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Accreditation Guidelines for Bone Densitometry[ TABLE OF CONTENTS ]

APPENDIX 7- Compliance testing of DXA equipment

The following program for equipment compliance testing is based in part on a protocol developed by the Department of Medical Physics of the New England Medical Center, Tufts University, MA, USA.

1. BMD Reproducibility

  • Examination of daily QA results for array (PA & lateral lumbar spine) and pencil beam (if relevant) modes, to determine whether the repeatability of the areal BMD results for the phantom have fallen within the manufacturer's limits of ± 1.5% of mean BMD.
  • Examination of longer term QA results based on multi-rule Shewart charting, running mean and/or Cusum plots (Appendix 5 and Appendix 6).

2. Accuracy of laser light positioning
[only necessary at commissioning (and this is normally performed by the manufacturer/installer)]
  • To assess the accuracy of the laser light position indicator, two wires meeting at right angles are positioned approximately 1 mm to the right (facing the table) of where the point beam of laser light intercepts the scanning cushion. A PA array spine is carried out to where the wires lie in the field of view. The wires may be imaged on the computer monitor and ideally should lie at the centre of the transverse scan lines and at the starting point of the longitudinal scan motion. A reasonable positioning accuracy is within 5 mm of the start point.

3. Accuracy of scan line spacing and step spacing
[only necessary at commissioning (and this is normally performed by the manufacturer/installer)]
  • To test the accuracy of the scan line spacing and step spacing, two rulers of set lengths are placed at right angles on the scanning table and an AP lumbar spine scan performed to image them. The incremental distances in each direction can be determined from this image by noting, the scan length and width, the number of required scan lines, and number of scan steps within one line. The calculated line spacings and step spacings should be within 2%.

4. Accuracy of indicated scan time
[only necessary at commissioning (and this is normally performed by the manufacturer/installer)]
  • The timing of a scan such as that indicated in Sect. 4 above is measured from when the X-ray beam light first comes on to when it goes out. The measurement is repeated. The measurements should agree to within 3%.

5. Patient Free Air Entrance Exposures
[only necessary at commissioning or at major service/tube replacement (and this is normally performed by the manufacturer/installer]
  • Patient free air entrance exposures are measured directly for standard PA scans by placing a calibrated radiation monitor ion chamber directly on the table cushion and scanning the chamber. For lateral scans (in some models) the entrance exposure can be calculated from the PA exposure values using the "inverse squared distance" correction, provided that the generator technique is the same for the PA and lateral scans

6. X-ray scatter measurements: exposure from individual scans
[only necessary at commissioning or at major service/tube replacement (and this is normally performed by the manufacturer/installer]
  • Patients are simulated using the appropriate equipment phantom (approximately 17 cms cubic), or else a block of perspex or the equivalent. Measurements are made in two positions: (i) 1 metre from the front edge of the patient table (and nearest the phantom); (ii) at the position where the technologist sits when operating the unit.
  • A calibrated integration radiation survey meter is required to make the measurements. Measurements should be made in the AP and lateral aspects (if both are relevant).
  • With the X-ray beam switched off, three background exposure measurements are made using the survey meter in integration mode, and averaged. These measurements include both the actual background radiation plus readings due to any current leakage from the survey instrument itself. The result (suitably weighted) is deducted from any measurements taken with the X-rays on, or else the monitor zero reading is adjusted to account for "background".
  • With the X-ray beam on, scatter measurements from the phantom (under scanning conditions) are made for PA spine and lateral spine (if relevant) clinical protocols at the operator's seat and at 1 metre from the front edge of the table closest to the phantom. The PA hip integrated dose from scatter is assumed to be the same per unit time as that of the spine. If this is thought not to be the case, then a hip scan scatter measurement should be made as well. Note that for equipment with multiple scan modes representing different scan resolutions, data should be collected for each scan mode. The exception is the whole body scan.
  • Scattered dose from the whole body scan need not be measured directly since the scatter is expected to be much lower than for the spine and hip protocols, due to the very low entrance exposures. These latter exposures are measured according to the protocol of Sect. 6. The whole body scatter dose may be estimated from (say) the spine AP dose and the ratio of the patient free air exposures for the PA spinal and whole body scans, obtained from Sect. 6.

7. X-ray exposure measurements: weekly and annual doses to staff

  • These may be estimated for each relevant staff member, from the doses for the individual scans (Sect. 6), the number of scans of each type performed each week and each year, and the work practices of each staff member (eg their habitual locations within the scanning room).
  • The doses acquired by individual staff members on an hourly and yearly basis must comply with limits set by the State Regulatory Authority. Usually these are "<20 µSv per hour" and "<1 mSv per year". In the unlikely event that these limits could be exceeded (particularly the second), staff work practices should be altered to avoid that possibility, on advice from (and confirmatory surveillance by) the Unit's radiation safety officer. Staff should be aware of radiation safety regulations and recommendations as they pertain to pregnancy.

<< Appendix 6 - Construction of a Cusum plot
Appendix 8 - Elements of a reference library >>
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