A New Technique For CT Measurement Of Renal Concentration And Dilution Function

The kidney has various physiological functions,including filtration,concentration and dilution,endocrine function,etc.There are many clinical tests on glomerular filtration rate（GFR）,and most of them are direct quantitative indicators.The concentrating and diluting functions of the renal tubules and collecting ducts determine the body’s water-electrilyte metabolism and homeostasis,not only important physiological indicators of the human body but also have important clinical value.Although there are various clinical methods to measure renal concentration and dilution function,including urine specific gravity,urine osmolality（U-osm）,free water clearance rate,etc,these indicators are often affected by a variety of factors and constraints can not directly reflect the renal concentration and dilution capacity,there is no clinical method to directly measure renal concentration and dilution function,in addition,the iodine contrast agent used in CT urography（computer tomography urography,CTU）has similar renal excretion characteristics as inulin,and can be freely filtered through the glomerulus without being reabsorbed or secreted by the renal tubules.The increase of the CT value is linear with the concentration of iodine contrast agent,so that the renal concentrating（diluting）capacity can be calculated by measuring the CT value（concentration of iodine contrast agent）of urine in the renal pelvis and blood in the aorta.The purpose of this study is to measure renal concentrating coefficient（RCC）using CTU image data,and to develop a new technique that can directly measure renal concentrating and diluting function and enable separate measurement of the two kidneys（split-nephron measurement）.The study prospectively included 80 subjects who were clinically required to undergo CTU examination.All subjects fasted for 3 h before the examination,voided urine 20 min before the examination,drank 8ml/Kg of water,and drew 3 ml of venous blood to measure the hematocrit（HCT）.The CTU images were acquired with a delay of 8 min after iodine contrast injection,and urine was collected from the subjects after completion of the examination to measure U-osm.The renal concentration coefficient（RCC）was equal to the CT value of urine in the CTU pyel divided by the CT-enhanced value of intra-aortic plasma during the same period,the CT-enhanced value of intra-aortic plasma is equal to the difference between the delayed and flat intra-aortic（blood）CT values divided by（1-HCT）.The overall concentrating（diluting）capacity of both kidneys was assessed by the renal concentrating coefficient average（RCCA）of the bilateral renal RCC.Independent samples Student-t test and receiver operating characteristic curve（ROC）were used to analyze and compare the differences in birenal RCCA and U-osm between control and furosemide loading groups,and the ability of birenal RCCA and U-osm to discriminate between the two groups of subjects,and all subjects were analyzed for Pearson correlation between birenal RCCA and U-osm.The results showed a moderate correlation（r=0.75,P<0.001）between birenal RCCA（y-axis）and U-osm（x-axis）with a linear equation of y=0.0561x-9.451.birenal RCCA in the control group（n=40）and furosemide loading group（n=40）were expressed as mean±standard deviation of（14.88±5.19）,（5.67±2.11）,and independent sample Student-t test analysis of the two-renal RCCA（t=10.38,P<0.001）was considered significantly different between the two groups.The U-osm in the control and furosemide loading groups was expressed as（396.1±84.96）m Osm/kg.H₂O and（307.2±44.61）m Osm/kg.H₂O,respectively,and the independent sample Student-t test analysis of the two-renal U-osm（t=5.85,P<0.001）was considered to have a significant difference.The area under curve（AUC）was 0.958 with a 95%confidence interval of[0.918,0.998],P<0.001,which was statistically significant,when the critical value（Cutoff value）of RCCA of both kidneys was 9.13,distinguishing the two groups of subjects with the best youden index（0.75）,corresponding to a sensitivity of 85%and a specificity of 90%.The ROC curve for U-osm showed an AUC of 0.817,95%confidence interval[0.720,0.915],P<0.001,which was statistically significant,and a Cutoff value of 341.5 m Osm/kg.H₂O had the best youden index（0.52）,corresponding to a sensitivity of 75%and a specificity of 77.5%.The single kidney RCC in the control group（n=80）and the furosemide loading group（n=80）,expressed as mean±standard deviation,were（14.88±5.35）and（5.67±2.17）,respectively,using the individual kidney as the study unit,and the independent sample Student-t test analyzed that the single kidney RCC was significantly different between the two groups（t=14.24,P<0.001）.The concentration（dilution）function index of the kidney RCCA proposed in this study has a moderate correlation with the commonly used clinical index U-osm,indicating that U-osm and the concentration（dilution）function of the kidney are correlated but do not accurately reflect the concentration（dilution）function of the kidney,and RCC can better judge and describe the concentration（dilution）function status of the kidney than U-osm.We have developed a new technique that can directly measure the concentration（dilution）function of the kidney using the image data of CTU without increasing the radiation dose,examination time and examination cost of the subject,and can also perform the measurement of split kidney,which has a good potential for clinical application.