MRI-Based Liver Iron Content Determination at 3T in Regularly Transfused Patients by Signal Intensity Ratio Using an Alternative Analysis Approach Based on R₂* Theory

Abstract

Objectives: To evaluate the feasibility of addressing liver iron content (LIC) in regularly transfused patients by MR imaging at 3 T based on the signal intensity ratio (SIR). An innovative data analysis approach was developed for this purpose.

Methods: 47 consecutive examinations of regularly transfused patients were included. In all cases, we expected high LIC levels. Patients were scanned with MRI at 3 T with multi-echo gradient echo sequences (GRE) at four different flip angles between 20° and 90° with echo times (TE) ranging from 0.9 to 9.8 ms. Spin-echo protocols were acquired to determine the LIC with a reference MRI method working at 1.5 T. 3 T GRE data were analyzed using the liver-to-muscle SIR. Since the method known for 1.5 T was not expected to be applicable for analyzing 3 T data, theoretic dependence of the SIR on the LIC was derived from the equation describing R₂* signal decay. Obtained SIR values were correlated to reference LIC to get a relation for calculating LIC from SIR quantities. LIC values and their uncertainties were determined from GRE data and correlated to LIC reference values. For two LIC thresholds, the diagnostic accuracy was determined.

Results: LIC was reliably determined from SIR in our patient cohort even for large LIC values. Median of LIC uncertainties was 10 %, and the diagnostic accuracy was 0.92 and 0.91, respectively.

Conclusion: Determination of even high LIC, resulting in small SIR values, is feasible at 3 T using appropriate SIR analysis.

Key Points:

• Determination of Liver Iron Concentration (LIC) based on GRE MRI at 3T is feasible even for high LIC levels using Signal Intensiy Ratios.

• Relative uncertainty of LIC determined with 3T GRE MRI was below 13 % in most cases.

• The patient-management relevant threshold (LIC = 80 µmol/g (4.5 mg/g)) yielded an accuracy of .92 in our cohort.

• The proposed method is quick and simple, both in terms of data acquisition and analysis.

Citation Format:

• Wunderlich AP, Cario H, Bommer M et al. MRI-Based Liver Iron Content Determination at 3T in Regularly Transfused Patients by Signal Intensity Ratio Using an Alternative Analysis Approach Based on R₂* Theory. Fortschr Röntgenstr 2016; 188: 846 – 852

Zusammenfassung

Ziel: Evaluierung der Möglichkeit, den Leber-Eisengehalt (LIC, von liver iron conentration) regulär transfundierter Patienten mittels MRI-Signalintensitätsverhältnissen (SIR, kurz nach dem engl. Signal Intensity Ratio) bei 3 T zu bestimmen. Hierzu wurde ein neuartiger Ansatz der Datenanalyse erarbeitet.

Methoden: 47 konsekutive Untersuchungen regulär transfundierter Patienten wurden in die Studie eingeschlossen. Bei ihnen wurde ein hoher Lebereisengehalt erwartet. Die Untersuchung erfolgte an einem 3T-MRI-Scanner mit Multi-Echo-Gradienten-Echo-(GRE)-Sequenzen mit vier verschiedenen Flipwinkeln zwischen 20° und 90° mit Echozeiten (TE) von 0,9 bis 9,8 ms. Eine MRI-Methode basierend auf Spin-Echo-Protokollen bei 1,5 T diente als Referenz zur Bestimmung des Leber-Eisengehalts. Für die Analyse der 3T-GRE-Daten wurde das Signal-Intensitätsverhältnis (SIR) zwischen Leber- und Muskelgewebe ermittelt. Da zu erwarten war, dass sich die für 1,5 T bekannte Methode nicht für die Auswertung von 3T-Daten eignete, wurde zur Ermittlung einer passenden Umrechnungsvorschrift der theoretische Zusammenhang zwischen SIR und LIC aus der Formel für den R₂*-Signalzerfall abgeleitet. Aus dem Zusammenhang der SIR-Werte mit den LIC-Referenzwerten wurde eine Relation ermittelt, um den LIC und dessen Unsicherheit aus den GRE-Daten zu bestimmen. Diese Ergebnisse wurden mit den LIC-Referenzwerten korreliert und für zwei LIC-Schwellwerte die diagnostische Genauigkeit berechnet.

Ergebnisse: Der LIC konnte in unserem Patientenkollektiv auch für hohe Eisenüberladung verlässlich aus den SIR-Werten bestimmt werden. Der Median der Unsicherheit der ermittelten LIC-Werte lag bei 10 %, die diagnostische Genauigkeit bei 0,92 bzw. 0,91.

Schlussfolgerung: Selbst hohe LIC-Werte, bei denen kleine SIR auftreten, können bei 3 T verlässlich bestimmt werden.

Kernaussagen:

• Bestimmung des Leber-Eisengehalts (LIC, kurz für Liver Iron Conentration) basierend auf GRE MRT bei 3T mittels Signal-Intensitäts-Verhältnissen ist möglich, auch für hohe Leber-Eisen-Werte.

• Die realtive Unsicherheit des mit der vorgeschlagenen Methode bestimmten Leber-Eisengehalts war in den meisten Fällen kleiner als 13 %.

Bei der für das Patienten-Management relevanten LIC-Schwelle von 80 µmol/g (4,5 mg/g) ergab sich für unser Patientenkollektiv eine Genauigkeit von 0,92.

• Die dargestellte Methode ist schnell und einfach, sowohl die Daten-Akquisition als auch die Auswertung.

Key words

abdomen - hematologic diseases - physics - hepatic iron - MR imaging - thalassemia

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