropriate radiation dose and the value could be over or under exposed

Exposure Indicator Range

• Each imaging system has a range

Overexposure

• Overexposure causes loss of soft tissue details

Underexposure

• Underexposure increases noise

Noise

• Noise are random grainy static-like patterns that degrade image quality

Computer Noise

• Computer Noise are distortions caused by software processing

Electronic Noise

• Electronic Noise is caused by interference in the detector system

Material Mottle

• Material Mottle is variations in phosphor screen materials that cause uneven brightness and occur in CR and DR systems

Quantum Mottle

• Quantum Mottle is grainy appearance due to insufficient X-ray photons reaching the detector

Contrast

• Contrast is the difference between black, white, and gray shades in an image
• Adequate contrast ensures clear visualization of anatomical structures
• Low contrast makes it difficult to differentiate tissues
• High contrast may lose soft tissue details

Recorded Detail

• Recorded detail is the sharpness of the image and how well fine details are visualized
• Recorded detail is affected by motion blur, pixel size, and system resolution

Artifacts

• Artifacts are unwanted distortions that can mimic pathology

Patient Artifacts

• Patient Artifacts include motion blur (from breathing, movement), jewelry, buttons, medical devices

Equipment Artifacts

• Equipment Artifacts are defects in grids, collimators, and "Grid cut off" in X-ray imaging

Exposure Artifacts

• Exposure Artifacts exist by overexposure or underexposure leading to loss of detail
• Incorrect technique setting (kVp, mAs) affects contrast and noise

Processing Artifacts

• Processing Artifacts result from errors in digital image processing

Moire Effect

• Moire Effect are stripped pattern artifacts caused by interference between the X-ray grid and detector pixel and exists in CR

Quality Assurance and Maintenance

• Quality assurance ensures standards are being met
• Maintenance Issues are a malfunction requiring work on equipment

Technologist Responsibility

• Technologist responsibility is a proper operation by doing regular check, calibration, and maintenance to produce high-quality images

Daily QC Duties

• Daily QC Duties include Inspect/Clean Cassettes, Inspect Hinged/Latch, Verify Interfaces/ Network, and Inspect Laser Printer

Weekly QC Duties

• Weekly QC Duties are: Clean CR cassette and inspect DR image receptor for dirt, Clean air intakes of CR reader-air, Clean CRT screen, -Keyboard,-Mouse,

Monthly QC Duties

• Monthly QC Duties include Reject Analysis

Reject Analysis

• Reject Analysis require that data be identified so that repeat exposure,number of repeats, and the technologist responsible for repeat can be analyzed
• personal repeat log be kept by each technologist
• A technologist is assigned to coordinate analysis of images

Service Personnel QC

• Service Personnel QC test reproducibility

Service Personnel Responsibilities

• The Service Personnel Responsibilities include X-ray generator, tube, and reader, Phosphor accuracy testing, Image processing/display functions, and Reader erasure

Radiation Physicist Responsibilities

• Radiation Physicist Responsibilities include multiple practices may be involved

Semiannual/Annual Review

• Semiannual/Annual Review include Reestablished baseline values and Check EI to accuracy