We measure the effective dose from an imaging procedure in an international unit called the Sievert. In imaging, doses are actually quite small compared to a Sievert, so we routinely use thousandths of a Sievert (milliSieverts, mSv) as our measuring stick. The average chest CT delivers a radiation dose of 7mSv. Arguably this is already low dose –it is the equivalent of 2 years or less of radiation received just from living on earth, and there is no evidence that doses this low correlate to any negative outcomes. The American Association of Physicists in Medicine (AAPM) has issued a strongly worded position statement to this effect: “Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent. Predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged.” AAPM Policy PP 25-A LINK.
That being said, in radiology we strive to make radiation as low as reasonably achievable (ALARA), delivering the lowest dose possible without rendering image quality non-diagnostic. In the screening population, therefore, it is important to lower our doses when compared to a routine chest CT. Our doses are always under 1mSv and usually in the 0.5-0.7mSv range, and so we are often delivering a dose 10 times lower than that of a routine chest CT. Our doses usually are in the ballpark (+/-20%) of the dose from screening mammography. What is sacrificed by this dose lowering? The lungs are filled with air and are relatively easy to pass X-rays through. The “meatier” parts like the heart and adjacent soft tissues, the bones, and the soft tissues of the chest wall and back require higher levels of X-ray penetration. Since in screening we are evaluating the lungs we use a dose that provides diagnostic quality evaluation of the lungs but limits evaluation of the mediastinum and heart, as well as the superficial soft tissues and upper abdomen. This is why it’s important that patients be asymptomatic and only undergo a low-dose chest CT for the purposes of finding lung cancer. A low dose chest CT cannot replace a routine dose chest CT for evaluation of lymphadenopathy, vascular problems, thyroid problems, and upper abdominal pathology, among others. But for lung nodules and lung cancer, it works just fine.
None. No intravenous contrast is administered, so there is no need for IV placement. The scans are performed in just a few seconds, so usually do not aggravate claustrophobia. No implants or devices contraindicate CT. For a video describing lung cancer screening and showing a CT scan as performed, please see this video from our friends at Charlotte Radiology on PBS Charlotte
The ACR has set standards for lung screening CT and has set up Standards for dose and technique . In short, the kVp should be between 100 and 140 and the resultant CTDIvol should be equal to or less than 3 mGy. Our practice uses the protocol below:
Coronal reformats, sagittal reformats, HRCT recons at intervals, and 20mm MIP images are produced on every case.
We have experimented with 80 kVp and gotten good quality images, but the ACR requires a minimum kVp of 100 for center of excellence designation. Most of our scanners are GE, either Brightspeed 16 or Lightspeed 64. Using the technique above, our doses are universally below 1mSv and usually around 0.5-0.6mSv with a CTDIvol around 1 mGy.