Understanding Dose Area Product and Air Kerma

Dose Area Product (DAP) and Air Kerma (AK) are measurements commonly used to represent the dose your patient receives during an X-ray procedure. What the terms mean is often not well understood. What actions can be taken to reduce the numbers is important to review.

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How do DAP and AK differ?

Consider the analogy of the garden hose spraying water through a window, where the hose is the X-ray tube, the water represents radiation from the X-ray tube, and the window is the collimator. Water only passes through the open part of the window.

What is Dose Area Product?

Dose Area Product (DAP) is an output measurement of the total amount of radiation delivered to the patient. Think of it as the total amount (volume) of water coming through the window. The amount is the same regardless of whether it is measured close to the hose (3) or further away (4). Only the area measured changes (grows broader) further from the hose.

What is Air Kerma?

Air Kerma (AK) is a dose (rate) measurement of radiation at a specific defined position such as a point on the patient’s skin. Think of it as the amount of water from the hose hitting a one square centimeter cup (1cm2). If the cup is close to the hose (5) it will fill quickly (a high AK measurement). If it is farther away (6) it will fill slowly (a low AK measurement).
X-ray tube
Collimator
DAP chambers
AK dose meters
Note: A “DAP chamber” is a dose meter but it measures all radiation passing through it rather than sampling 1cm2
Air Kerma is the measurement of the dose the patient’s skin recieves and Dose Area Product is a measurement of all dose exposure.

What do the numbers tell us?

DAP measurement provides a general dose reference and guideline for risk, but does not tell how much dose was received by the patient in a particular spot. AK is very accurate for dose measured at a particular spot (i.e. the reference point – 15cm towards the tube from the isocenter) and in 10 different projections using Philips Zone Dose Reporting, but does not offer a picture of overall radiation dose. DAP and AK compliment each other. DAP and AK readouts taken during the procedure can be used to help minimize radiation. System operation plays an extremely important role. How was dose applied? How was it distributed? You have control.

Radiation exposure readouts
with Zone Reporting

During radiation - DoseRate per second or minute
DAP
56 mGycm 2/S

1.9

min
Time

61.709

mGy/min
AK

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What you can do to manage patient dose

Limit fluoro and exposure time Select the lowest ‘acceptable’ image quality dose setting for fluoro and exposure if available.
Keep source to image distance (SID) down. An increase of just 20cm results in a 60%+ increase in AK dose to the patient.
Collimate whenever possible. If you don’t need to see it, don’t radiate it. Collimation greatly reduces DAP and scatter radiation too.
Increase Field of View (FOV) and collimate. You must to both. If you just increase FOV, AK goes down, but DAP goes up.
Do not lower table below isocenter if not necessary. Also, minimize angles — steeper angles put the X-ray tube closer to the patient, increasing AK.
Always keep detector as close to the patient as possible. Dose is increased as the detector moves away to maintain image quality.
If you can’t reduce it — move it! Radiation injury can be avoided. Do whatever is needed to change the entrance point on the patient.

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