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PERIPHERAL VASCULAR ULTRASOUND
REPETITIVE STRAIN INJURY AND
OCCUPATIONAL HAZARDS
Sonographers are at high risk of developing occupational injuries due to prolonged periods of bad posture during ultrasound examinations. Back problems and repetitive strain injuries of the wrist and shoulder are increasingly common. To mini- mize the risk it is essential that vascular ultrasound units be equipped with variable height examination tables that have adjustable upper and lower sec- tions. Ideally, it should be possible to tilt the table, especially for venous examinations. The operator’s chair should have a variable height adjustment, adjustable back rest and swivel capability. Sonog- raphers should vary the workload and types of scans performed during the day and take regular breaks. The ability to scan with either hand also reduces strain on one side. The probe should not be gripped too hard, and excessive pressure should not be used to make contact between the patient and the probe. Most vascular examinations can be performed with relatively light probe con- tact. If the sonographer develops problems, these should be treated at an early stage as long-term chronic problems may be difficult to resolve. Care should be taken when moving apparatus around the hospital, as it is easy to strain muscles, trap hands between the scanner and door frames or run over toes.
SAFETY OF DIAGNOSTIC ULTRASOUND
During the scan, the patient is exposed to ultra- sound energy, and it is therefore important that the sonographer be aware of the possible risks and how to minimize them. Over the years there has been a steady increase in the output power generated by ultrasound systems. The potential risks have been regularly assessed by various safety committees, including that of the World Federation of Ultra- sound in Medicine and Biology (WFUMB 1998). Information on safety issues can be found on the website of the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB 2002). The British Medical Ultrasound Society has also produced a statement on the safe use and potential hazards of diagnostic ultrasound equip- ment (BMUS Safety Group 2002).
Table 7.1 The upper limits of exposure required by the United States Food and Drug Administration
Application
All except ophthalmology
Ophthalmology
Derated Ispta (mW cm2)
720
50
Derated Isppa (W cm2)
190
NS
MI TI
1.9 (6.0)*
0.23 1.0
*The upper limit of 6.0 is advisory. At least one of the quantities MI and Isppa must be less than the specified limit. NS: not specified.
It is believed that the two main potential risks of tissue damage due to ultrasound exposure are tis- sue heating and cavitation. Cavitation refers to the formation, growth, oscillation and violent collapse of small, gas-filled cavities within the ultrasound beam. Inertial cavitation—that is, large variation in size and possible violent collapse of bubbles— occurs above a threshold of negative acoustic pres- sure (Duck & Shaw 2003).
Ultrasound intensity
The intensity is the energy crossing a unit area (usually 1 cm2) in unit time. The spatial peak tem- poral average intensity, Ispta, is the peak within the beam averaged over time. Another value of inten- sity that is used is the spatial peak pulse average intensity, Isppa, which is the spatial peak intensity averaged over the duration of the pulse. These have been used by the Food and Drug Administration (FDA) in the United States to define the upper limit of exposure produced by ultrasound systems for diagnostic use (Table 7.1). Manufacturers often supply data on the maximum Ispta and Isppa in the operator’s manual.
Mechanical and thermal indices
The output power produced by a system will vary with the modality used and the control settings. So that the ultrasound user can be aware of the poten- tial risks of any given scanner set-up, two new, potentially more meaningful indices have been developed. These are the thermal index (TI) and the