How bold is blood oxygenation level-dependent (BOLD) magnetic resonance imaging of the kidney? Opportunities, challenges and future directions

Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. Yet, in vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Many of the established approaches are invasive, hence not applicable in humans. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) offers an alternative. BOLD-MRI is non-invasive and indicative of renal tissue oxygenation. Nonetheless, recent (pre-) clinical studies revived the question as to how bold renal BOLD-MRI really is. This review aimed to deliver some answers. It is designed to inspire the renal physiology, nephrology and imaging communities to foster explorations into the assessment of renal oxygenation and haemodynamics by exploiting the powers of MRI. For this purpose, the specifics of renal oxygenation and perfusion are outlined. The fundamentals of BOLD-MRI are summarized. The link between tissue oxygenation and the oxygenation-sensitive MR biomarker T2∗ is outlined. The merits and limitations of renal BOLD-MRI in animal and human studies are surveyed together with their clinical implications. Explorations into detailing the relation between renal T2∗ and renal tissue partial pressure of oxygen (pO2 ) are discussed with a focus on factors confounding the T2∗ vs. tissue pO2 relation. Multi-modality in vivo approaches suitable for detailing the role of the confounding factors that govern T2∗ are considered. A schematic approach describing the link between renal perfusion, oxygenation, tissue compartments and renal T2∗ is proposed. Future directions of MRI assessment of renal oxygenation and perfusion are explored.

Early effects of an x-ray contrast medium on renal T(2) */T(2) MRI as compared to short-term hyperoxia, hypoxia and aortic occlusion in rats

AIM

X-ray contrast media (CM) can cause acute kidney injury (AKI). Medullary hypoxia is pivotal in CM-induced AKI, as indicated by invasively and pin-point measured tissue oxygenation. MRI provides spatially resolved blood oxygenation level-dependent data using T2 * and T2 mapping. We studied CM effects on renal T2 */T2 and benchmarked them against short periods of hyperoxia, hypoxia and aortic occlusion (AO).

METHODS

Rats were equipped with carotid artery catheters (tip towards aorta) and supra-renal aortic occluders. T2 */T2 mapping was performed using a 9.4-T animal scanner. CM (1.5 mL iodixanol) was injected into the thoracic aorta with the animal in the scanner followed by 2 h of T2 */T2 mapping. For T2 */T2 assessment, regions of interest in the cortex (C), outer medulla (OM), inner medulla (IM) and papilla (P) were determined according to morphological features.

RESULTS

Hyperoxia increased T2 * in C (by 17%) and all medullary layers (25-35%). Hypoxia decreased T2 * in C (40%) and all medullary layers (55-60%). AO decreased T2 * in C (18%) and all medullary layers (30-40%). Upon injection of CM, T2 * increased transiently, then decreased, reaching values 10-20% below baseline in C and OM and 30-40% below baseline in IM and P.

CONCLUSION

T2 * mapping corroborates data previously obtained with invasive methods and demonstrates that CM injection affects renal medullary oxygenation. CM-induced T2 * decrease in OM was small vs. hypoxia and aortic occlusion. T2 * decrease obtained for hypoxia was more pronounced than for AO. This indicates that T2 * may not accurately reflect blood oxygenation under certain conditions.

Linking non-invasive parametric MRI with invasive physiological measurements (MR-PHYSIOL): towards a hybrid and integrated approach for investigation of acute kidney injury in rats

Acute kidney injury of various origins shares a common link in the pathophysiological chain of events: imbalance between renal medullary oxygen delivery and oxygen demand. For in vivo assessment of kidney haemodynamics and oxygenation in animals, quantitative but invasive physiological methods are established. A very limited number of studies attempted to link these invasive methods with parametric Magnetic Resonance Imaging (MRI) of the kidney. Moreover, the validity of parametric MRI (pMRI) as a surrogate marker for renal tissue perfusion and renal oxygenation has not been systematically examined yet. For this reason, we set out to combine invasive techniques and non-invasive MRI in an integrated hybrid setup (MR-PHYSIOL) with the ultimate goal to calibrate, monitor and interpret parametric MR and physiological parameters by means of standardized interventions. Here we present a first report on the current status of this multi-modality approach. For this purpose, we first highlight key characteristics of renal perfusion and oxygenation. Second, concepts for in vivo characterization of renal perfusion and oxygenation are surveyed together with the capabilities of MRI for probing blood oxygenation-dependent tissue stages. Practical concerns evoked by the use of strong magnetic fields in MRI and interferences between MRI and invasive physiological probes are discussed. Technical solutions that balance the needs of in vivo physiological measurements together with the constraints dictated by small bore MR scanners are presented. An early implementation of the integrated MR-PHYSIOL approach is demonstrated including brief interventions of hypoxia and hyperoxia.

