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. 2022 Jan 1;51(1):20210140.
doi: 10.1259/dmfr.20210140. Epub 2021 Jun 19.

Can gray values be converted to Hounsfield units? A systematic review

Marjorie Eguren  1 , Anderson Holguin  2 , Karla Diaz  3 , Jose Vidalon  4 , Carlos Linan  5 , Camila Pacheco-Pereira  6 , Manuel Oscar Lagravere Vich  7
Affiliations

Can gray values be converted to Hounsfield units? A systematic review

Marjorie Eguren et al. Dentomaxillofac Radiol. .

Abstract

Objectives: The purpose of this systematic review was to answer the focus question: "Could the gray values (GVs) from CBCT (cone beam computed tomography) be converted to Hounsfield units (HUs) in multidetector computed tomography (MDCT)?"

Methods: The included studies try to answer the research question according to the PICO strategy. Studies were gathered by searching several electronic databases and partial grey literature up to January 2021 without language or time restrictions. The methodological assessment of the studies was performed using The Oral Health Assessment Tool (OHAT) for in vitro studies and the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) for in vivo studies. The Grading of Recommendations Assessment, Development and Evaluation (GRADE system) instrument was applied to assess the level of evidence across the studies.

Results: 2710 articles were obtained in Phase 1, and 623 citations remained after removing duplicates. Only three studies were included in this review using a two-phase selection process and after applying the eligibility criteria. All studies were methodologically acceptable, although in general terms with low risks of bias. There are some included studies with quite low and limited evidence estimations and recommendation forces; evidencing the need for clinical studies with diagnostic capacity to support its use.

Conclusions: This systematic review demonstrated that the GVs from CBCT cannot be converted to HUs due to the lack of clinical studies with diagnostic capacity to support its use. However, it is evidenced that three conversion steps (equipment calibration, prediction equation models, and a standard formula (converting GVs to HUs)) are needed to obtain pseudo Hounsfield values instead of only obtaining them from a regression or directly from the software.

Keywords: Bone density; Cone-beam computed tomography; Hounsfield unit; Multidetector Computed Tomography; diagnostic imaging; systematic review.

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Conflict of interest statement

Conflict of Interest: The authors declare no potential conflict of interest with respect to the authorship and/or publication of this article. There was no funding for this research project.

Figures

Figure 1.
Figure 1.
Flow Diagram of the Literature
Figure 2.
Figure 2.
QUADAS-2 Summary
See this image and copyright information in PMC

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