Renal blood flow: measurement in vivo with rapid spiral MR imaging

PURPOSE: To evaluate the use of stent-grafts for the percutaneous closure of arteriovenous fistulas that develop after cardiac catheterization. MATERIALS AND METHODS: From January 1994 to November 1997, 14 arteriovenous fistulas in 13 patients (eight men, five women; age range, 46-65 years; mean age, 53.5 years) were treated. Eleven fistulas were situated between the deep femoral artery and the common femoral vein, and three fistulas were between the superficial femoral artery and the common femoral vein. All fistulas were closed with stent-grafts positioned in the artery at the level of the fistula. RESULTS: The percutaneous treatment of arteriovenous fistulas was successful in all cases. The findings at angiography performed after the procedure demonstrated the closure of the fistulas and the correct positioning of the prostheses; veins were no longer visible. One complication occurred--a partial thrombosis of the common femoral vein at the puncture site after manual compression. CONCLUSION: On the basis of the preliminary data, the authors believe that the percutaneous closure of arteriovenous fistulas with stent-grafts is a safe and effective alternative to conventional surgery.     

Validation of volume flow measurements with cine phase-contrast MR imaging for peripheral arterial waveforms

Shock wave lithotripsy (SWL) has made a revolution in the treatment of urolithiasis. Recent reports suggesting that stone features for SWL have changed during the last years have forced us to compare our initial and last 250 patients treated at our ESWL unit in terms of stone and patient characteristics. We found that the number of ureteric stones and small calyceal stones have increased significantly with time whereas the number of larger stones undergoing SWL has decreased significantly. We believe that this change in stone features is caused by the changing trends in the treatment of stone disease by incorporating other therapeutic options and modifying the SWL indications according to patient characteristics We also believe that prophylactic lithotripsy for asymptomatic calyceal stones still remains to be a debatable issue.     

Functional magnetic resonance imaging of regional cerebral blood oxygenation changes during breath holding

BACKGROUND and PURPOSE: Recently, noninvasive MRI methods have been developed that are now capable of detecting and mapping regional hemodynamic responses to various stress tests, which involve the use of vasoactive substances such as acetazolamide or inhalation of carbon dioxide. The aim of this study was to assess regional cerebral blood oxygenation changes during breath holding at 1.5 T. METHODS: In 6 healthy volunteers, T2*-weighted gradient echo images were acquired for a total dynamic scanning time of 10 minutes during alternating periods of breath holding and normal breathing at 40-second intervals after inspiration, at 30-second intervals after expiration, and at 18 seconds after expiration. To quantify the relative signal changes, 2.5-minute baseline image sampling with normal breathing was carried out. RESULTS: Repeated challenges of breath holding of various durations induced an overall rise in blood oxygen level-dependent (BOLD) signal intensities. In general, BOLD signal intensity increases were greatest in gray matter and nonsignificant in white matter. Depending on the breath-holding duration and techniques, BOLD signal intensity increases of all activated pixels varied from 0.8% to 3.5%. CONCLUSIONS: The present study demonstrates that cerebral blood oxygenation changes during breath holding can be detected by means of fMRI at 1.5 T. The breath-holding test, a short and noninvasive method to study cerebral hemodynamics with fMRI, could become a useful alternative to the acetazolamide or CO2 test.     

Coarctation of the aorta: collateral flow assessment with phase-contrast MR angiography

OBJECTIVE: The purpose of this report is to describe a new use of MR imaging in coarctation of the aorta. The specific question addressed was how well collateral blood flow in intercostal arteries, as determined by phase-contrast MR angiography, correlated with findings during surgery or catheterization in patients with coarctation of the aorta. CONCLUSION: Phase-contrast MR angiography is an excellent technique for detecting the presence or absence of collateral blood flow in the intercostal arteries of patients with coarctation of the aorta. Knowing whether collateral blood flow is present in patients with narrowing of the juxtaductal aorta should help assess the clinical hemodynamic significance of the coarctation.     

