Coronary CT Angiograms: Radiation Exposure

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March 31, 2011 by dailybolusoflr

Coronary CT Angiogram to evaluate patients with CP in the ED

 

·         This is a relatively new modality used to evaluate patients in the ED with chest pain. 

·         From a radiation standpoint, there is a range of radiation exposure to patients based on how the CT is performed. 

 

There are two approaches for CCTA:

—  Retrospective ECG Gating (which delivers about 10-15mSv)

       Segments cardiac cycle (R-R Interval) into 10 phases (0-90%)

       Scans the entire time, but uses information obtained during certain times to form the images.

—  Best time to obtain information is 40-80%

       Tube Modulation is possible which decreases the radiation output during the less useful times in the cycle.  So, for phases 0-39% the radiation output would be deceased, increased during phases 40-80% and then decreased again until 90%.

—  Prospective ECG Gating (2.5-7mSv)

       Approximates location of the 10 phases in the cardiac cycle from estimate based on HR and RR interval

       Collects data only from certain phases- the radiation output is OFF during the less useful cycles

       This significantly decreases the radiation dose but due to the lack of imaging for the entire cycle, does limit information obtained

—  LVEF cannot be calculated from prospectively gated ECG CCTAs

       These images can be compromised by patients with ectopy or irregular HRs. 

 

For comparison:

 

·         Rest-stress (sestamibi)                                  9.4 mSv

·         Rest-stress (thallium)                                     40 mSv

·         Angio                                                                    7 mSv

·         Angio with stent or angioplasty                 15 mSv

·         CCTA retrospective gating                            10-15 mSv

·         CCTA prospective gating                               2.5-7 mSv

 

The ‘what, when, where, who and how?’ of cardiac computed tomography in 2009: guidelines for the clinician. The Canadian Journal of Cardiology 25, 135-139.

 

Linda Regan, MD FACEP

Program Director, Emergency Medicine Residency

Johns Hopkins Medical Institutions

 

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