Every digital radiography (DR) company offers technical charts with panels. These x-ray prescriptions have evolved over time from the days of film and have become more sensitive with the development of DR panels. Most likely, DR companies have spent a great deal of time testing and optimizing these technologies to provide the best image quality for their panels. But what are these knobs (kVp and mAs)? How do they affect image quality for your patients of all species and sizes? What is the impact on patient dose, personnel dose, and panel dose?
◭ X-rays and how they are produced
◭Turn the knob: kVp and mAs
◭Dose to patients and staff
◭DR panel sensitivity
X-rays and how they are generated
In short, diagnostic X-rays are produced by accelerating electrons at high voltages and bombarding them onto a heavy metal target (usually tungsten). The energy of these colliding electrons is converted into an X-ray spectrum.
This occurs inside the X-ray tube. The familiar "kVp" setting is used to control this high voltage and the synthetic energy of the resulting X-rays.
The peak of this large voltage between the cathode and anode is what we call kVp (kV is kilovolts, p is peak). This determines the maximum energy of the electrons when they collide with the anode target, which in turn determines the maximum energy of the X-rays produced. Thus, for a 90 kVp tube, the maximum X-ray energy produced is 90 keV (kilo-electron volts).
However, not all X-rays have this peak energy. In fact, most of them do not. The generated X-ray spectra range from 0 to the maximum, regardless of the kVp setting. The average X-ray energy is only about 1/3 to 1/2 of kVp, including some internal filtering of the truly low-energy X-rays. Finally, the kVp setting determines the energy distribution of the X-rays, which is really what we are most interested in.
This energy distribution is important because it determines the overall penetration of the X-rays. For the scope of this paper, it can be argued that higher energy X-rays are less likely to be absorbed and therefore penetrate deeper into the tissue.
Current (milliAmperes, mA) refers only to the number of electrons used per exposure throughout the process (i.e., the length of time, in milliseconds, that the kVp is on, connecting the cathode and anode). The product of current (mA) and exposure time (seconds) is the familiar mAs and translates into the number of X-rays produced per second. For a given exposure time, mAs (current times time) provides the number of X-rays used during that exposure.
Turning the knob: kVp and mAs
So, what is the radiographic image you will be taking that will allow you to increase or decrease either of these settings? The answer is the size of the patient and the body part of interest (chest, abdomen, or musculoskeletal system).
In the era of film radiography, detailed technical charts were required to produce consistent radiographic exposures from patient to patient and from body part to body part. These technical charts specified incremental changes in kVp based on patient thickness. For example, the thickness needs to be varied by 2-4 kVp per centimeter (cm). in addition, the kVp range used depends on the body part of interest. For example, the thoracic technique will use a relatively high kVp range and relatively low mAs (e.g., 80-110 kVp and 3mAs), which is necessary to increase pulmonary vascular prominence. Skeletal radiography at the other end of the kVp and mAs spectrum (e.g., 50-70 kVp and 10-15 mAs) allows absorption of more energetic lower-energy X-rays, producing fine skeletal detail.
The take-home message is that fairly precise exposure techniques are required to produce radiographs of consistent quality. Failure to do so results in multiple retakes at the cost of inefficient use of time and increased X-ray exposure for patients and personnel.
Today's world of digital radiology has completely simplified this process while producing consistent and superior diagnostic images. In fact, the most modern DR systems, equipped with new, more sensitive panels, have integrated their technical charts to divide most anatomical options into two categories: cranial/extremity (i.e., the smallest anatomical structures surrounded by air) and "everything else" . Then, as the patient size (usually summarized by weight) increases, you only need to increase the technique slightly.
For patients and staff
Dose Calculating dose without making a lot of assumptions about the distance from the x-ray machine to the patient and the detector can be tricky. However, a good rule of thumb is that the patient dose is quadratically proportional to the kVp setting and linearly proportional to the mAs setting. Wait! What! In English, please!!! Let's start with mA, because its quantitative nature should be very intuitive. If you double the number of X-rays used (remember, this is what mAs stands for), your patient dose will be doubled. Simple. But for the kVp setting, the patient dose increases as the square of the kVp: a 20% increase in kVp (e.g., from 100 kVp to 120 kVp) increases the dose by about 44%.
Thus, while, yes, the dose is more sensitive to changes in kVp, the kVp range used varies only between 80-120 kVp (~50%), while the mAs value varies from 2 to more than 30 (1500%). The first level of examination used to compare the two different techniques to see which produces more dose.
Dose ratio of technique A to technique B = (kVp_A/kVp_B)^2 x (mAs_A/mAs_B)
Technique A: 100 kVp @ 5 mAs (typical technique for large abdominal setup)
Technology B: 89 kVp @ 15 mAs (example technology from other manufacturers)
Dose ratio of A to B = (100/89)^2 x (5/15) = 42%, which means that technology A produces a 42% reduction in dose.
The dose is proportional to your staff in the form of scattered radiation, but the magnitude is less than the patient dose. So yes, a slightly higher x-ray technique will deliver slightly more scattered radiation from the patient (and possibly to your staff). However, any radiologist (or radiation safety officer) will tell you that by implementing basic radiation safety protocols, you can avoid the radiation exposure to your staff.
DR Panel Sensitivity
DR panels have evolved over the past 15 years with better scintillators, more efficient electronics and more advanced image processing. This has led to exposure techniques that can significantly reduce mAs. Unfortunately, this is not the same as kVp because you still need to go through the same 95 pounds of Rottweiler hips as you did 15 years ago.
The technology chart also provides a good comparison tool for measuring manufacturers over time (as technology advances) and between manufacturers. There are a lot of marketing resources being spent on "low dose" messages these days. However, if X-ray technology has not changed as a result, then you are not realizing the true benefits of high sensitivity, low dose products.
Kilovoltage potentials (kVp) and mAs are familiar to all of us veterinarians and remain the basis for radiographic imaging. Thankfully, the days of complex and demanding technical charts are gone. The advent and continuous improvement of digital radiography has resulted in highly reproducible diagnostic images, generated using fewer mAs, while exposing patients and personnel to less radiation.