The study protocol was approved by the local authorities (Niedersächsisches Landesamt für Verbraucherschutz und Lebensmittelsicherheit, Oldenburg, Germany (Reference number 9-42502-05-11A171).
During one year, 21 healthy dogs aged 5.3 ± 4.2 years and having a bodyweight of 16.1 ± 6.1kilogramms (kg) were enrolled in this study (11 male, 10 female). To prove healthiness, each dog underwent a general examination, as well as analysis of blood profile, echocardiography, electrocardiography, abdominal ultrasound and urine analysis (taken by zystocentesis) including the urine protein to creatinine (u p/c) ratio.
For the last 12 hours prior to CEUS examination as well as blood sampling for the determination of the plasma creatinine clearance, dogs had permanent access to fresh water but no feed intake was allowed.
For measuring the plasma creatinine clearance, an 18- or 22-gauge vein catheter (Vasovet, B.Braun, Melsungen, Germany) was placed in the vena cephalica antebrachii and the dogs were infused with an isotonic electrolyte solution (Sterofundin®, B Braun AG, Melsungen, Germany) at a rate of 5 ml/kg/hour for 8 hours. The creatinine solution was mixed at least 24 hours before application as formerly described by Mohr (3). For the creatinine clearance test reference blood samples were taken at time zero. Subsequently, the creatinine solution was applied intravenously with 2 mg/m² body surface. After that, blood samples were taken three, five and eight hours after creatinine application and were centrifuged at 4400 rounds per minute (rpm). Serum was collected and sent to an external laboratory (IDEXX Laboratories - IDEXX Vet Med Labor, Ludwigsburg, Germany) for further analysis of the creatinine clearance and estimation of the glomerular filtration rate (GFR).
Immediately before and after contrast-enhanced ultrasound examination, the systolic blood pressure of the dogs was measured in right lateral recumbency by means of the Doppler method (Doppler Flow Detector, Eickemeyer® Medizintechnik für Tierärzte KG, Tuttlingen, Germany). For this purpose, a blood pressure cuff 40% of the size of the tail root’s width was placed directly under the tail root (8). Each
47 measuring method was repeated five times, followed by the calculation of the mean systolic blood pressure value from three measurement values differing less than 10 mmHg.
Ultrasound examination
Ultrasound examination was performed using the ultrasonographic device Logiq E9 (GE Healthcare, General Electric, Fairfield, Munich, Germany) equipped with a convex array probe (1-5 MHz) and a linear array probe (5-9 MHz). The mechanical index (MI) was less than 0.1 and the acoustic output less than 7%. The gain was set at 20% and a penetration depth of 6 cm was chosen. These settings achieved a frame rate of 11 frames /s. There was one focal zone set at the bottom of the screen.
Scans were performed with an amplitude inversion technique. Each dog was scanned by the same experienced examiner (SH) with the described settings.
For the examination dogs were placed in dorsal recumbency while the head was slightly elevated. The examiner depicted both kidneys at a depth of 1 – 2 cm, central and as parallel as possible to the probe, avoiding intestinal loops or ribs between the kidney and the abdominal wall.
Contrast ultrasound examination was carried out by means of linear and convex array probes for both kidneys of each dog. First, the kidney was displayed in a longitudinal position (occasionally, an intercostal approach of the right lateral abdomen was necessary to depict the right kidney) in B-Mode ultrasound. Then, the contrast mode was activated. Contrast medium, SonoVue® (Bracco Imaging GmbH, Constance, Germany), was mixed immediately prior to usage according to the manufacturer’s specifications. A countdown timer was used to ensure an accurate contrast media application; when the timer reached zero, the contrast media was applied. The injection was carried out at a dosage of 0.06 ml/kg bodyweight using the straight opening of a three-way stopcock (3 Way Stopcock, Fresenius Kabi Deutschland GmbH, Bad Homburg, Germany) of a vein catheter (Vasovet, B.Braun, Melsungen, Germany) placed in the V. cephalica antebrachii of the right forelimb.
