Quality Control Notes

Quality Control

• Quality Assurance: A managerial philosophy encompassing all aspects of patient care, image production, and image interpretation.
• Quality Control: Calibration and monitoring of equipment, some tests require special equipment and should be done by a medical physicist.

Radiographic Equipment Testing

Components of a Quality Control Program

• Acceptance testing at the time of purchase.
• Establishment of baseline performance parameters.
• Diagnosis of correctable deviations in performance.
• Documentation of actual corrections made.
• Guidelines for testing and suggested parameters of acceptance have been published by the American Association of Physicists in Medicine (AAPM) and other related organizations.

Equipment Tests: Frequency and Parameters

Radiographic Units

• Exposure Timer:
  • Checked by a physicist using a photodiode device or ion chamber.
  • Should be checked any time a major repair or change has occurred.
  • Guideline: ± 5%
• mA Linearity:
  • The accuracy of a particular mA station relative to the two adjacent mA stations in radiation output.
  • Must be within 10% of each other.
  • The least reliable “technique” factor and most difficult to calibrate.
  • Guideline: The 10% rule applies only to adjacent stations.

Calculating Percentage of mA Linearity

1. Subtract the difference in output between the two stations, divide this by the lesser of the two, and multiply by 100.

   • Example:
     • For 100 mA station at 1 second: 6.4 mGy is measured.
     • For 200 mA station at 1 second: 11.2 mGy is measured.

2. Divide mAs values into gray measurements to give you mGy/mAs.

   • Ex: 6.4/100 = 0.064 mGy/mAs

3. Convert mGy/mAs values to µGy/mAs

   • Ex: 0.064 mGy = 64 µGy

4. Plug in values into the formula:

   \frac{64 \frac{µGy}{mAs} - 56 \frac{µGy}{mAs}}{56 \frac{µGy}{mAs}} = \frac{0.8}{56} = 0.143

   0.143 \times 100 = 14.3

   The 200 mA station is 14% out of calibration, outside the 10% limit.

• Exposure Reproducibility:
  • The ability to repeat the same overall technique settings and obtain the same results in actual exposure.
  • Ten exposures are made using identical technique settings.
  • The average exposure is calculated by summing and dividing.
  • Each individual exposure is then checked against the average.
  • Guideline: ± 5% from average.
• Half-Value Layer:
  • Defined as the thickness of absorber material (usually aluminum) required to reduce the intensity of the x-ray beam to one-half its original.
  • The only true measure of the actual penetration capability of the x-ray beam.

Half-Value Layer

• The most important quality of the x-ray beam is its ability to penetrate through the human body to produce an adequate image.
• Beam penetration may be raised by:
  1. Adding filtration, which raises the minimum energies.
  2. Increasing kVp, which raises the maximum energies.
• Any ion chamber can be used with a series of aluminum slabs of varying thicknesses to determine the HVL.
• Minimum required HVLs are provided in tables published by regulatory agencies and scientific groups, particularly the NCRP (National Council of Radiation Protection and Measurement), Report #102.
• The HVL must be specified for each type of generator and for each level of kVp.
• High HVL indicates that the x-ray beam is highly penetrating.
• If the measured HVL falls below the required amount, it can indicate that either:
  1. The calibration of the kVp control is off.
  2. There is insufficient filtration in the x-ray beam.
• The kVp can be ruled out as the cause by using electronic calibration tools.
• Once the kVp is calibrated, the only remaining cause of insufficient HVL can be insufficient filtration.
• Insufficient HVL overrules the general minimum filtration rule - If kVp is accurate and the HVL is insufficient, filtration must be added until the HVL is adequate, even if it is more than 2.5 mm Al.
• kVp Calibration:
  • For digital equipment, done by physicist using:
    • Ion chamber or photodiode with filtration
    • Voltage diode instrument or oscilloscope
  • Guideline: ± 5%
• Collimator:
  • It is not uncommon for edges of actual x-ray field to be as much as 1 cm off from the edges of the projected light field due to collimator:
    • Belt or gear slippage
    • Mirror crooked
  • Guideline: ± 2% of SID
• Distance:
  • A simple tape ruler can be used.
  • Measure from the red “+” sign or other mark on the anode end of the x-ray tube housing, which designates the exact location of the focal spot.
  • Guideline: ± 2% of SID
• Focal Spot Blooming, Size and Condition:
  • Due to the complexity of x-ray tube manufacture, regulations allow actual focal spots under 0.8 mm to be 50% larger, and those over 0.8 mm to be 40% larger, than the nominal (advertised) size.
  • Actual focal spot size is larger at higher mA stations, due to blooming of the electron cloud from mutual repulsion of more electrons.
  • Sudden deviations in FS size indicate damage to the anode.