High-resolution cardiac PET in rabbits: imaging and quantitation of myocardial blood flow

A high-resolution PET system for small animals was tested for its applicability to the investigation of regional myocardial blood flow (MBF) in rabbits. METHODS: Nineteen measurements were performed in 10 closed-chest anesthetized rabbits at baseline and during infusions of adenosine (0.2 mg/kg/min) and propranolol (0.20-1.20 mg slow infusion) to obtain a wide range of MBF. Myocardial blood flow was assessed both by dynamic 13N-ammonia PET and by colored microspheres. Blood was withdrawn directly from the femoral artery, and arterial 13N activity was measured by coincidence type gamma detection system for the input function. Nitrogen-13 myocardial uptake was calculated by dividing the myocardial 13N activity by the integral value of the input function. RESULTS: Three or four contiguous cross-sectional myocardial images were obtained after 13N-ammonia injection. The left ventricular wall and cardiac cavity were clearly visualized. Moreover, initial passage of the tracer through the heart was obtained with serial 10-sec PET images. Nitrogen-13 myocardial uptake correlated well with flow measured with microspheres (r = 0.88). CONCLUSION: Our cardiac PET system can be used for in vivo imaging and quantitation of MBF in small animals and may play an important role in the future study of animal models of cardiovascular diseases.     

Skeletal muscle contraction: analysis with use of velocity distributions from phase-contrast MR imaging

PURPOSE: Velocity gradient data from phase-contrast magnetic resonance (MR) imaging were tested for the ability to calculate tensile strain and shear strain (deformation) during cyclical motion of skeletal muscle. MATERIALS AND METHODS: Strain data were derived from in vitro and in vivo phase-contrast MR velocity maps. A motion phantom designed to cyclically compress and expand a specimen of skeletal muscle provided a standard of reference to validate deformation, translation, and rotation measurements. The authors studied anterior and posterior muscle compartments of the lower extremity in three healthy volunteers during ankle dorsiflexion and plantar flexion against various resistances and the forearms of five healthy volunteers during flexion and extension of the fingers. RESULTS: The mean in vitro tracking error was 0.5 mm. The gastrocnemius muscle area in vivo changed 20% for both the minimum and maximum force conditions and therefore did not appear to be a good predictor of force. CONCLUSION: Phase-contrast MR imaging provides quantitative data on muscle contraction and demonstrates that shear and tensile strain can be measured and separated from translation and rotation of muscle.     

Gd-enhanced 3D phase-contrast MR angiography and dynamic perfusion imaging in the diagnosis of renal artery stenosis

The objective of this study was to investigate the role of contrast enhancement using a three-dimensional (3D) phase-contrast (PC) magnetic resonance (MR) sequence (3D PC-MRA) and to assess the value of a dynamic MR perfusion study of the kidneys to determine the hemodynamic relevance of unilateral renal artery stenosis (RAS). Seventeen patients with unilateral RAS were examined on a standard 1.0 T imaging system using a phase shift and magnitude sensitive 3D PC sequence (TR=160 ms, TE=9 ms, venc. 30 cm/s). Following the initial pre-contrast 3D PC-MRA a dynamic first pass perfusion study was performed using a Turbo-FLASH 2D sequence (TR=4.5 ms, TE=2.2 ms, TI=400 ms) after bolus injection of 0.15 mmol gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)/kg body weight. The 3D PC-MRA was then repeated during infusion of 0.15 mmol Gd-DTPA/kg body weight. Evaluation by three independent readers was based on maximum intensity projection images. Source images were rendered on request. Signal intensity (SI) over time curves of the renal cortex were obtained from the dynamic perfusion study and analyzed for maximum signal enhancement as well as temporal relationship to the aortic SI curve. Results from 3D PC-MRA revealed a sensitivity (pre-/post-contrast) of 100%/89%, specificity of 76%/63%, positive predictive value of 80%/69 %, negative predictive value of 90%/78%, and accuracy of 85%/75% (p=0.07). Interobserver agreement was kappa=0.61/kappa=0.47 (pre/post Gd-DTPA), respectively. Increased signal-to-noise was present in all segments of the renal arteries post contrast (p=0.0003). This came along with image degradation due to aliasing and elevated SI of venous flow that partially obscured the renal arteries. Dynamic SI curves showed a significantly decreased maximum SI in RAS (p=0.01-0.001). A temporal delay of cortical signal intensity enhancement could not be confirmed in this setting. Gd-enhanced 3D PC-MRA did not yield a superior diagnostic value in the diagnosis of RAS compared to pre-contrast measurements. Dynamic perfusion imaging of the kidneys, in combination with 3D PC-MRA, can contribute additional information in suspected unilateral RAS.     