Subsequently, 2 ml of 0.9% sodium chloride solution were administered through the other opening. In the following, three consecutive loops each lasting 60 seconds were recorded for each examination. This procedure was conducted in a
48 standardised manner and with the same speed in each dog. After the third loop measured with the linear probe, the acoustic power (MI) was elevated and the remaining microbubbles were destroyed. When there was no noteworthy residual contrast medium left, the MI was set under 0.1 again and the described procedure was repeated for the convex probe.
ROI placement and perfusion quantification
Data analyses of the recorded CEUS examinations were performed using the internal software of the Logiq E9. Therefore, nine regions of interest (ROI) were placed in the kidney; three in the cortex near the probe (near field cortex), three medullary and three in the cortex distant from the probe (far field cortex). Each triplet of ROIs was set at the same depth and with a distance of 1.5 cm between each other, whereas the middle ROI was placed in the centre of the longitudinal axis of the kidney (Fig. 1).
The software measured the pixel intensity for these ROIs over time and created a smoothed time intensity curve (TIC) of the echo signal in dB. In case of possible movements of the ROI due to respiration, the user was able to correct the ROI position manually, in order to ensure a consistent positioning at the same point within the renal tissue throughout the examination. Measurement data were exported to an Excel spreadsheet, including time-dependent data of contrast media distribution within each ROI. The following parameters were assessed:
■ Peak enhancement (PE in dB)
■ Area under the curve (WiAUC in dB)
■ Time to peak (TTP in s)
■ Wash-in Gradient (WiGrad in dB)
■ Arrival Time (AT in s)
In order to investigate the intra- and interobserver variability, CEUS was repeated three successive times in five dogs (using the linear probe and the left kidney).
Analyses were performed by two examiners (J.D. and S.H.) and using two different ROI sizes (approximately 0.03 cm2 vs. 0.12 cm2).
49 Analysis of TICs
In the following, statistical comparisons of the number of evaluable TICs between the different regions of the kidney were performed. Therefore, following regions were evaluated: Far field cortex, near field cortex and medulla, each region analysed for left, middle and right ROI in the left and the right kidney. All analyses were performed for the linear and the convex probe.
Every TIC underwent a testing for its evaluability. TIC evaluability was assessed by means of a scheme which allowed to rank the different curves according to their specific curve progression into different categories.Following categories (Fig. 2) for evaluable as well as for nonevaluable curves were created:
Category A summarised all curves that were evaluable and used for further analysis.
Category B covered the nonevaluable curves which showed multiple peaks.
Category C comprised nonevaluable curves showing no or barely any enhancement.
Category D and E included all nonevaluable TICs with low and late, or only late peaks (Fig. 3). The cut-off values for the categories D and E were defined as follows: Peaks with cortical PE less than 14 dB were considered as low;
cortical TTP > 20s and medullary TTP > 30 s were considered as late peaks.
For each analysed ROI the category of the associated TIC was registered in a spreadsheet software program (Microsoft Excel, Microsoft Corporation, Redmond, USA). Furthermore, perfusion parameter values of the same parameters were compared for two ROI sizes and for two examiners.
Statistical analysis
For evaluating the most favourable combination of probe and ROI placement, the software SAS (SAS Institute Inc., Cary, North Carolina, USA) was used to perform statistical analysis. In order to test for significant differences in the number of feasible curves (category A) between different positions or between the two kinds of probes, a McNemar's test was conducted. P-values of < 0.05 were considered significant. The repeated measurements by the same examiner using three consecutive clips were
50 compared with an intraclass correlation coefficient, considering the influence of the repeated measurements and the dogs on the variance. The differences between the two observers and between the results achieved with different ROI-sizes were analysed using a Wilcoxon signed rank test for the abnormally distributed results and a Student's t-test for the normally distributed results. The distribution was determined using the Kolmogorov-Smirnov test.
A Spearman correlation test was performed to look for an interrelation between clinical parameters (urea, creatinine, GFR and urine specific gravity) and perfusion parameters measured with CEUS.