Photo of John NoblittThere are many areas where the laws of physics may be applied to medical instrumentation. One of these principles is the inverse square law. The website HyperPhysics describes the inverse square law as follows:

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound, or radiation obey the inverse square law.

So, how does this information affect you in your job as an HTM professional? The phenomenon of this law describes why TDS, or time, distance, and shielding, play such a big factor in radiation safety.

Theory Into Practice

Figure 1. The inverse square law states that if you double the distance between an object and the source of radiation, the intensity of the radiation will become one fourth the original amount.

Figure 1. The inverse square law states that doubling the distance from a source of radiation reduces radiation intensity by 75%.

Imagine, if you will, substituting the light bulb in Figure 1 with an x-ray tube and applying the inverse square law to the amount of radiation exposure the patient or the x-ray technician receives. From this image, it is easy to see that if you double the distance from the source of radiation, the intensity of the radiation will become one fourth the original amount.

Note that if you were to measure the intensity of the radiation at a point—say, 1 foot away from the source—your result would be 20 mR. If you were to measure the intensity of radiation at 2 feet away, your result would now be 5 mR. At 4 feet away, it would be 1.25 mR, and at 8 feet, it would measure just 0.312 mR.

So in a span of only 8 feet, the intensity of radiation fell from a level of 20 mR to 0.312 mR. This tremendous reduction in radiation intensity did not require any special devices, just distance. Taking advantage of the inverse square law is a very powerful tool in radiation safety.

This law would apply to all sources of radiation, such as a microwave oven. Microwave ovens rarely have problems with radiation, but if you are old enough, you have probably seen warnings signs pertaining to pacemaker patients and microwaves. Medical device manufacturers have resolved most of the concerns about microwaves interfering with pacemaker functionality by shielding the devices. However, by applying the inverse square law, one could reduce the likelihood of interference by an incredible amount by keeping the pacemaker patient as far from the microwave as possible.

Practical Applications

Now let’s look at how the inverse square law applies to gravity. The law of gravity states that “any two masses attract each other with a force equal to a constant (constant of gravitation) multiplied by the product of the two masses and divided by the square of the distance between them” (source:

Figure 2. The inverse square law can be used to administer two intravenous solutions through a single-channel IV pump.

Figure 2. The inverse square law is taken into account in the placement of IV bags.

So, where might the inverse square law play into your day-to-day activities as a biomedical technician? Figure 2 shows a typical piggyback set-up for administering two solutions intravenously to a patient. Note that the piggyback bag (often antibiotics or lipids) is set higher than the primary solution bag. This arrangement gives the piggyback bag higher pressure to push the IV solution into the IV pump.

This set-up and the law of gravity allow a second solution to be administered to a patient using a single-channel IV pump. For instance, imagine that a patient is on a slow infusion of saline solution and the physician orders antibiotics to be administered intravenously. This set-up is often used to administer the antibiotic first and then return to giving the patient the slow, steady drip of saline solution from the primary bag.

The law of gravity provides the higher bag with more pressure, so all the solution in the piggyback bag flows through the Y connection into the single-channel IV pump first. Once all the fluid is released from the secondary or piggyback bag, the primary bag will again begin to infuse at the rate at which the pump was originally set.

Understanding the inverse square law can help you with your daily duties as a biomedical technician. Reviewing this principle and other laws of physics should be completed as preparation for all three of the certification exams (CBET, CRES, and CLES).

I hope you find this information useful in your preparation for your exam.

Review Questions

1) How many channels must an IV pump have to administer two solutions to a patient?
a) 2
b) 6
c) 1
d) 3

2) In radiation safety, TDS refers to:
a) Time, diameter, shielding
b) Time, distance, shielding
c) Termination, diameter, shielding
d) Time, dose, sum

3) The inverse square law states that as the distance from the source is doubled, the intensity of the source is reduced to this level.
a) .25
b) .33
c) 1/3
d) 1/8

4) If you measured 1/9 the amount of radiation in your last measurement compared to your first radiation measurement, how many times have you doubled the distance from the first measurement?
a) 1
b) ¼
c) 40
d) 2

John Noblitt, MAEd, CBET, is the BMET program director at Caldwell Community College and Technical Institute, Hudson, NC. For more information, contact editorial director John Bethune at [email protected].




Answers: 1—C, 2—B, 3—A, 4—D