Quantification of variability sources and evaluation of myocarditis extent from MRI LGE images

Kralj, Lana (author), Jeraj, Robert (mentor), Kirn, Borut (co-mentor)

Introduction Clinical experts determine the extent and distribution of myocardial damage in myocarditis by quantitatively analyzing magnetic resonance late gadolinium enhancement (MRI LGE) images. The variability of results originates from two quantitative imaging steps: image acquisition and image analysis. The latter is studied in this master's thesis to rank all analysis steps with respect to their contribution to overall variability. Methods Whilst analyzing MRI LGE images of 35 myocarditis patients with the clinical experts, I determined three analysis steps, all three of them being selection of reference regions of interest (ROI): delineation of myocardium (ROI_myoc), reference pathological or healthy tissue (ROI_ref), and excluded tissue (ROI_excl). To visualize ROI’s and calculate LGE extent, I developed a quantitative MRI LGE image analysis tool. Using developed tool, four experts analyzed five images in three sessions, whereby LGE extent was quantified with two methods: 3σ, which uses signal intensity threshold of three standard deviations above ROI_ref defined in normal myocardium, and FWHM, which uses threshold of 50% of the maximum signal intensity within the scar-defined ROI_ref for pathology determination. Results and discussion I observed artefacts and image resolution impacted ROI_myoc and ROI_excl delineation, and that image contrast was crucial when selecting ROI_ref. These factors, in combination with subjective contour delineation during the analysis process, caused variations in ROI regions selection. The variability of ROI_myoc and ROI_excl was significantly smaller when using the FWHM method. Furthermore, ROI_ref variability was significantly greater than ROI_myoc variability when using the 3σ method. It was demonstrated that overall variability may exceed clinically acceptable 5% limit, with the risk being greater when using the 3σ method, which could lead to misevaluation of the adverse effects risk. Relative shares of ROI variabilities showed that ROI_ref contributes on average 1.8-times more to overall variability than ROI_myoc in the 3σ method. Thereby, clinical experts should be especially careful when selecting ROI_ref.

• Language: English
• Keywords: myocarditis, quantitative MRI imaging, MRI LGE image analysis, variability, quantification methods, region of interest (ROI)
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2021
• PID: 20.500.12556/RUL-130268
• COBISS.SI-ID: 75976195
• Publication date in RUL: 12.09.2021

Secondary language

• Language: Slovenian
• Title: Kvantifikacija izvorov variabilnosti in določanje obsega miokarditisa iz MRI LGE slik

Abstract

Uvod Obseg in lokacija prizadetega tkiva se pri bolnikih z miokarditisom ocenita na podlagi kvantitativne analize MRI LGE slik (magnetnoresonančne slike s poznim kopičenjem kontrasta gadolinija). Zaradi negotovosti, ki nastanejo v dveh korakih kvantitativnega MRI slikanja, to sta zajem in analiza slik, rezultati analize variirajo. Cilj magistrskega dela je določiti variabilnosti korakov analize in ovrednotiti prispevek vsakega koraka k celotni variabilnosti. Metode V sodelovanju s kliničnimi strokovnjaki smo analizirali MRI LGE slike 35 bolnikov z miokarditisom. V procesu sem določila tri korake analize. Vsak korak ustreza izbiri določenega območja interesa (ROI): izbira območja miokardija ROI_myoc, izbira referenčnega zdravega ali patološkega tkiva ROI_ref in izbira izključitvenih območij ROI_excl. Razvila sem orodje za kvantitativno analizo MRI LGE slik, ki je omogočilo vizualizacijo območij ROI ter izračun obsega LGE-ja. S pomočjo novega orodja so štirje strokovnjaki v treh sejah analizirali pet slik. Obseg LGE-ja smo izračunali s pomočjo dveh kvantifikacijskih metod: 3σ, kjer je prag za izračun patologije definiran kot tri standardne deviacije nad povprečno vrednostjo intenzitete signala znotraj ROI_ref, ki se izbere v zdravem tkivu, in FWHM, kjer je prag enak 50% maksimalne intenzitete znotraj ROI_ref, ki se izbere znotraj brazgotine. Rezultati in razprava Na občtrovanje območij ROI_myoc in ROI_excl vplivajo artefakti in ločljivost slike, medtem ko je pri izbiri območja ROI_ref najbolj pomemben dejavnik kakovosti slike kontrast. Navedeni dejavniki ter subjektivno določanje kontur pri analizi povzročijo variacije v izbiri območij ROI. Variabilnost območij ROI_myoc in ROI_excl, pridobljena z metodo FWHM, je značilno manjša. Variabilnost območja ROI_ref je v primeru uporabe metode 3σ značilno večja kot variabilnost območja ROI_myoc. Izkaže se, da lahko splošna variabilnost preseže klinično sprejemljivo mejo 5%, medtem ko je tveganje večje pri uporabi metode 3σ, kar lahko privede do napačne ocene tveganja neželenih stranskih učinkov miokarditisa. Relativni deleži variabilnosti ROI pokažejo, da ROI_ref pri metodi 3σ prispeva v povprečju 1.8-krat več k splošni variabilnosti kot ROI_myoc. Klinični strokovnjaki morajo tako pri izbiri ROI_ref biti še posebej previdni.

• Keywords: miokarditis, kvantitativno MRI slikanje, analiza MRI LGE slik, variabilnost, metode kvantifikacije, območje interesa (ROI)