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7- Compliance testing of DXA equipment
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
- Examination of longer term QA results based on multi-rule Shewart
charting, running mean and/or Cusum plots (Appendix 5 and
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.
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%.
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]
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
- 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
- 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.