Three-vessel study of cerebral blood flow using phase-contrast magnetic resonance imaging: effect of physical characteristics

The development of phase-contrast magnetic resonance imaging (P-C MRI) provides a noninvasive method for measurement of volumetric blood flow (VFR). We performed P-C MRI to study the effects of physical characteristics on cerebral blood flow. VFR of the left and right internal carotid arteries and basilar artery were measured using P-C MRI and total cerebral blood flow (tCBF) was calculated by summing up the VFR values in the three vessels. Moreover, we investigated the changes in these blood flows as influenced by age, head size, height, weight, body surface area, and handedness. The blood flows were 142 +/- 58 ml/min (mean +/- standard deviation) in the basilar artery; and 229 +/- 86 ml/min in the left, and 223 +/- 58 ml/min in the right internal carotid artery; and tCBF was 617 +/- 128 ml/min. Significant increases were observed in head size-related change of VFR in the basilar artery (p = .028) and height-related change of tCBF (p = .045). The other characteristics did not significantly influence any VFR. The results suggest that head size and height may reflect CBF, and that these effects should be considered when changes of CBF are diagnosed. Phase-contrast MRI is useful for a noninvasive and rapid analysis of cerebral VFR and has potential for clinical use.     

Alterations in pulmonary artery flow patterns and shear stress determined with three-dimensional phase-contrast magnetic resonance imaging in Fontan patients

OBJECTIVE: This study compares in vivo pulmonary blood flow patterns and shear stresses in patients with either the direct atrium-pulmonary artery connection or the bicaval tunnel connection of the Fontan procedure to those in normal volunteers. Comparisons were made with the use of three-dimensional phase contrast magnetic resonance imaging. METHODS: Three-dimensional velocities, flows, and pulmonary artery cross-sectional areas were measured in both pulmonary arteries of each subject. Axial, circumferential, and radial shear stresses were calculated with the use of velocities and estimates of viscosity. RESULTS: The axial velocities were not significantly different between subject groups. However, the flows and cross-sectional areas were higher in the normal group than in the two patient groups in both pulmonary arteries. The group with the bicaval connection had circular swirling in the cross section of both pulmonary arteries, causing higher shear stresses than in the controls. The disorder caused by the connection of the atrium to the pulmonary artery caused an increase in some shear stresses over the controls, but not higher than those found in the group having a bicaval tunnel. CONCLUSIONS: We found that pulmonary flow was equally reduced compared with normal flow in both patient groups. This reduction in flow can be attributed in part to the reduced size of the pulmonary arteries in both patient groups without change in axial velocity. We also found higher shear stress acting on the wall of the vessels in the patients having a bicaval tunnel, which may alter endothelial function and affect the longevity of the repair.     

Changes in blood flow velocity and diameter of the middle cerebral artery during hyperventilation: assessment with MR and transcranial Doppler sonography

PURPOSE: To compare blood flow velocity changes within the middle cerebral artery (MCA) during hyperventilation, as measured with by both transcranial Doppler sonography and MR imaging, with the diameter of the MCA as measured with MR imaging alone. METHODS: The studies were performed in six healthy volunteers ranging in age from 22 to 31 years (mean, 27 years). Transcranial Doppler sonography was carried out with a range-gated 2-MHz transducer. MR examinations were done on a 1.5-T imaging unit. MR angiography was performed using the time-of-flight technique. MR flow measurements were carried out by using the phase-mapping technique with an ECG-triggered phase-contrast sequence. RESULTS: During hyperventilation, the mean blood flow velocity of the proximal MCA declined by 49.6% +/- 5.7 (mean +/- standard deviation) as measured with Doppler sonography, and by 47% +/- 4.6 as measured with MR flow calculation. The diameter of the MCA (3.4 +/- 0.3 mm) remained unchanged on MR imaging studies (3.3 +/- 0.3 mm). CONCLUSION: We found a good correlation between relative flow velocity changes measured by transcranial Doppler sonography and MR techniques. MR imaging revealed no significant changes in the diameter of the proximal MCA during normal versus hyperventilation. Relative changes in flow velocity in the MCA would thereby reflect relative changes in cerebral blood flow, at least during hyperventilation.     

