From the September 2019 issue of HealthCare Business News magazine
By Nakul Gupta
Computed Tomography (CT) has transformed medical care worldwide since its development in the 1970s, and the technology has advanced considerably since then, allowing for an ever increasing number of applications.
CT utilizes a rotating X-ray tube detector array in order to generate cross-sectional images of the body, typically utilizing an X-ray beam covering a broad energy range, but with a peak energy of 120 kilo-electron volts (120 kVp), although this may be higher or lower in certain clinical situations. The image created assigns grayscale values based on the attenuation of the beam (i.e., how much was absorbed or scattered in the patient). While traditional CT is performed using a single peak energy level, spectral (or “dual energy”) CT refers to methods that obtain information regarding tissue attenuation at 2 or more energy levels. Broadly speaking, this may be achieved by either varying the energy level of the beam generated by the X-ray tube, or by discriminating between the energies of the incident X- photons at the detector. This allows for multiple new types of images to be generated and multiple new applications.
Material density images
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Iodine has been employed for many decades as a means to generate contrast in X-ray-based imaging modalities, including CT. One of the primary advantages of spectral CT is that it allows separation of the total attenuation of the tissue into that which is attributable to iodine, and that which is attributable to water (the primary constituent of biological tissues) through a process called material decomposition. This allows quantification of iodine content, and generation of iodine overlays or iodine basis images.
Iodine quantification is an active area of research as a quantitative imaging biomarker, and is being studied as an indicator of treatment response in various cancers, for example. Additionally, there is a growing body of evidence that iodine images can help discriminate between benign and potentially malignant incidental renal masses on routine abdominal contrast enhanced CT’s — a common occurrence. Typically, these incidental findings would require bringing the patient back for a follow up multiphase scan with and without contrast. With iodine images it is now possible to characterize the majority of these lesions at the initial scan, saving time, cost, and radiation dose to the patient.
Another area where material decomposition is poised to bring value to CT is in bone marrow edema imaging. In this case, calcium is used as one of the basis materials, and by removing its contribution to the image, it becomes possible to visualize bone marrow edema, which could previously only be done with MRI. Although research is ongoing and this is still in its infancy, many centers are beginning to employ it in the clinical routine. These are typically performed in the trauma setting, such as when evaluating spinal compression fractures. Currently, determining the acuity of spinal compression fractures at CT can be unreliable, and many patients go on to have an MR, resulting in additional cost and possibly length of stay. With spectral CT, it may be possible to answer this question up front, reducing both cost and length of stay.