Measurement of renal blood flow by phase-contrast magnetic resonance imaging during septic acute kidney injury: a pilot investigation.

OBJECTIVE: In septic patients, decreased renal perfusion is considered to play a major role in the pathogenesis of acute kidney injury. However, the accurate measurement of renal blood flow in such patients is problematic and invasive. We sought to overcome such obstacles by measuring renal blood flow in septic patients with acute kidney injury using cine phase-contrast magnetic resonance imaging. DESIGN: Pilot observational study. SETTING: University-affiliated general adult intensive care unit. PATIENTS: Ten adult patients with established septic acute kidney injury and 11 normal volunteers. INTERVENTIONS: Cine phase-contrast magnetic resonance imaging measurement of renal blood flow and cardiac output. MEASUREMENTS AND MAIN RESULTS: The median age of the study patients was 62.5 yrs and eight were male. At the time of magnetic resonance imaging, eight patients were mechanically ventilated, nine were on continuous hemofiltration, and five required vasopressors. Cine phase-contrast magnetic resonance imaging examinations were carried out without complication. Median renal blood flow was 482 mL/min (range 335-1137) in septic acute kidney injury and 1260 mL/min (range 791-1750) in healthy controls (p = .003). Renal blood flow indexed to body surface area was 244 mL/min/m2 (range 165-662) in septic acute kidney injury and 525 mL/min/m2 (range 438-869) in controls (p = .004). In patients with septic acute kidney injury, median cardiac index was 3.5 L/min/m2 (range 1.6-8.7), and median renal fraction of cardiac output was only 7.1% (range 4.4-10.8). There was no rank correlation between renal blood flow index and creatinine clearance in patients with septic acute kidney injury (r = .26, p = .45). CONCLUSIONS: Cine phase-contrast magnetic resonance imaging can be used to noninvasively and safely assess renal perfusion during critical illness in man. Near-simultaneous accurate measurement of cardiac output enables organ blood flow to be assessed in the context of the global circulation. Renal blood flow seems consistently reduced as a fraction of cardiac output in established septic acute kidney injury. Cine phase-contrast magnetic resonance imaging may be a valuable tool to further investigate renal blood flow and the effects of therapies on renal blood flow in critical illness.

Ciné phase-contrast magnetic resonance imaging for the measurement of renal blood flow.

During critical illness, reductions in renal blood flow (RBF) are believed to be a major cause of kidney dysfunction, and therapy is often aimed at restoration of RBF. Despite this, our ability to measure RBF during critical illness has been limited by the invasiveness of the available techniques. Ciné Phase-Contrast Magnetic Resonance Imaging (CPC-MRI) represents an entirely noninvasive, contrast-free method of measuring blood flow with the potential of enabling the measurement of blood flow to major organs including the kidney. We have recently assessed the feasibility of measuring RBF by means of CPC-MRI in 2 critically ill patients with septic acute kidney injury and were able to compare such measurements to those obtained in a normal volunteer.

Assessment of suitability of thrombolysis in middle cerebral artery infarction: a proof of concept study of a stereologically-based technique.

BACKGROUND: The extent of cerebral ischemia, assessed by the Alberta Stroke Program Early CT Score (ASPECTS) method and unaided visual determination of the CT Summit Criterion, correlates with increased risk of intracerebral hemorrhage following rt-PA administration. Concerns about the accuracy of the unaided visual assessment in the estimation of infarct size and the conservative nature of the ASPECTS method led us to develop a new method (MCAGrid) based on stereological grid counting and a digital atlas of the middle cerebral artery (MCA) infarct territory. METHODS: We tested the hypotheses that the stereological method increases the accuracy of infarct estimation and that the number of patients deemed eligible for thrombolysis is greater with this method than with existing methods. Four experienced radiologists with extensive neuroradiological experience examined the CT images of 19 patients with MCA territory stroke and determined patient eligibility for thrombolysis by: unaided visual determination of the CT Summit Criterion, MCAGrid, and the ASPECTS score. The chi(2) test was used to compare the differences in the number of patients deemed 'eligible' for thrombolysis by the 3 imaging methods. Further, the unaided visual assessment and MCAGrid were compared with volumes calculated following manual segmentation of infarct, and the sensitivity, specificity and positive and negative likelihood ratios for these techniques were calculated. RESULTS: In general, MCAGrid was better than unaided visual assessment in the prediction of >1/3 involvement of the MCA territory by infarct. The number of patients considered as 'eligible' for thrombolysis based on imaging criteria was significantly lower when ASPECTS criteria (15/76) were used than when unaided visual determination of the CT Summit Criterion (32/76; p < 0.01) or MCAGrid (59/76; p < 0.001) criteria were used. CONCLUSION: The choice of methods for rating infarct extent affects the number of patients 'eligible' for thrombolysis significantly. Furthermore, MCAGrid increased the accuracy with which infarct extent was estimated. These results provide justification for a prospective study of this technique in the setting of acute stroke.

The ASPECTS template is weighted in favor of the striatocapsular region.

The extent of cerebral infarction correlates with increased risk of intracerebral hemorrhage (ICH) following recombinant tissue plasminogen activator (rt-PA) administration. The Alberta Stroke Program Early CT Score (ASPECTS) is a widely used, validated method which assesses involvement of 10 selected regions of the MCA territory. An ASPECTS score >7 is associated with a higher risk of ICH following thrombolysis than lower scores. To understand the internal structure of the ASPECTS template better, we estimated the infarct volume corresponding to each region. We hypothesized that, in the ASPECTS scoring system, the striatocapsular region is weighted disproportionally. Four experienced radiologists rated individual ASPECTS regions on subacute CT images (day 5-day 10) of 19 patients with MCA territory stroke. Infarct volume was determined from manual segmentation of infarcts on CT images. Linear regression was used to estimate the regional volume associated with each ASPECTS region. The ASPECTS regions are weighted unequally with the striatocapsular region accounting for 21% of the MCA territory infarct volume. Together, the 10 ASPECTS regions account for approximately 51% of the maximum MCA infarct territory volume. These findings should provide impetus for research to develop a scoring system explicitly based on regional hemorrhage risk as an aid to selecting patients for thrombolysis.

Hypertensive encephalopathy.

The case report of a patient with hypertensive encephalopathy is presented here. The findings of reversible bilateral thalamic changes on T2-weighted MR imaging is an atypical manifestation of the reversible posterior leucoencephalopathy syndrome (RPLS), which is the neuroradiological correlate of hypertensive encephalopathy. The importance of prompt and accurate diagnosis of hypertensive encephalopathy / RPLS is stressed.