Wednesday, May 14, 2014
Radiation: Rationem Practicam, A New JTI Blog Series by Mannu Kalra
Part I: Chest versus Abdomen: A Regional Advantage
Although the chest is by no means a simple region in terms of its complex anatomy and a host of pathologies, from a CT radiation dose utilization point of view, the chest is an easier place to be when compared to the abdomen, for example. The lungs with their air component cause little x-ray beam attenuation while offering high tissue contrast for the lesions. The mediastinal vessels and fat also contribute to high tissue contrast, particularly for contrast-enhanced chest CT. These attributes distinguish the chest from other body regions and offer the potential for substantial radiation dose reduction without sacrificing diagnostic yield or information.
One can actually learn at least a couple of important lessons from these peculiarities of the thorax. Firstly, a routine or “general purpose” chest should not be scanned with the same scan protocols or radiation doses as the routine abdomen. Operating on the wrong side of this principle can over-dose the chest or under-dose the abdomen.
Secondly, anyone can look at the dose information page (many CT scanners generate this page to summarize important CT dose indicators or descriptors such as CT dose index volume (CTDI vol) and dose length product (DLP)) or better still at the structured DICOM information summary (this one summarizes all scan parameters and dose metrics in a long “exportable” report, but generally available only on some newer scanners) of their patients who had simultaneous chest and abdomen CT. A mere glance can tell if the dose is being appropriately used for scanning. If the chest CT doses are higher than those for the abdomen, things are just not right! On the other hand, if the reverse is true for these doses, it is rather reassuring to know that a key principle of dose reduction is being followed.
Indeed in certain ways, dose reduction may actually enhance the diagnostic quality of a chest CT. Remember: CT pulmonary embolism at low kilovoltage – radiation dose goes lower, but contrast enhancement in pulmonary vessels goes way up. The ability to confidently detect small lung nodules at extremely low radiation dose CT has tilted the balance of lung cancer screening CT favorably by reducing associated radiation risks. These regional advantages must never be forgotten as we image one of the most important regions of the body that delivers oxygen to the blood and then helps take this essential life element to the rest of the body.
Revisit the JTI blog in June for Part II of this series, which will review personalizing chest CT protocols.
Email Dr. Kalra at MKALRA@mgh.harvard.edu
with your comments and questions.