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Measuring Single-Kidney Glomerular Filtration Rate on Single-Detector Helical CT Using a Two-Point Patlak Plot Technique in Patients with Increased Interstitial Space

¹ Department of Diagnostic Radiology, Justus-Liebig Universität Giessen, Klinikstr. 36, Giessen 35385, Germany.
² Department of Urology, Justus-Liebig Universität Giessen, Giessen 35385, Germany.
³ Department of Clinical Pharmacology, Justus-Liebig Universität Giessen, Giessen 35385, Germany.

View larger version (130K): [in a new window]   Fig. 1. —74-year-old man with bladder carcinoma obstructing both ureters; he was treated with percutaneous nephrostomy on right and left sides. CT scan obtained during parenchymal phase shows one region of interest delineating parenchyma of right kidney (outlined in white) and second region of interest inside aortic lumen (outlined in white).

View larger version (8K): [in a new window]   Fig. 2. —Three-compartment model shows contrast medium distribution in kidney. Contrast medium is injected into vascular space. Bidirectional flow into and out of interstitial space is proportional to difference in contrast medium concentration between two compartments. Unidirectional contrast medium flow from vascular space into nephron is proportional to plasma concentration (i.e., clearance definition). Parameter definitions are given in Appendix 1.

View larger version (13K): [in a new window]   Fig. 3. —Graph shows correlation of total glomerular filtration rates (GFR) (•) between iopromide excretory clearance and plasma clearance in 18 patients. Correlation coefficient was r = 0.94, with regression line of y = -6 + 0.97 x x. (SDyx = 11.4 mL/min). Solid line represents correlation line, and dashed line represents line of identity.

View larger version (13K): [in a new window]   Fig. 4. —Graph shows correlation of total glomerular filtration rates (GFR) (•) in 20 patients as determined by CT and plasma clearance. Correlation coefficient was r = 0.80, with regression line of y = 35 + 0.79 x x. (SDyx = 18.5 mL/min). Solid line represents correlation line, and dashed line represents line of identity.

View larger version (14K): [in a new window]   Fig. 5. —Graph shows correlation of single-kidney glomerular filtration rates (GFR) as determined with CT and excretory clearance in 34 kidneys. Correlation coefficient was r=0.81, with regression line of y=20 + 0.84 x x (SDyx = 10.9 mL/min). Solid line represents correlation line, dashed line represents line of identity. Six kidneys without acute disease are close to line of identity. Kidney conditions are indicated by following symbols: = no disease, • = untreated hydronephrosis, = hydronephrosis and percutaneous nephrostomy (PCN), = pyelonephritis, = pyelonephritis and PCN, + = pyelonephritis and hydronephrosis.

View larger version (16K): [in a new window]   Fig. 6. —Graph shows difference between single-kidney glomerular filtration rates (GFR) as measured with CT and with excretory clearance versus their means. Dashed line represents mean difference (D=4.1 mL/min), and solid line represents limits of agreement (SD = 2.8 mL/min). Kidney conditions are indicated by following symbols: = no disease, • = untreated hydronephrosis, = hydronephrosis and percutaneous nephrostomy (PCN), = pyelonephritis, = pyelonephritis and PCN, + = pyelonephritis and hydronephrosis.

View larger version (14K): [in a new window]   Fig. 7. —Graph shows correlation between single-kidney parenchymal volume and discrepancy of single-kidney glomerular filtration rate (GFR) as measured with CT and excretory clearance. Solid line represents line of correlation. Overestimation of GFR with CT was calculated as difference of excretory GFR and CT GFR for 34 single kidneys (•) (r=0.55; p<0.0008).

View larger version (151K): [in a new window]   Fig. 8A. —68-year-old man with chronic pyelonephritis, hydronephrosis, and cystitis. Left kidney had been treated by percutaneous nephrostomy. Volume of right kidney was 395 mL, and that of left kidney was 260 mL. Contrast medium uptake on right side was reduced. Glomerular filtration rate (GFR) as measured with CT was considerably overestimated in both kidneys. For right kidney, excretory GFR = 3 mL/min and CT GFR = 47 mL/min; for left kidney, excretory GFR = 10 mL/min and CT GFR = 44 mL/min. Axial CT scan (A) and coronal reconstructed image derived from CT scans (B) obtained during parenchymal phase (115 sec after contrast injection) show markedly swollen kidneys, especially on right side.

View larger version (82K): [in a new window]   Fig. 8B. —68-year-old man with chronic pyelonephritis, hydronephrosis, and cystitis. Left kidney had been treated by percutaneous nephrostomy. Volume of right kidney was 395 mL, and that of left kidney was 260 mL. Contrast medium uptake on right side was reduced. Glomerular filtration rate (GFR) as measured with CT was considerably overestimated in both kidneys. For right kidney, excretory GFR = 3 mL/min and CT GFR = 47 mL/min; for left kidney, excretory GFR = 10 mL/min and CT GFR = 44 mL/min. Axial CT scan (A) and coronal reconstructed image derived from CT scans (B) obtained during parenchymal phase (115 sec after contrast injection) show markedly swollen kidneys, especially on right side.

