Intradiurnal Fluctuations of Off-Resonance Saturation Effects in Healthy Human Achilles Tendons Assessed with a 3D Ultrashort Echo Time MRI Sequence at 3 Tesla

Abstract

Purpose: The purpose of this study was to evaluate whether gravitational interstitial fluid accumulation in healthy subjects has an impact on off-resonance saturation ratios (OSR) or the volume of the Achilles tendon after a prolonged time of reduced levels of physical activity.

Materials and Methods: 7 healthy volunteers were repeatedly investigated on 3 consecutive days on a 3 T whole body MR scanner using an ultrashort echo time (UTE) imaging sequence with a Gaussian off-resonance saturation pulse at a frequency offset of 2000 Hz to calculate OSR values. For accurate volumetric quantification of the Achilles tendon, a newly developed contour detection snake algorithm was applied on high-resolution isotropic T2-weighted SPACE sequence datasets. Single-measure intraclass correlation coefficients (ICC) were calculated to estimate test-retest reliability.

Results: For OSR and tendon volume measurements on three consecutive days, excellent reproducibility could be achieved with ICC values above 0.96 and 0.97, respectively. Comparing the results of all three days, a statistically significant mean individual percentage decrease (– 4.1 ± 1.5 %; p = 0.001) of calculated tendon OSR values was found for the evening measurements. No statistically significant difference between tendon volumes in the morning and the evening could be detected (p = 0.589).

Conclusion: The results of this in-vivo study demonstrate a significant influence of gravitational interstitial fluid accumulation after reduced physical activity on OSR values in the Achilles tendon, but not on tendon volume. Taken together with the demonstrated excellent reproducibility, these findings are important for future studies investigating temporal changes of the Achilles tendon microstructure.

Key Points:

• Tendon volume measurements and intratendinous OSR-values demonstrated a high day-to-day reproducibility.

• Gravitational fluid shifts might partly explain the known variation of OSR- measurements in healthy tendons.

• Physiological interstitial fluid shifts of the lower extremity can influence highly sensitive quantitative MR-parameters (as for example in this study used OSR-measurements) even in healthy individuals.

Citation Format:

• Grosse U., Syha R., Partovi S. et al. Intradiurnal Fluctuations of Off-Resonance Saturation Effects in Healthy Human Achilles Tendons Assessed with a 3D Ultrashort Echo Time MRI Sequence at 3 Tesla. Fortschr Röntgenstr 2015; 187: 1003 – 1010

Zusammenfassung

Ziel: In dieser Studie an gesunden Probanden sollten die Auswirkungen des orthostatischen Ödems durch reduzierte körperliche Aktivität auf intratendinöse Magnetisierungstransfer- (MT-) Effekte (oder allgemeiner Off-Resonanz-Sättigungs-Ratios („OSR“)) und das Volumen der Achillessehne untersucht werden.

Material und Methoden: Sieben gesunde Probanden wurden an drei aufeinander folgenden Tagen jeweils morgens und abends mittels einer 3D-Ultrashort-echo-time (UTE) Sequenz mit 2000 Hz Off-Resonanz-Sättigungspulsen zur Kalkulation von OSR-Werten an einem 3 T Ganzkörper-MRT untersucht. Für eine möglichst exakte Volumenquantifizierung der Achillessehne in hochaufgelösten isotropen T2-SPACE Sequenzen kam ein neu entwickelter „Snake“- Konturerkennungsalgorithmus zum Einsatz. Zusätzlich wurden Einzelmessungs-Intraklassen-Korrelationskoeffizienten (engl. ICC) als quantitatives Reliabilitätsmaß zur Abschätzung der Test-Retest-Reliabilität berechnet.

Ergebnisse: Sowohl für die OSR- Werte als auch das Sehnenvolumen konnte eine exzellente Reproduzierbarkeit mit ICC-Werten größer 0,96 bzw. 0,97 über die drei untersuchten aufeinander folgenden Messtage gezeigt werden. Bei der Betrachtung aller Messzeitpunkte wurde eine statistisch signifikante Abnahme der intratendinösen OSR-Werte (– 4,1 ± 1,5 %; p = 0,001) am Abend gefunden, während das ermittelte Sehnenvolumen hier keine signifikante Änderung zeigte (p = 0,589).

Schlussfolgerung: Die Ergebnisse dieser in-vivo Studie zeigen einen messbaren Einfluss des orthostatischen Ödems durch reduzierte körperliche Aktivität auf intratendinöse OSR-Werte aber nicht das Sehnenvolumen. Zusammen mit der aufzeigten exzellenten Reproduzierbarkeit sind diese Ergebnisse besonders für zukünftige Longitudinalstudien und die Untersuchung von zeitlichen Veränderungen der tendinösen Mikrostruktur von Bedeutung.

Kernaussagen:

• Messungen des Sehnenvolumens und der intratendinösen OSR-Werte zeigten im kurzfristigen täglichen Verlauf eine sehr gute Reproduzierbarkeit.

• Das orthostatische Ödem der unteren Extremität ist wahrscheinlich teilweise für die in früheren Studien festgestellten Messwertschwankungen von intratendinösen OSR-Werten in gesunden Sehnen verantwortlich.

• Physiologische Flüssigkeitsverschiebungen an der unteren Extremität können sogar in gesunden Probanden hochsensible quantitative MR-Messmethoden (wie zum Beispiel in dieser Studie benutzte OSR-Werte) beeinflussen.

Key words

tendons - MR imaging - tissue characterization

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