Greg Freiherr has reported on developments in radiology since 1983. He runs the consulting service, The Freiherr Group.
Do We Have the Courage to Scan Gently?
"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius — and a lot of courage — to move in the opposite direction." —Albert Einstein
We fear what we can’t see and what we don’t know. It’s why children don’t want to go into a dark, unfinished basement. It’s what makes scary movies what they are.
Radiation is like that. It’s invisible. And, while many of us would say we understand it, we really don’t. We know radiation is part of the electromagnetic spectrum and that X makes us more susceptible to cancer. But exactly how? And to what extent? Are some of us more vulnerable? If so, why? How much radiation increases the likelihood of cancer? Over what period? Beginning at what age?
Asking these questions is a good thing. Recognizing that we don’t have the answers is scary. That, too, is a good thing.
Manufacturers are building CT scanners now that are extraordinarily stingy in the dose of X radiation they deliver. But it’s going to take awhile to get these scanners diffused through the installed base. In the meantime, there is much we can do to cut patient dose.
The methods do not involve bigger, more powerful CT scanners and they are only marginally more complex. But they require effort — and courage — to put in place, courage because the accepted way to improve image quality during the last 100-plus years has been to turn the voltage up, not down.
In March, at the European Congress of Radiology, Professor Thomas Albrecht, M.D., from the Institut für Radiologie und Interventionelle Therapie Vivantes, Klinikum Neukölln in Berlin, Germany, showed how low-voltage CT, enhanced with contrast media, could reduce patient radiation dose while actually improving contrast enhancement. Image quality improves because the iodine in the contrast media absorbs more X-rays. This occurs as the mean energy of the X-ray beam, due to every less voltage, gets closer to the k-edge of iodine, which is 33.2 keV.
High kVp protocols generate high photon energies. At 140 kVp, for example, the keV is 61.5. At 80 kVp , the keV is 43.7. The latter is much closer to the k-edge of iodine.
By shifting the mean energy of the X-ray beam closer to this k-edge, more X-rays are absorbed by the contrast medium, thereby improving image contrast. Meanwhile, the lower tube voltage produces fewer X-rays, exposing the patient to less ionizing radiation. The reduction in patient radiation dose is remarkable.
Going from 120 kVp to 100 kVp can cut patient radiation dose from between 20 and 50 percent. Reducing tube voltage to 80 kVp can cut dose by as much as 70 percent.
Unfortunately, it’s not just a matter of turning down the voltage. To maintain image quality, tube current must increase about 50 percent. The exact increase varies according to body area and the scanner.
Similarly, radiation dose reduction to the patient depends on such factors as body habitus and body area. For example, thin patients reap a greater dose reduction than the obese. And dose savings also are most pronounced in the chest.
Such personalization of the CT exam requires a thorough understanding of the underlying physics of CT, as well as the habitus of the patient. But the increased safety of the patient, as well as the improved image quality, seem worth the effort.