SPECT cerebral blood flow, MR imaging, and neuropsychological findings in traumatic head injury.

Investigated 16 patients with diffuse or contusional brain damage and 8 patients with focal lesions 5–22 mo postinjury, using single proton emission computed tomography (SPECT) cerebral blood flow (CBF) measurements and neuropsychological examination. All Ss were aged 16–64 yrs. Compared with 16 controls, the diffuse group showed significant differences on 13 of 24 measures after correction for premorbid differences, whereas the focal group was significantly impaired on only 3 tests after correction. SPECT apparently identified abnormalities not demonstrated on magnetic resonance (MR) imaging and vice versa. Abnormal regional CBF seemed to be related to neuropsychological defects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Renal artery stenosis and accessory renal arteries: accuracy of detection and visualization with gadolinium-enhanced breath-hold MR angiography

Extracellular volume (ECV) of arms, trunk, and legs determined from segmental bioimpedance data in 11 healthy men (31.6 +/- 7 yr) obtained at the end of a 30-min equilibration phase in the supine body position was compared with ECV determined from whole body measurements (ECVWB). ECV was calculated from extracellular resistance (RECV) identified from the bioimpedance spectrum for a range of 10 frequencies. Whole body RECV (527.6 +/- 55.6 Omega) was equal to the sum of RECV in the arms, trunk, and legs (241.6 +/- 36. 3, 49.2 +/- 5.1, and 236.3 +/- 25.5 Omega, respectively). The sum of equilibrated ECV in arms (1.31 +/- 0.25 liters), trunk (10.08 +/- 1.65 liters), and legs (2.80 +/- 0.82 liters) was smaller than ECVWB (20.90 +/- 2.59 liters). In six subjects who changed from a standing to a supine body position, ECV decreased in arms (-2.59 +/- 2.51%, P = NS) and legs (-10.96 +/- 3.02%, P < 0.05) but increased in the trunk (+4.2 +/- 3.2%, P < 0.05). ECVWB also decreased (-4.98 +/- 1. 41%, P < 0.05). However, the sum of segmental extracellular volumes remained unchanged (-0.06 +/- 0.07%, P = NS). The sum of segmental ECVs is not sensitive to changes in body position, which otherwise interferes with the estimation of ECV in bioimpedance analysis when ECVWB is used.     

Evaluation of subclavian steal with two-dimensional phase-contrast and two-dimensional time-of-flight MR angiography

We describe two MR angiographic methods of diagnosing subclavian steal in each of three patients. By using phase-directional information from a single two-dimensional phase-contrast sequence, we were able to show that the direction of flow in the affected vertebral artery was reversed. The same vertebral artery showed no signal on a 2-D time-of-flight sequence with a concatenated presaturation pulse applied above each section.     

MR first pass imaging: quantitative assessment of transmural perfusion and collateral flow

Recent advances with fast switching gradient coils, and the optimization of magnetic resonance techniques for multislice imaging have made it possible to apply models of contrast agent transit for the quantification of myocardial perfusion, and determination of the transmural distribution of blood flow. This article summarizes some of these recent developments and presents examples of quantitative, multi-slice myocardial perfusion imaging studies in patients and animal models. Multi-slice, true first pass imaging, with high temporal resolution, and T1-weighted, arrhythmia insensitive contrast enhancement is used for the quantification of perfusion changes accompanying mild to severe ischemia. The first pass imaging technique and the modeling approach are sufficiently robust for fitting of tissue residue curves corresponding to a wide, physiologically realistic range of myocardial blood flows. In animals this was validated by comparison to blood flow measurements with radiolabeled microspheres as gold standard. It is demonstrated that with the proposed modeling approach one can determine the myocardial perfusion reserve from two consecutive MR first pass measurements under resting and hyperemic conditions. In patients with microvascular dysfunction the MR studies show for the first time that the myocardial perfusion reserve correlates with Doppler flow measurements (linear regression with slope of 1.02 +/- 0.09; r = 0.80). Since perfusion limitations usually begin in the subendocardium as coronary flow is gradually reduced, first pass imaging with the prerequisitie spatial and temporal resolution allows early detection of a mild coronary stenosis.     

