Review ArticleThe reliability of CT numbers as absolute values for diagnostic scanning, dental imaging, and radiation therapy simulation: A narrative review
Introduction
Computed Tomography (CT) is a vital imaging modality in medical imaging and has replaced many diagnostic radiographic procedures as well as being key in the simulation of radiation therapy treatment. In CT, all body tissues can be expressed in Hounsfield units (HU), also referred to as CT numbers, which reflect the value of the attenuation coefficient of any specified tissue relative to the attenuation coefficient of water, where vacuum is arbitrarily defined to be a -1000 HU and water as zero HU [1].
Several clinical outcomes rely on the accuracy of CT number values to identify internal structures and differentiate pathological tissue from adjacent healthy tissue [2]. The reliance on CT numbers for diagnosis underlines the importance of accurate performance for radiological diagnosis. Thus, variation within and between scanners and technical factors that impact accuracy should be taken into account when making quantitative measurements that depend on quantitative HU values for diagnosis; examples include adrenal and renal lesions [3], [4], [5], [6], renal stone composition [7], cerebral venous thrombosis [8], airway and parenchymal abnormalities [9], the extent of coronary atherosclerosis [10], 11, 12, and hepatic stenosis [13,14].
Cone beam CT (CBCT) also relies on CT numbers for diagnosis and treatment planning in several dentomaxillofacial clinical applications [15]. In fact, the accuracy of CT numbers can affect the assessment of bone mineralisation at potential implant sites using computer-aided automatic segmentation [15,16].
In radiation therapy, the dose calculation treatment planning systems (TPS) are highly dependent upon CT imaging data and are used to delineate body contour, shape, the density of internal organs and dose coverage for targeting tumours adjacent to critical organs [17]. The value of CT numbers represent tissue electron density (ED) and directly relate to the attenuation characteristic of the X-ray beam in the photon path length within a particular tissue [18]. Accurate dose calculation is possible only when the correct CT number, expressed in HU, and relative electron density (RED) is established in TPS [19]. Changes to the measured HU for a specific tissue, compared say to the HU value in the TPS calibration, can impact material assignment in CT pre-processing and influence therapeutic doses calculated by the TPS [20].
In brief, CT number accuracy is critical to achieving optimal efficiency of several imaging applications, dental treatment planning, and radiation therapy dose calculation. This review will discuss the current literature concerning four key factors (energy dependence, body size and anatomical location and patient centring) that influence the reliability of CT numbers as an absolute value.
Section snippets
Methodology
A review of the current literature was undertaken across Medline (PubMed), Google scholar, and Ovid databases to investigate this topic with the keywords. Broad search terms of CT number variability, CT number accuracy and uniformity, tube voltage, patient positioning, patient off-centring, and size dependence (including synonyms) were employed. Only studies with unrestricted access to their full text and written in English were included. The refined publications from the identified search
Discussion
The primary objective of the review was to highlight the importance of CT number accuracy in clinical practice. Furthermore, this review intended to explore the effect of applied energy, body size, anatomical location, and patient centring on CT number reliability.
Summary of evidence
Linearity is an important parameter of image quality that affects radiation dose calculation and quantitative diagnostic imaging. Various authorities are known to provide CT number tolerance and acceptance limits that are not the same. Generally, CT numbers for peripheral ROIs should not deviate by more than 4 HU compared to the central ROI [29,30]. A CT number is considered energy-dependent for any material other than air and water, based on the physical principles of photon interaction
Acknowledgments
The authors owe deep gratitude to Dr. Andrew Kilgour for his valuable discussions and consultations. I would like to give my special thanks to The Hashemite University for their generosity with my PhD scholarship.
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Contributors: All authors provided critical feedback, helped shape the research, and approved the manuscript's final version.
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