Focal Spot Check

• The “Slit” Camera:
  • A lead template with finely cut slits of decreasing width.
  • Lines are placed perpendicular to the FS dimension being measured.
  • Exposure is scanned to find the blur point in each direction.
  • A table converts the blurred grouping into LP/mm.
• LP/mm = Line Pairs per Millimeter
  • A measure of sharpness or resolution.
  • The greater the resolution, the smaller the focal spot.
  • Once a baseline measurement is recorded, any sudden loss of resolution indicates damage to the focal spot.
• Automatic Exposure Control (AEC):
  • Reproducibility should fall within ± 10%.
  • AECs should also be linear within 20% between different rooms.
  • Back-up timer can be tested by placing leaded rubber sheets over all detectors and seeing if the exposure terminates automatically at the back-up time or at 600 mAs.

Fluoroscopic Units

• Since patient exposure levels can be very high, QC tests must be performed regularly and accurately by a medical physicist, to include “spot-film” devices exposure levels and collimation, and AEC function.
• A medical physicist should perform calibration checks after any major change or repair of the tube, generator or console.
• Simple observational tests for distortion can be performed by the radiographer, including checks for pincushion and S-distortion, vignetting and veiling glare, described in Chapter 38.
• Annual checks should include:
  1. Minimum source-to-table distance of:
     • 38 cm (15 inches) for stationary units
     • 30 cm (12 inches) for mobile units
  2. 2 mm lead equivalent shielding in fluoro tower
  3. Proper filtration
  4. Proper automatic collimation
  5. Presence of bucky slot cover
  6. Presence of fluoro curtain
  7. Function of cumulative timer (5-minute)
  8. Output within guidelines (<10 R/min)
• In some states, the radiographer must record the amount of fluoroscopic beam-on time for each procedure, and in some, the number of overhead and spot exposures taken.

Monitoring of Digital Acquisition Systems

• Radiographers can monitor sudden changes in field uniformity, erasure thoroughness, intrinsic noise, and spatial resolution by visual checks.
• Field Uniformity:
  • Digital detectors are all inherently non-uniform.
  • Take a flat-field exposure with no object in the beam at a moderate technique.
  • Open collimation to cover the entire field, and use a long 180 cm SID.
  • Scan the resulting image for lighter areas or white specks.
• Erasure Thoroughness and “Ghosting”:
  • Ghost images can indicate:
    1. Bulbs in the reader are burned out.
    2. Loss of lamp intensity in the reader.
    3. Too short exposure to lamps in the reader.
  • They can also occur from extreme overexposure during the previous procedure (when residual electric charge is trapped).
  • Expose the imaging plate with a step-wedge or other homogeneous object in the beam, (and process if CR).
  • Immediately re-expose the same plate without the object in the beam and with 1 inch (2.5 cm) collimation in from each edge.
  • Scan the resulting image for any ghost image of the object.
• Intrinsic (Dark) Noise:
  • Erase and then immediately process a single plate without exposing it to an x-ray beam.
  • Scan the resulting image for any unusual amounts of mottle or noise compared to a baseline image.
• Spatial Resolution:
  • A flat wire mesh can be exposed, and the image examined for any distortion or blur.
  • A lead foil “bar” test pattern can be used to obtain spatial resolution in LP/mm