The step response: a method to characterize mechanisms of renal blood flow autoregulation

Response of renal vasculature to changes in renal perfusion pressure (RPP) involves mechanisms with different frequency characteristics. Autoregulation of renal blood flow (RBF) is mediated by the rapid myogenic response, by the slower tubuloglomerular feedback (TGF) mechanism, and, possibly, by an even slower third mechanism. To evaluate the individual contribution of these mechanisms to RBF autoregulation, we analyzed the response of RBF to a step increase in RPP. In anesthetized rats, the suprarenal aorta was occluded for 30 s, and then the occlusion was released to induce a step increase in RPP. Three dampened oscillations were observed; their oscillation periods ranged from 9.5 to 13 s, from 34.2 to 38.6 s, and from 100.5 to 132.2 s, respectively. The two faster oscillations correspond with previously reported data on the myogenic mechanism and the TGF. In accordance, after furosemide, the amplitude of the intermediate oscillation was significantly reduced. Inhibition of nitric oxide synthesis by Nomega-nitro-l-arginine methyl ester significantly increased the amplitude of the 10-s oscillation. It is concluded that the parameters of the dampened oscillations induced by the step increase in RPP reflect properties of autoregulatory mechanisms. The oscillation period characterizes the individual mechanism, the dampening is a measure for the stability of the regulation, and the square of the amplitudes characterizes the power of the respective mechanism. In addition to the myogenic response and the TGF, a third rather slow mechanism of RBF autoregulation exists.

Influence of baroreflex on volume elasticity of heart and aorta in the rabbit

Optimal ventriculoaortic coupling includes tuning of elastic properties. The ratio of effective arterial elastance and left ventricular endsystolic elastance is often taken as a measure for mechanical and energetical efficiency. The present study determined the time course of ventricular and aortic volume elasticity (VE = dp/dV) throughout a complete heartbeat. This was achieved by using changes of eigenfrequency of two catheter-transducer systems under closed chest conditions in rabbits. Short-term VE modulation was studied by a baroreflex response, as induced by pressure changes applied to the carotid sinus. Long-term changes were studied in atherosclerotic rabbits (12 wk of high-cholesterol feeding). The time course and mean values of ventricular and aortic VE were changed by the baroreflex stimulus. Cholesterol feeding diminished the response. The degree of ventriculoaortic coupling, as quantified by VE(Aorta)/VE(Ventricle) ratio, varied during a single ejection period. The large span allows either maximal energetical efficiency or maximal stroke work. Although normal rabbits adjusted their ventriculoaortic coupling during baroreflex input, the cholesterol-fed rabbits failed to do so.

Coupling of left ventricular and aortic volume elasticity in the rabbit

Changes in volume elasticity (VE) of the left ventricle and aorta could be important for blood flow. A procedure is presented to rapidly assess VE of the left ventricle and aorta by analyzing changes in the eigenfrequency. Six control rabbits and 11 rabbits with atheromatosis (12 wk of high-cholesterol feeding) were studied. In control rabbits, during the first half of the systole, left ventricular VE continuously increased to +43% (P < 0.05). Then VE gradually declined to an end-diastolic minimum (20% of the average systolic levels, P < 0.05). Aortic VE changes were in the opposite direction to the ventricle. Aortic VE continuously decreased throughout the systole; the last value was 20% lower than at the beginning of the systole (P < 0.05). Conversely, diastolic VE of the aorta took on greater values. This inverse time course between ventricle and aorta may reduce energy requirements for conveying blood. High cholesterol-fed rabbits did not reveal the inverse behavior of ventricular and aortic VE, e.g., aortic VE increased during the systole (119%, P < 0.05).

Accuracy of screening compression ultrasonography and clinical examination for the diagnosis of deep vein thrombosis after total hip or knee arthroplasty

OBJECTIVE

To determine whether compression ultrasonography or clinical examination should be considered as screening tests for the diagnosis of deep vein thrombosis (DVT) after total hip or knee arthroplasty in patients receiving warfarin prophylaxis postoperatively.

DESIGN

A prospective cohort study.

SETTING

A single tertiary care orthopedic centre.

