In 1979, the Nobel Prize in Physiology or Medicine was awarded jointly to Godfrey Newbold Hounsfield and Allan M Cormack1 for their integral roles in the development of computer assisted tomography. Four decades later, CAT or CT imaging is still recognized as a major advance in medical diagnostics. The technology behind this area of radiology continues to evolve, adding to its value in early detection and treatment of disease. Now, dual-energy computed tomography (DECT) is poised to change the way radiologists see internal organs.
Single energy CT imaging
X-rays, the earliest form of radiology, are pictures of structures inside the body. While this information greatly supplements external examination and reduces the need for exploratory procedures, it has limitations. X-rays capture two-dimensional images, so structures appear stacked, with the uppermost (such as rib bones) obscuring what lies underneath (heart or lungs).
Standard computed tomography largely removes that constraint by eliminating overlapping. A single energy CT scan captures a multitude of cross-sectional slice images from many angles, as one polychromatic X-ray tube rotates around the patient.2 Robust computer software stores, sorts, and analyzes the pictures, reconstructing a new, three-dimensional image. With overlying structures removed, targeted internal anatomy becomes more apparent in this reconstructed image. A radiologist, a doctor trained in interpreting CT scans, uses these images to determine important characteristics of bodily structures such as shape, size, density, and texture. The information helps to establish if a medical concern exists, and if so, its location and progression.
However, in a 2017 report in The Korean Journal of Radiology3, authors Hyun Woo Goo, MD, PhD and Jin Mo Goo, MD, PhD said, “CT has an inherent limitation in soft tissue differentiation because the pixel value or CT number entirely depends on the linear attenuation coefficient (µ) which has considerable overlap between different body materials. The linear attenuation coefficient is a result of two physical interactions between X-ray photons, i.e., the sum of photoelectric absorption that is predominant under low energy and Compton scattering that is predominant under high energy. Compton scattering strongly depends on the electron density of the material. The photoelectric effect is proportional to the cube of the atomic number (Z) and inversely proportional to the cube of the incident photon energy (E). Only a few heavy atoms, such as calcium, iodine, barium, and xenon, having strong photoelectric effect can be easily differentiated from other body tissue having similarly weak photoelectric effect.”
Enter dual-energy computed tomography (DECT)
Dual-energy CT scanning is a newer evolution of computed tomography technology. It utilizes the technique of standard CT, adding a second, less powerful X-ray.4 With this extra energy source, the impact of some chemical substances on scans can be selectively enhanced or reduced. That makes dual technology appropriate for a wider range of applications.
For example, iodine is commonly introduced into the body as an X-ray contrast agent. A dual-energy CT scan can increase its effect to produces clearer images of blood vessels in angiography. The effect of the contrast agent can also be minimized, allowing the radiologist to compare results with and without the agent, in a single procedure.
DECT still faces technological hurdles, however. The June 2018 edition of Radiology Business5 shares the insight of Bhavik N. Patel, MD, MBA associated with Stanford University, and Daniele Marin, MD, who works with Duke University.
They said, “Dual-energy computed tomography has gained much promise as a diagnostic modality. Through simultaneous and near-simultaneous low- and high-energy acquisition, it offers several advantages and capabilities over conventional polychromatic images. [We] review common sources of image quality degradation that may exist because of the inherent dual-energy technology and acquisition, and known strategies that overcome these limitations that have allowed routine usage of DECT in busy clinical practices.”5
A DECT scanner uses a rapid kilovolt switching system which can compromise image quality and ability to filter data. Limited field of view is also an inherent hindrance. Patel and Marin, however, believe these setbacks can be overcome.5 They suggest an asymmetrical sampling methodology, with projections at low and high tube voltages for clearer images. Though this modification creates additional “noise” in images, it can be offset with an adaptive statistical iterative reconstruction technique. The field of view issue can be addressed through technologist training, ensuring subjects are properly positioned for DECT.
Benefits of CT imaging
Why the ongoing push to continually advance computed tomography technology and techniques? The answer is found in one simple word – people.
Consider some of the down-to-earth ways progress in CT has made medical management more effective2 for the patients it serves:
- Rapid emergency room assessment
- Indicating when specialized attention, such as intensive care, is needed
- Determining when surgery is necessary
- Reducing the need for exploratory procedures
- Shorter hospital stays
- Improved detection, diagnosis, and treatment of cancer
- Guided treatment for cardiac disease, stroke, injuries, and other conditions
Medical imaging is not science for the sake of science. The objective is to improve and preserve the lives of humans.
- The Nobel Prize. (1979) The Nobel Prize in Physiology or Medicine 1979 was awarded jointly to Allan M. Cormack and Godfrey N. Hounsfield "for the development of computer assisted tomography" [Press release]. Retrieved from https://www.nobelprize.org/prizes/medicine/1979/press-release/ May 24, 2019.
- org. What are the benefits of CT Scans? https://www.radiologyinfo.org/en/info.cfm?pg=safety-hiw_04 Web. May 24, 2019.
- Korean Journal of Radiology. (2017) Dual-Energy CT: New Horizon in Medical Imaging. https://synapse.koreamed.org/DOIx.php?id=10.3348/kjr.2017.18.4.555 May 24, 2019.
- Inside Radiology. Dual Energy CT Scan. https://www.insideradiology.com.au/dual-energy-ct-scan/ Web. May 24, 2019.
- Radiology Business. How radiologists can improve image quality with dual-energy CT. https://www.radiologybusiness.com/topics/quality/how-radiologists-can-improve-image-quality-dual-energy-ct May 24, 2019.