Monitoring of Electronic Image Display Systems

• The display monitor is the weakest link in the imaging chain.
• Consistency between all monitors within a workstation is essential.
• All should have the same luminance, be set at the same contrast, and be cleaned monthly.
• Guidelines and tests are available from several groups, including:
  • AAPM (American Association of Physicists in Medicine)
  • SMPTE (Society of Motion Picture and Television Engineers)
  • ACR (American College of Radiology)
    *Various test patterns are available from Task Group 18 of the AAPM.
    *This universal, generic test pattern is available from SMPTE (The Society of Motion Picture and Television Engineers).
    *It includes checks for most image qualities all in one template
  • Class 1 display monitors:
    • are those used for diagnosis.
    • These are subject to much more stringent guidelines than Class 2 monitors
  • Class 2 display monitors:
    • may be used for display and a limited amount of image manipulation, but not for documented diagnosis
• Luminance *The rate of light emitted from an LCD, LED, or CRT monitor *The unit for luminous flux (flow) as perceived by the human eye is the lumen *1 lumen = 0.0015 Watts of power *For the unit lumen to be meaningful, we must define the area of space that this amount of power is emitted across, or its concentration in space – This is the steradian *A steradian is the cone swept out by a 3D (solid) angle such that the area of its base is equal to the square of its radius from the light source (r^2) *There are roughly 6 steradians in a hemisphere (12.57 in a sphere) *The Candela *The total rate of light emitted by a typical candle in all directions *The lumen refers to the light flux a typical candle emits within just one steradian *The candela describes the power of the light source, whereas the lumen describes the power of the light flow (flux) traveling through space
  • 1 Cd = 1 Lm/sr
    *Examples of Luminance:
    *Maximum for an LCD = 800 Lm
    *Maximum for a CRT = 600 Lm
    *Maximum for a conventional viewbox = 3000 Lm
    *Typical range preferred by radiologists = 500-600 Lm on electronic display monitors
    *Measuring Luminance
    *LCDs need only be checked once each year
    *The photometer is a device used to measure the light intensity from a display monitor
    *Photometer read-outs are usually in units of lux (to be defined shortly), lumens or candela / m^2
    *Illuminance
    *Refers to the rate of light striking a surface, or how well objects in our field of view are illuminated by a light source
    *e.g., the luminance of a desk lamp illuminates a paper on the desk in front of it
    *The unit for illuminance is the lux, defined as 1 lumen per square meter of surface area
    *1 lux = 1 Lm / m^2
    * Typical office lighting: 75-100 lux
    * Radiologic reading room lighting: 2-25 lux
    * A radiologic reading room should have less than ¼ normal office lighting: 25 lux or less
    *Luminance and Contrast Tests
    *Maximum luminance: Check with photometer
    *Compare with baseline over time: Check LCD yearly
    *Luminance response: A monitor’s ability to accurately display different levels of brightness from a test pattern
    *Essentially identical to a contrast test
    *Essentially identical to a contrast test
    *DICOM GSDF (GRAYSCALE STANDARD DISPLAY FUNCTION)
    *The DICOM GSDF is based on human perception, with increments of brightness called JND’s (Just noticeable differences) à AAPM recommends that luminance response should fall within 10% of the GSDF standard
    *Adjacent squares on the SMPTE test pattern should measure 10% difference between each other on a photometer, within 10% accuracy
    *Luminance Ratio: Maximum luminance divided by minimum luminance L{MAX} / L{MIN}
    *An LCD is not capable of producing a true black level while it is energized
    *On the SMPTE universal test pattern, black-on-white and white-on-black bars are used to measure LR
    *Luminance Uniformity: Consistency of a single brightness level displayed across the area of the display screen
    *AAPM recommends that luminance uniformity not deviate more than 30% from the average
    *Luminance Uniformity is checked at 5 locations across the screen
    *Ambient Lighting and Reflectance
    *Diffuse reflectance is the ratio of the brightness of the general ambient light in the room that is being reflected off the surface of the display screen to the output brightness of the monitor itself
    *Specular reflectance refers to the reflection of a specific, localized light source behind the viewer of the display monitor, and is defined as the ratio of the reflected brightness to the brightness of that light source
    *In a diagnostic reading room, ambient lighting must be dimmed to a point where both diffuse reflectance and specular reflectance are below any noticeable level
    *LCDs generally have low reflectance
    *Electronic Display Resolution
    *Both the ACR and the AAPM recommend a minimum resolution for electronically displayed images of 2.5 LP/mm
    *On any test pattern for spatial resolution, series of high-contrast bars of diminishing width are used:
    *Horizontal bars measure vertical resolution
    *Vertical bars measure horizontal resolution
    *The universal SMPTE pattern has resolution bars in 5 locations: Center, and 4 corners
    *Each series consists of a set of high-contrast bars and a set of low -contrast bars
    *Dead and Stuck Pixels (LCD)
    *The LCD monitor can be visually inspected for faulty pixels and subpixels with the aid of a simple magnifying glass
    *For most LCDs, one pixel is just the size of a 12-point font period or the dot of an “i”
    *A defect smaller than this would be caused by the failure of a subpixel
    *There are three rectangular subpixels in each pixel
    *A truly dead pixel appears as a white spot against a solid black background
    *A stuck pixel appears as a black spot against a solid white background
    *A stuck pixel is being continuously supplied with electricity
    *For a Class 1 LCD monitor, the AAPM recommends as “passing criteria:
    *15 or fewer bad pixels across the entire screen
    *Three or fewer bad pixels within any 1-cm circle
    *Three or fewer bad adjacent pixels
    *Less stringent guidelines apply for class 2 monitors
    *Viewing Angle Dependence (LCD)
    *More expensive LCD monitors employ pixel elements that are slightly angled in order to increase the effective viewing angle
    *VAD can be measured using a photometer at a precisely maintained distance from the center of the monitor screen, as a percentage of the perpendicular luminance
    *Rapid loss of luminance at increasing viewing angles is one of the major disadvantages of LCDs

Repeat Analysis in the Digital World

• An unfortunate aspect of digital systems is that since rejected images are usually erased and permanently lost, there is no way to track them and follow up with targeted “repeat” analysis.
• Because the advent of digital imaging has dramatically reduced the rate of repeated exposures due to poor technical quality, we can state that most retakes are now due to positioning errors.
• A realistic target for diagnostic x-ray departments is to maintain an overall repeat rate of 3-5% due to technologist errors.