X-ray units can be conveniently divided into portable, mobile and fixed units, and each of these groups vary in their size, portability and output. The source of electrical energy from the mains supply is in the form of an alternating current, and each type of X-ray unit deals with this fluctuating electrical supply in a slightly different way. However, the end result is that throughout the duration of any exposure with any of these types of machines, the actual energy of the X-ray beam is not constant, and overall will be less than the kV setting on the console suggests. Portable machines typically show an average drop of at least 30%, fixed units approximately 5%, and mobile units vary depending upon the type of generator used. Recently, high-frequency generators have entered the veterinary market and are used in all three types of X-ray units. High-frequency generators produce X-rays far more efficiently, only require a normal mains electrical supply (unlike the three-phase supply required for the older fixed units), and do not suffer any significant reduction in kV during the exposure. Whichever type of unit is being considered, there will be an advantage if it contains a high-frequency generator. This advantage is greatest with portable units and less so with fixed units.
Kv, mA, mAs or kW?
A radiographic exposure is dependent upon the number of X-ray photons as well as the energy of those photons. The former is determined by the cathode filament current (mA) and the duration of the exposure (s), while the latter is determined by the potential difference between the cathode and the anode (kV).
In small animal radiography the kV setting for most exposures taken would typically be in the range of 45-90 kV. As most (if not all) X-ray machines available in the trade stands today are able to exceed this range, the maximum kV of a machine should not affect the choice of X-ray unit.
The mAs of an exposure is the product of mA and time. Whether mA is high and time is short, or vice versa, if the product is the same (and the kV is unchanged), then the exposure is identical. As short exposures are desirable, it makes good sense to purchase a machine with as much ma (not mAs!) as you can afford. Brochures that give only the mAs of a unit are not helpful, as it is possible to have a low mA that can take very long exposures, far in excess of anything practical for veterinary radiography.
Recently, some manufacturers are providing details of a machine's kW output. This reflects the maximum safe output of the transformer's secondary windings and is a product of the kV x mA, divided by 1000. This figure can be deceiving sometimes, as some machines (typically some portable machines) cannot reach their highest mA setting when high kVs are used. Therefore, the maximum kW given may only be attainable at reduced kV settings. As a rough guide, the maximum mA is usually around 10-20 times the quoted kW figure.
What is the Expected Caseload?
For general small animal practice where there is a reasonable proportion of large dog spines, abdomens and chests to radiograph, any X-ray unit with a maximum mA less than 150 will present imaging challenges, and the ideal unit would be capable of at least 250 mA. For a cat-only practice, a maximum of 50 mA would be more than adequate. If an X-ray machine is to be used at multiple sites, then portability becomes the deciding factor. However, the lower output of portable units (typically less than 60 mA maximum) means that compromises will be made with larger patients. If a portable unit is chosen, it should supply as much mA as you can afford, and the advantages of high-frequency technology mean that no other type of portable unit should be considered.
What About Price?
When comparing new prices portable units are generally cheapest to purchase. This may, however, prove a false economy now that higher-powered, high-frequency units capable of over 200 mA (that can come integrated with an X-ray table), are now readily available in the veterinary marketplace at not too much extra cost.
Flexibility in Setting Exposure Factors
It is helpful if the kV settings are independent of the mA settings, and this is generally true of fixed units, mobile units and many portable units. Some portable units still have very limited settings and the kV may be linked to the mA. This type of unit gives no real flexibility when deciding upon the correct exposures for a particular region and the author would encourage you not to choose such a machine. They may be suitable for a caseload that has minimal variability (e.g., a cat-only practice, or for radiographing distal limbs of horses).
How Much Space?
It may be that the deciding factor in which type of X-ray unit to purchase is where it will be located. Portable units have the advantage that they take up very little space, can be wall mounted, and may be the only option if the radiography room is small and/or used for other procedures throughout the day. However, these historical advantages are not as great with the recent introduction of higher-powered high-frequency units that are integrated with relatively discrete X-ray tables.
Requirements of the HSE
All practising veterinarians should be familiar with the Guidance Notes for the Safe Use of Ionising Radiations in Veterinary Practice. If purchasing a radiographic unit for the first time in a practice (where no radiographs have been taken before), the Health & Safety Executive must be notified in writing at least 28 days in advance of the start of work. When a unit is simply replacing a previous machine, no such notification is required (unless the particulars given in the original notification have changed), although the practice's Radiation Protection Advisor (RPA) should be informed.
The X-ray unit should be maintained and checked in accordance with the advice of the manufacturer, supplier and/or the practice's RPA. These maintenance arrangements should be included in the practice's Local Rules (formulated in consultation with the RPA).
References
1. Barthez PY, Manwaring N, et al. Comparison of single-phase and high-frequency generators for x-ray units. Veterinary Radiology & Ultrasound 2002; 43: 118-122.
2. Guidance Notes for the Safe Use of Ionising Radiations in Veterinary Practice (Ionising Radiation Regulations, 1999). British Veterinary Association, June 2002.