PATIENTS

One hundred and eleven patients who underwent elective total hip or knee arthroplasty were enrolled. Postoperatively the warfarin dose was adjusted daily to maintain the international normalized ratio between 1.8 and 2.5. Eighty-six patients successfully completed the study protocol.

INTERVENTION

Before they were discharged from hospital, patients were assessed for DVT by clinical examination, bilateral compression ultrasonography of the proximal venous system and bilateral contrast venography.

RESULTS

DVT was found in 29 patients (34%; 95% confidence interval [CI] 24% to 45%), and 6 patients (7%; 95% CI 3% to 15%) had proximal DVT. DVT developed in 18 (40%) of 45 patients who underwent total knee arthroplasty and in 11 (27%) of 41 patients who underwent total hip arthroplasty. The sensitivity of compression ultrasonography for the diagnosis of proximal DVT was 83% (95% CI 36% to 99%) and the specificity was 98% (95% CI 91% to 99%). The positive predictive value of compression ultrasonography was 71%. In contrast, clinical examination for DVT had a sensitivity of 11% (95% CI 2% to 28%) and a positive predictive value of 25%.

CONCLUSIONS

DVT is a common complication after total hip or knee arthroplasty. Compression ultrasonography appears to be a relatively accurate noninvasive test for diagnosing postoperative proximal DVT. In contrast, clinical examination is a very insensitive test. Whether routine use of screening compression ultrasonography will reduce the morbidity of venous thromboembolism after joint arthroplasty requires confirmation in a prospective trial involving long-term follow-up of patients.

Ultrasonographic screening before hospital discharge for deep venous thrombosis after arthroplasty: the post-arthroplasty screening study. A randomized, controlled trial

BACKGROUND

The clinical significance of asymptomatic deep venous thrombosis that develops after joint arthroplasty and the value of screening tests to detect thrombi are uncertain.

OBJECTIVES

To determine 1) the rate of symptomatic deep venous thrombosis or pulmonary embolism occurring after hospitalization for joint arthroplasty and 2) the value of screening compression ultrasonography.

DESIGN

Double-blind, randomized, controlled trial.

SETTING

Tertiary care hospital.

PATIENTS

1024 patients undergoing elective total hip or knee arthroplasty who received warfarin prophylaxis.

INTERVENTION

Patients were randomly assigned to undergo either bilateral compression ultrasonography or a sham procedure before hospital discharge. Patients with a diagnosis of asymptomatic deep venous thrombosis were treated after discharge with standard anticoagulant therapy; other patients had warfarin therapy discontinued at discharge. All patients were followed for 90 days.

RESULTS

In the screening group, asymptomatic proximal deep venous thrombosis was detected in 13 of 518 patients (2.5%). Another 4 patients subsequently developed symptomatic proximal deep venous thrombosis, and 1 patient treated for asymptomatic deep venous thrombosis developed major bleeding, for a total outcome event rate of 1.0% (5 of 518 patients). In the placebo group, 3 patients developed symptomatic proximal deep venous thrombosis and 2 had nonfatal pulmonary embolism, for a total event rate of 1.0% (5 of 506 patients) (difference, 0 percentage points [95% CI, -1.2 to 1.2 percentage points]).

CONCLUSIONS

In patients undergoing total hip or knee arthroplasty, the use of warfarin prophylaxis during hospitalization results in a very low rate of symptomatic deep venous thrombosis or pulmonary embolism after hospital discharge. The use of screening compression ultrasonography at hospital discharge does not seem to be justified in this setting.

Ventricular volume elasticity: a component of cardiovascular control?

According to Koepchen, stimulation of different receptors within a complex network produces typical patterns of effector responses. We investigated whether the dynamical volume elasticity of the heart chamber is an independent component of cardiovascular control. The measurement of the dynamical volume elasticity was calculated by changes of the eigenfrequency of a modified catheter transducer system. The carotid baro- and chemoreflex affected end-diastolic pressure, contractility and calculated diastolic and systolic elasticity to different extents. In conclusion, cardiac volume elasticity is an independent element interacting with other parameters in cardiovascular control.

