Ultrasound Strain Relaxation Time Ratio: A Quantitative Marker for the Assessment of Cortical Inflammation/Edema in Renal Allografts

Abstract

Purpose: To evaluate the ability of ultrasound strain relaxation time ratio to assess cortical inflammation/edema in renal allografts.

Materials and Methods: We prospectively assessed renal allograft cortical inflammation/edema in 16 renal transplants using ultrasound elasticity imaging and correlated the findings with kidney biopsy. Strain relaxation times in the renal cortex and reference soft tissue were produced by free-hand compression with the ultrasound transducer and estimated with 2 D speckle tracking. Compression was performed in 3-second compression-relaxation cycles (push for 1 second, constant pressure for 1 second, and release for 1 second). We propose a strain relaxation time ratio (time of cortical strain to return to zero/time of the reference strain return to zero) to assess the relationship of compression-induced time-dependent strain relaxation in the cortex and reference tissue. 16 patients were divided into a group with ≤ 25 % (n = 8) and a group with > 26 % (n = 8) cortical inflammation/edema based on the Banff score. A t-test was used to examine the difference in the strain relaxation time ratio between the two groups. The diagnostic accuracy, inter-rater reliability, and reproducibility of this technique in discriminating between the groups were tested.

Results: The strain relaxation time ratio of cortex/reference tissue was significantly higher in patients with > 26 % than in patients with ≤ 25 % cortical inflammation/edema (1.15 ± 0.10 vs. 0.91 ± 0.08, P = 0.0002). The strain relaxation time ratio has high reliability (Pearson correlation coefficient, R² = 0.93), reproducibility (intraclass correlation coefficient = 0.98, P = 0.000), and accuracy (area under curve = 1) in determining > 26 % renal cortical inflammation/edema.

Conclusion: The strain relaxation time ratio of cortex/reference tissue can be used as a quantitative marker for the assessment of cortical inflammation/edema in renal allografts.

Zusammenfassung

Ziel: Kann der Ultraschall-Strain-Relaxationszeit-Quotient kortikale Inflammation/Ödem bei Nierenallotransplantaten bestimmen?

Material und Methoden: Wir bewerteten prospektiv kortikale Inflammation/Ödem bei 16 Nierenallotransplantaten mittels Ultraschallelastrografie und verglichen die Befunde mit der Nierenbiopsie. Strain-Relaxationszeiten der Nierenrinde und der Weichteile als Referenz wurden durch Freihandkompression mit dem Ultraschallkopf erzeugt und mit 2-D-Speckle-Tracking bestimmt. Die Kompression wurde in 3-sekündigen Kompressions-Relaxations-Zyklen durchgeführt (Push 1 Sek., konstanter Druck 1 Sek. und Release 1 Sek. lang). Wir schlagen vor, den Strain-Relaxationszeit-Quotienten (Zeit des kortikalen Strains bis zur Nullrückkehr/Zeit des Referenzstrains bis zur Nullrückkehr) in Beziehung zu kompressionsinduzierter zeitabhängiger Strain-Relaxation in Rinde und Referenzgewebe zu bewerten. 16 Patienten wurden auf Basis der Banff-Klassifikation in eine Gruppe mit ≤ 25 % (n = 8) kortikaler Inflammation/Ödem und eine weitere mit > 26 % (n = 8) eingeteilt. Um den Unterschied der Strain-Relaxationszeit-Quotienten beider Gruppen zu ermitteln wurde der t-Test angewandt. Die diagnostische Genauigkeit, Interrater-Reliabilität und Reproduzierbarkeit wurden untersucht.

Ergebnisse: Der Strain-Relaxationszeit-Quotient von Rinden- zu Referenzgewebe war bei Patienten mit kortikaler Inflammation/Ödem > 26 % im Vergleich zu denen ≤ 25 % signifikant erhöht (1,15 ± 0,10 vs. 0,91 ± 0,08, p = 0,0002). Der Strain-Relaxationszeit-Quotient hat eine hohe Verlässlichkeit (Pearson’s Korrelationskoeffizient, R² = 0,93), hohe Reproduzierbarkeit (Intraklasse-Korrelationskoeffizient = 0,98; p = 0,000) und große Genauigkeit bei der Festlegung von > 26 % Inflammation/Ödem.

Schlussfolgerung: Der Strain-Relaxationszeit-Quotient von Rinden- zu Referenzgewebe kann als quantitativer Marker für die Bewertung kortikaler Inflammation/Ödem bei renalen Allotransplantaten eingesetzt werden.

Key words

ultrasound - abdomen - renal allograft - ultrasound elasticity imaging - strain relaxation time

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