Radiation Exposure in Cardiology Testing: How much is too much?

Author(s): 

Morton Kern, MD, Clinical Editor,
Clinical Professor of Medicine,
Associate Chief Cardiology,
University of California Irvine,
Orange, California

Daily non-occupational exposure:

It is well-known that there is naturally occurring background radiation exposure for everyone, everywhere. The average person in the United States receives an approximate 3 mSv per year from naturally occurring radioactive background activity from land and atmospheric sources. For example, for those individuals with atmospheric exposure at high elevation locations such as New Mexico and Colorado, there is an increase of approximately 1.5 mSv more per year than at sea level. Individuals flying on commercial airplanes on coast-to-coast round trip flights are known to accumulate another 0.03 mSv, since the high altitude fails to filter out other sources of background radiation. In addition, radon gas in homes accounts for approximately 2 mSv per year as a background exposure. In simple terms, the radiation exposure from one chest x-ray is equivalent to the amount of radiation exposure from our natural surroundings over a 10-day period.1 Table 1 provides an excellent comparison in the amount of radiation which you might expect to experience.

The amount of radiation from diagnostic and therapeutic interventional radiology procedures has an increased exposure relative to some of the other procedures that are performed. The risks and benefits of undergoing these interventional procedures certainly favor the opportunity to avoid surgery and improve patient outcomes. The risk of the radiation exposure is relatively minimal compared to the other likely adverse outcome of not treating such patients. The CT angiography used to diagnose coronary artery disease (CAD) before coronary interventions has a higher radiation exposure with increased late risk but the risk/benefit is favorable for the indicated procedure. The radiation exposure from CTA is approximately 5 times that of coronary angiography and varies in intensity from 8 to 20 mSv.

Factors influencing radiation exposure:

The radiation dose depends on the type of examination, patient size, equipment technique and other factors. With respect to the radiation dose, the entrance exposure and skin entrance exposure rates are those which are most sensitive and need the most care in management. For recurrent and repetitive patient exposures, these individuals must not only have measurements of these exposure rates but also have direct monitoring of skin to limit harmful activity.

Skin effects and radiation exposure:

Skin effects after single radiation exposure increase with duration and intensity and the changes one might witness are as seen in Table 2.

In the cath lab and interventional suite, high-dose fluoroscopy may sometimes be needed. Remember the dose is approximately 10-fold for skin entrance exposures compared to normal fluoroscopy (10-50 mGy/min vs. 100-200 mGy/min, also roughly 1-5 rads per minute for normal fluoro and 10-20 rads per minute for high dose/cine). Comparing radiation exposure for diagnostic coronary angiography to coronary interventional procedures, a diagnostic study may have a total exposure of 900-1900 mGy, whereas the coronary interventional procedure often ranges from 2400-5500 mGy. With radiofrequency catheter ablation procedures, the fluoro time may increase from approximately 5 minutes for diagnostic study to 42 minutes for a radiofrequency catheter ablation study. Compare this with the average 22 minutes of fluoro time for coronary interventional procedures.

The take-home message:

The cath lab should consider the risk/benefits of the procedures (including CTA when asked) and always employ dose-reduction techniques, some of which are suggested as follows:

1. Use intermittent fluoroscopy. Stay off the fluoro pedal whenever possible.

2. Remove the x-ray grids when appropriate to reduce scatter.

3. Use last image hold technology with electronic collimation.

4. Employ automatic adjustment of beam quality to limit Kvp and Ma.

5. Reduce image magnification when possible. (Note that at a normal mode, an entrance skin dose increases 2.4 times when magnification increases from 23-cm field to a 15-cm field and increases 4.4 times the dose when the magnification goes to an 11-cm field.)

6. Use pulsed fluoroscopy. (Dose reduction of approximately 20% over continuous fluoro dose at 30 pulses per second and reduction to 80% at 15 pulses per second)

When the time comes that an x-ray dose of CT angiography is minimal, this will likely turn out to be our best non-invasive screening tool for CAD. In the meantime, wear sun block.

References: 
1. Radiology Info: The radiology information source for patients. Available at: http://www.radiologyinfo.org. Accessed January 10, 2005.

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