[Kidney function in the course of compensatory hypertrophy following unilateral nephrectomy in the Wistar rat]

Unilateral nephrectomy was performed in 109 male Wistar rats, 35 animals served as controls. Determinations of glomerular filtration rate (GFR) using slope clearance of 99mTc-DTPA and of tubular function using an orally water-loading test were made. The tests were performed 2 and 5 days as well as 2, 4, 5 and 6 weeks after nephrectomy. In all examinations the GFR was more than 50% of the level of two-kidney control animals. Fractional maximum diuresis was increased in the first 2 examinations after nephrectomy from 8.9 +/- 2.1 to 15.4 +/- 4.2% of the GFR and fractional clearance of osmotically free water from 6.8 +/- 1.9 to 10.9 +/- 3.5%. In the fourth and sixth week the proximal tubular reabsorption was increased and the glomerulo-tubular balance had been recovered. However, the minimal urine osmolarity was increased to 107.8 +/- 17.7 mmol/l and the osmotic load of the nephrons remained elevated. Possible clinical implications of the results are discussed.

Determination of the acid-base status in blood of patients on chronic haemodialysis and of bicarbonate in dialysate: comparison of three techniques and their mathematically derived values

The study deals with the comparison of acid-base parameters in blood of patients on chronic haemodialysis and of bicarbonate dialysate determined by Gas-Check AVL 945, equilibration technique (ET), and a titrimetric method. The results show that an acceptable agreement exists between AVL and ET with respect to measurements of pH, pCO2, HCO3-, and base excess. However, the values obtained for total buffer base related to the actual haemoglobin concentration are significantly lower (P less than 0.001) when determined by AVL. A titrimetric method is proposed for routine measurement of HCO3- in bicarbonate dialysate. Values obtained using this method are 3-4 mmol/l higher than those determined by AVL and ET. However, when the values for pK1' and for the solubility coefficient used in the Henderson-Hasselbalch equation are replaced by those for saline-bicarbonate solutions, results obtained using the titrimetric determined values agree well with those obtained by AVL and ET.

Erythrocyte sodium transport in normotensive and hypertensive patients on chronic hemodialysis, acute effect of treatment

Patients on chronic hemodialysis were divided into two groups: normotensive patients (group I) and renal hypertensive patients treated with antihypertensives (group II). The sodium and potassium contents in red blood cells ([Na+]i, [K+]i), ouabain-resistant net sodium uptake (ORNa+ uptake, phi Na), the relative ORNa+ uptake (k), the mean cell hemoglobin concentration (MCHC), and acid-base status were examined just before and after dialysis. The results indicate that in treated renal hypertensive patients k is stimulated and causes lower red blood cell sodium content. The reason for this increase remains obscure: the pattern of alterations of the sodium transport components during dialysis is similar in all patients: [Na+]i and phi Na increase significantly during dialysis, and the increases in [Na+]i are closely related to increases in pH and bicarbonate.

[Relation of glucose tolerance and intracellular sodium concentration]

In rabbits with standard diet, in cholesterolaemic rabbits, in pregnant women with physiological hyperinsulinism and in a control group with lower peripheral insulin concentrations after glucose load negative correlations were found between the intracellular sodium content and the KG-value of the intravenous glucose tolerance test. The hormonally regulated blood glucose regulating circle was a prerequisite for these relations, since in vitro a high sodium concentration in the erythrocytes was connected with a high glucose uptake rate. The findings in the total organism were explained by cell membrane properties which stabilise the level of the intracellular sodium concentration as well as are important for functions of the blood glucose regulating circle. As a link between the carbohydrate metabolism and the electrolyte metabolism the SH-groups of the cell membrane were discussed. This concept was used for a pathophysiological explanation of the higher erythrocytic sodium concentrations proved in manifest diabetics.

[Correlation of erythrocyte sodium concentration and the KG-value in variable peripheral insulin levels]

In pregnant women with physiological hyperinsulinism and in a control group with lower peripheral insulin concentrations after glucose load was investigated, whether differences in the peripheral insulin content involve different relations between the KG-value and the sodium concentration of the erythrocytes. Despite the differences in the peripheral insulin content during the intravenous glucose tolerance test negative correlations between the KG-value and the sodium concentration of the erythrocytes were found in the groups investigated. From this was concluded that these correlations are not based on changes of the peripheral insulin level, as they were established in the present examinations. A conception was presented issuing from the fact that the negative correlation between the KG-value and the sodium concentration is based on properties of the cell membrane which influence the intracellular sodium concentration as well as the effect of insulin in the cell.

