Emerging techniques in diagnostic imaging

Dual-energy computed tomography

Medical imaging with CT has benefited from the development of multidetector technology, leading to faster scanning of the abdomen with higher-resolution images, resulting in submillimeter isotropic image voxels that allow for multiplanar reformatting and three-dimensional volume rendering (see Chapter 13 ). New dose modulation techniques have also reduced the radiation dose exposure for patients. An exciting innovative technique in CT imaging relevant to hepatopancreatobiliary tumors is dual-energy CT (DECT). ^, To understand the utility of DECT, a few physical principles underlying CT imaging are worth reviewing.

CT is based on the application of x-rays, which represent electromagnetic waves (photons) of very high energy and very short wavelengths that can pass through most objects, allowing us to “see” through the body. The degree of x-ray attenuation by different elements in our body is proportional to the number of electrons present. The higher the atomic number of an element, the greater the number of electrons present that can interact with x-rays. Put simply, x-rays will be more frequently scattered or absorbed by the photoelectric effect when they travel through bones, which are high in calcium atoms (Ca ²⁰ ), than when traveling through other soft tissues made predominantly of hydrogen (H ¹ ), carbon (C ⁶ ), nitrogen (N ⁷ ), and oxygen (O ⁸ ), which are lower in atomic number. The density of atoms present is another factor contributing to x-ray attenuation; lungs are radiolucent (dark) on plain film and CT because of the much lower density of atoms (and electrons) present in the air within the lungs. On the other hand, the use of iodinated contrast agents in contrast-enhanced CT (CECT) is partly based on the high attenuation of iodine (I ⁵³ ), which can more easily scatter and absorb x-rays because of the higher number of electrons, thereby making vessels and enhancing organs brighter than surrounding tissues.

With single energy (or routine) CT, the x-rays are generated from accelerating electrons in a x-ray tube subject to a peak voltage, kVp, with a predictable energy spectrum of the emitted x-rays. The addition of a second x-ray beam with a different kVp allows us to create CT images based not only on electron density but also on the concentration of distinct elements present in the body. DECT thus takes advantage of the “signature” x-ray interaction profile of distinct elements, such as calcium (Ca ²⁰ ) and iodine (I ⁵³ ).

The potential applications of DECT are numerous but are primarily separated into two functions. The first is in changing the image contrast, such as by increasing the conspicuity of tumors that enhance using iodinated contrast. At lower x-ray energy, the attenuation of iodinated contrast is magnified compared with other elements in soft tissue, which can alter the conspicuity of subtle enhancing lesions. Potential applications include improving hepatocellular carcinoma (HCC) detection (see Chapter 89 ), liver metastasis detection (see Chapter 90, Chapter 91, Chapter 92 ), and pancreatic cancer or pancreatitis detection ^, ( Fig. 19.1 ; see Chapter 55, Chapter 56, Chapter 57, Chapter 58, Chapter 59 , 62 ). A second application of DECT relies on the quantification of specific elements, such as calcium or iodine. Quantifying iodine can improve our ability to measure treatment response, such as in therapies in which tumor vascularization (and enhancement) is affected and changes in tumor attenuation are informative. Fat quantification, in patients at risk for hepatic steatosis, for example, is also another potential application of this technique. Although DECT is gaining in popularity as the applications multiply, a number of obstacles remain, including standardization and differences in hardware and software between vendors. Nevertheless, the potential quantification of certain elements by DECT, including iodine uptake, is attractive to radiologists who are increasingly exploring quantitative tools in medical imaging.