View larger version (17K): [in a new window]   Fig. 9A. —Results of CT measurements and calculations with three-compartment model for contrast medium concentrations in vascular space and interstitium in 74-year-old patient with good renal function treated with right-sided percutaneous nephrostomy for ureteral stone. Volume of both kidneys was 578 mL. Glomerular filtration rate (GFR) measured by plasma clearance was 113 mL/min. CT GFR was 129 mL/min. Lower curve (A and B) represents contrast medium concentration in interstitial space. Parameters are more fully defined in Appendix 1. Graph shows contrast medium concentrations in vascular space as measured with CT, b(t) (). As long as contrast medium concentration in interstitial space, I(t) (), is lower than that in vascular space, contrast medium flows into interstitial space. Equilibrium is reached approximately 110 sec after contrast injection. = b(t) interpolated.

View larger version (20K): [in a new window]   Fig. 9B. —Results of CT measurements and calculations with three-compartment model for contrast medium concentrations in vascular space and interstitium in 74-year-old patient with good renal function treated with right-sided percutaneous nephrostomy for ureteral stone. Volume of both kidneys was 578 mL. Glomerular filtration rate (GFR) measured by plasma clearance was 113 mL/min. CT GFR was 129 mL/min. Lower curve (A and B) represents contrast medium concentration in interstitial space. Parameters are more fully defined in Appendix 1. Graph shows amount of contrast medium in vascular space, B(t) (); in interstitial space, I(t) (); and excreted into nephron based on model calculations and CT measurements. • = amount of contrast medium in nephron, N(t); = amount of contrast medium in kidney, K(t); = contrast medium measured with CT (KCT).

View larger version (13K): [in a new window]   Fig. 9C. —Results of CT measurements and calculations with three-compartment model for contrast medium concentrations in vascular space and interstitium in 74-year-old patient with good renal function treated with right-sided percutaneous nephrostomy for ureteral stone. Volume of both kidneys was 578 mL. Glomerular filtration rate (GFR) measured by plasma clearance was 113 mL/min. CT GFR was 129 mL/min. Lower curve (A and B) represents contrast medium concentration in interstitial space. Parameters are more fully defined in Appendix 1. Graph shows amount of contrast medium as Patlak plot [3] resulting from three-compartment model () calculations. Interstitial space in patient was calculated as 11% of whole-kidney volume. Consequently, amount of contrast medium in interstitial space is much smaller than amount of contrast medium excreted into nephron, and calculated Patlak plot is almost a straight line. GFR calculated with Patlak plot is close to true GFR. = CT measurement.

View larger version (15K): [in a new window]   Fig. 10A. —Contrast medium concentrations as results of three-compartment model calculations and CT measurements in patient with markedly swollen kidneys (shown in Figs. 8A, 8B) caused by bilateral pyelonephritis with hydronephrosis and treated with percutaneous nephrostomy on left side. Patient had low glomerular filtration rate (GFR). Interstitial space was calculated to be 46%. Parameters are more fully defined in Appendix 1. Graph shows that contrast medium concentrations in vascular space as measured with CT, b(t) (), and in interstitium as calculated by formula in Appendix 1, I(t) (), are similar to those in patient with small interstitial space. = b(t) interpolated.

View larger version (20K): [in a new window]   Fig. 10B. —Contrast medium concentrations as results of three-compartment model calculations and CT measurements in patient with markedly swollen kidneys (shown in Figs. 8A, 8B) caused by bilateral pyelonephritis with hydronephrosis and treated with percutaneous nephrostomy on left side. Patient had low glomerular filtration rate (GFR). Interstitial space was calculated to be 46%. Parameters are more fully defined in Appendix 1. Graph shows contrast medium amount in vascular space, B(t) (); in interstitial space, I(t) (); and excreted into nephron as result of model calculations and CT measurements. Amount of contrast medium in interstitial space is much greater than amount excreted into nephron. Most contrast medium measured with CT (KCT) represents contrast medium in interstitial space. Calculated Patlak plot [3] is curved line with slope that is initially steep and later flattens. • = amount of contrast medium in nephron, N(t); = amount of contrast medium in kidney, K(t); = contrast medium measured with CT (KCT).

View larger version (14K): [in a new window]   Fig. 10C. —Contrast medium concentrations as results of three-compartment model calculations and CT measurements in patient with markedly swollen kidneys (shown in Figs. 8A, 8B) caused by bilateral pyelonephritis with hydronephrosis and treated with percutaneous nephrostomy on left side. Patient had low glomerular filtration rate (GFR). Interstitial space was calculated to be 46%. Parameters are more fully defined in Appendix 1. Graph shows Patlak plot as result of three-compartment model calculations () and CT measurements (). GFR calculated with Patlak plot is much higher than true GFR in patient with increased interstitial space.

View larger version (11K): [in a new window]   Fig. 11. —Graph shows Patlak plot [3] of correlation between interstitial volume as calculated by three-compartment model and discrepancy of total glomerular filtration rate (GFR) as measured with CT and plasma clearance (n = 17 kidneys; coefficient correlation of r = 0.93). Each point represents data from total GFR of one patient; interstitial space represents sum of left and right kidneys. High correlation is found between interstitial space and overestimation of GFR as measured with CT.

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