The sublethal effects of a polychlorinated biphenyl (Aroclor 1254) diet on the Atlantic cod (Gadus morhua)

The effects of diet levels of 1, 5, 10, 25 and 50 micrograms Aroclor 1254/g on the Atlantic cod Gadus morhua were determined after a feeding period of 5 1/2 months. Altered steroid biosynthetic patterns in vitro were observed in the testes and head kidneys (adrenal homologue) of the fish that were fed various levels of PCB in vivo. Histological examination of tissues revealed abnormalities in the testes, gills, and livers of the PCB-fed fish. Various testicular abnormalities including slight-to-marked derangement of lobules, hyperplasia of lobule walls and disintegration and/or fatty necrosis of spermatogenic elements were observed in the testes of PCB-fed fish. Hyperplasia of the epithelial layer of the secondary lamellae was observed in the gills of fish on the 5 to 50 micrograms Aroclor 1254/g diet. Fatty degeneration was observed in the livers of all PCB-fed fish. The PCB content of testes, livers and head kidneys were directly proportional to the level of Aroclor 1254 in the diet. It is apparent that sublethal levels of PCB have a detrimental effect on the physiology of the Atlantic cod.

[Behavior of intracellular electrolytes in the diabetic metabolic state]

By animal experiments and examinations of test persons with healthy metabolism could be confirmed that a high intracellular content of sodium coincides with a low glucose assimilation coefficient. Moreover, the intracellular sodium concentration seems to have influence also on the peripheral insulin content after glucose load as it was shown by the negative correlation of the quotient Na(Plasma)/Na(Ery) with the insulin concentration surface in the rabbits. In maturity-onset diabetics with and without hypertension compared with a control group the intracellular sodium concentration was increased. This intracellular shift of electrolytes might have an influence of the utilisation of glucose in maturity-onset diabetes.

Open-heart surgery: somatic predictors of postoperative psychopathology

This study reports on some relationships between somatic conditions and postoperative psychopathological disturbances in 102 subjects from 20 to 65 years of age, undergoing open heart surgery. The overall incidence of postoperative psychiatric complications was 40%. Only two variables showed a significant correlation with the psychiatric criteria we used, independent of the different diagnoses: body weight and preoperative urea-N levels in serum. Some of the somatic predictors reported in the literature could be found to be specific for mitral or aortic valve disease. Predictors for psychiatric complications in patients with aortic valve replacement were: age, preoperative protein and urea-N levels in serum, and daytime sedation. The predictor in mitral valve replacement was the decrease in venous oxygenation (pulmonary artery) under physical strain.

Open-heart surgery from the psychological point of view and resulting therapeutic considerations

A total of 168 patients underwent thorough psychological investigation before and after open-heart surgery. The pre-operative psychological status and the psychosocial situation of the patients were studied by means of questionnaires and semi-standardized interviews. The post-operative psychic status was documented daily on the first four post-operative days and once after discharge from the intensive care unit, by means of a slightly modified version of the AMP rating scale. We found significant correlations between psychosocial problems, the patients' ability to cope with their illness, the surgical stress and the environmental stress of the intense care unit, and post-operative psychic disturbances. The results lead to deliberations on therapeutic consequences.

[Some psychological predictors for psychosis after open heart surgery (author's transl)]

In a study of postoperative psychosis after open heart surgery three psychopathological syndromes were identified which had different psychological predictors. Predictors of postoperative emotional disturbances are family problems and the lack of plans for the future, whereas patients with postoperative disorientations seem to have difficulties in their jobs and therefore feel distressed preoperatively. Predictors of the paranoid syndrome after the operation are: a high degree of fear in awaiting the operation and little confidence in the doctors. The social surroundings of these patients are often unstable and although they have no precise plans for the future they object to start working again after operation.

[Classification of psychopathological disorders after open-heart surgery (author's transl)]

A better discrimination of phenomenology and severeness of psychopathological disorders after open-heart surgery has been obtained: a) There is no equal distribution of degrees of severeness, a high correlation of severeness with duration of the disorders, a low correlation with age and no dependance of sex. b) Disorientation, clouding of consciousness and disorders of awareness arise earlier, delusions, hallucinations and paranoid ideas tentatively later after surgery. c) Three psychopathological syndromes have been identified. They appear in different frequency, intensity and can be combined.

[Some somatic and psychological predictors of psychopathological disorders after cardiac surgery (author's transl)]

Some new predictors of postoperative psychosis in open-heart surgery have been identified in a multifactorial study of somatic and psychological factors: Somatic predictors are lower body weight, higher blood levels of urea nitrogen and creatinine, preoperative use of tranquilizers, especially in patients with double valve replacement or congenital heart disease and preoperative cerebral embolism in mitral valve disease. Bypass-time has not proved to be a valid predictor. Psychological predictors are distress provoked by severity and duration of illness, repeated delay of surgery as well as the manner of psychological coping with the illness.