Glomerular filtration rate is an essential variable in evaluation of renal function. In dogs and cats, the determination of GFR with iohexol—an iodinated, water-soluble, nonionic, low-osmolar contrast medium—has become an accepted alternative to GFR determination with inulin and radioactive tracers.1–7 Calculation of GFR from contrast-enhanced CT data correlates well with findings of more established methods such as renal scintigraphy and assessment of plasma iohexol clearance or the gold-standard inulin clearance.8–10 Advantages of CT-GFR determination include the noninvasive measurement of contrast medium concentration in an ROI, possible morphological evaluation of the kidneys (eg, in cases of hydronephrosis, renal or ureteral calculi, and renal tumors) with good spatial resolution, and redundancy of blood or urine sample collection. The examination requires general anesthesia for immobilization of animals and exposes them to radiation.9,11 In dogs, GFR determined from a single CT slice underestimates the actual GFR, compared with determinations made on the basis of renal scintigraphy and measurements of plasma iohexol clearance.9,12 In pigs, single-slice CT-GFR correlates with inulin-derived GFR without bias for the right kidney and both kidneys.8 Threephase whole-kidney CT with Patlak plot analysis underestimates right kidney GFR and total inulin-derived GFR.13 In cats, a maximum difference between CT-GFR and GFR determined by plasma clearance testing or renal scintigraphy of up to 20% exceeds the definition of acceptable clinical use.14
The Patlak plot is a graphical representation of a 2-compartment mathematical model, which facilitates the calculation of CT-GFR by plotting the iodine content of each kidney for each measurement time point against the respective integrated iodine content of the aorta, both normalized to the respective iodine content in the aorta. The plot ideally becomes linear after the initial distribution phase, and the slope of the linear phase represents the whole blood clearance. Typical assessments of CT-GFR in animals use lower doses of contrast medium (150 to 350 mg of iodine/kg) than those used in routine contrast-enhanced abdominal CT examinations (700 mg of iodine/kg) and involve variable injection rates (CRIs between 0.25 and 4 mL/s). In people and dogs, the shape of the time-attenuation curve in the blood pool depends on the injection protocol. In most studies,9,11,13,15–19 CT-GFR has been based on a single-slice technique, which includes both kidneys in 1 CT slice. It is recommended to exclude the main vessels and the renal hilus. In patients with pathological changes in the kidneys, it may be impossible for the operator to find an adequate, single image plane without including main vessels and the renal hilus under practical conditions.
To our knowledge, there are no published GFR studies regarding the influence of the injected dose of contrast medium, the injection protocol, or the measurement technique on time-dependent attenuation in the ROIs. The objective of the study reported here was to examine the influence of the injection protocol and measurement technique on CT assessment of GFR in healthy Beagles. To this end, the effect of 3 contrast medium injection protocols with 1 of 2 contrast medium doses (350 or 700 mg of iodine/kg) on the calculation of GFR determined from attenuation values of contrast-enhanced CT was investigated. In addition, 4 techniques (STST, OTST, DRT, and VCT) to measure attenuation within the kidneys were evaluated. The study was performed to test the null hypothesis of no differences in the determined GFR values among injection and measurement techniques.
Supported in part by the Albert-Heim Foundation.
Presented in abstract form at the Conference of European Veterinary Diagnostic Imaging, Wroclaw, Poland, September 2016.
Whole blood iodine clearance
Plasma iodine clearance
Constant rate injection
Dorsal reconstruction technique
Exponentially decelerated injection
Glomerular filtration rate
Optimized transverse section technique
Region of interest
Standard transverse section technique
Volume calculation technique
Prequillan, Arovet AG, Dietikon, Switzerland.
Methadon Streuli, 10 mg/mL, Streuli Pharma, Uznach, Switzerland.
Propofol 1%, Fresenius Kabi AG, Stans, Switzerland.
Sevoflurane, Baxter AG, Volketswil, Switzerland.
Datex Ohmeda S5 Avance, Soma Technology Inc, Bloomfield, Conn.
Esmeron, MSD AG, Luzern, Switzerland.
Accupaque 350, 350 mg of I/mL, GE Healthcare, Glattbrugg, Switzerland.
Accutron CT-D Medtron Injector, SMD Medical Trade GmbH, Salenstein, Switzerland.
Brilliance CT 16-slice, Philips AG, Zurich, Switzerland.
OsiriX Imaging Software, version 4.1, 64-bit, Geneva, Switzerland.
MacMini (2010) 2.4 GHz Intel Core Duo, Apple Switzerland AG, Zurich, Switzerland.
EIZO MX210 (1,600 × 1,200) 21.5 in, EIZO AG, Wädenswil, Switzerland.
Microsoft Office Excel for Mac, Microsoft Schweiz GmbH, Wallisellen, Switzerland.
SPSS, IBM SPSS Statistics, IBM Schweiz AG, Zurich, Switzerland.
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