Renal ischemia-reperfusion injury: contribution of nitric oxide and renal blood flow

In nocturnal rodents, the c-fos gene is directly involved in the light mechanism of resetting of the suprachiasmatic nucleus (circadian clock). Light also induces c-fos expression in the retinal ganglion cell layer (GCL), but no attempt has been made to study the retinal responses to the phase-shifting effects of light. The expression of the Fos protein in each of the two populations of the GCL (displaced amacrine cells [DACs] and ganglion cells [GCs]) was analyzed in hamsters after light stimulation delivered early (circadian time [CT13]) and in the middle (CT18) of the subjective night. To evaluate as accurately as possible the number of GCs able to phase shift the locomotor activity rhythm (LAR), neonatal hamsters treated with monosodium glutamate (MSG) were also used, an in vivo model which displays retinal degeneration and LAR normally entrained by light. In nontreated hamsters, the number of Fos-immunoreactive (Fos-ir+) nuclei in the GCL was significantly higher at CT18 than at CT13. In MSG-treated hamsters, the number of Fos-ir+ nuclei was the same at both CTs and nonsignificantly different as those of nontreated hamsters at CT13. MSG treatment destroyed as many Fos-ir+ DACs as Fos-ir- DACs or Fos-ir+ GCs. Fos-ir+ GCs were less sensitive to neurotoxic than other GCs, as only 37% of them were destroyed by treatment versus 92% for Fos-ir- GCs. At CT18, a maximum of 3,500 GCs expressed Fos protein in nontreated hamsters versus only 2,200 in MSG-treated hamsters. This minor subgroup was sufficiently potent to normally synchronize the circadian rhythms to the Light/dark cycle in treated hamsters.     

Hepatic cavernous hemangioma: appearance on T2-weighted fast spin-echo MR imaging with and without fat suppression

OBJECTIVE: The goals of our study were to define the morphologic appearance of cavernous hemangioma of the liver on T2-weighted fast spin-echo MR imaging and to determine if the use of fat suppression may quantitatively and qualitatively modify the MR imaging appearance of cavernous hemangioma. SUBJECTS AND METHODS: Twenty-six patients with cavernous hemangiomas of the liver were prospectively studied with T2-weighted MR imaging with a fast spin-echo technique with and without fat suppression. Thirteen patients had known hemangiomas for more than 2 years, with no change in size or morphology during this period. The remaining 13 patients had diagnoses based on dynamic CT and sonography and an absence of change in the morphology and size of their lesions during follow-up of more than 6 months (range, 6-12 months) after the MR imaging studies. Values for signal intensity and contrast-to-noise (C/N) ratios in cavernous hemangiomas that were obtained with and without fat suppression were compared. Images were qualitatively analyzed separately at identical level and window settings by two interpreters for morphologic features of cavernous hemangiomas. RESULTS: No significant difference was found between signal intensity values obtained using the fat-suppressed fast spin-echo MR imaging technique (5.62 +/- 1.14 [SD]) and those obtained without fat suppression (5.51 +/- 1.23). Values for C/N ratios obtained with the fat-suppressed fast spin-echo MR imaging technique (20.13 +/- 7.63) were significantly superior to those obtained without fat suppression (16.59 +/- 5.31) (p < .001). On T2-weighted fast spin-echo MR imaging without fat suppression, 100% of cavernous hemangiomas were hyperintense relative to the spleen, 90% had well-defined and sharp margins, 55% were isointense to CSF, and 76% were homogeneous. Without fat suppression, 34% of cavernous hemangiomas showed the combination of isointensity to CSF, well-defined margins, and homogeneity. On T2-weighted fast spin-echo MR imaging with fat suppression, all cavernous hemangiomas showed this same combination of features. CONCLUSION: Seventy-six percent of hepatic cavernous hemangiomas were homogeneous on T2-weighted fast spin-echo MR imaging, and 55% were isointense to CSF. However, only 34% of hepatic cavernous hemangiomas showed typical features. Although fat suppression significantly increased the C/N ratio of cavernous hemangiomas of the liver, fat suppression did not affect their morphologic appearance on T2-weighted fast spin-echo MR imaging.     

Power Doppler imaging of carotid artery stenosis. Comparison with color Doppler flow imaging and angiography

BACKGROUND AND PURPOSE: Power Doppler imaging (PDI) is a new sonographic technique that has recently been introduced for vascular application. Since the technical principles of PDI may provide increased sensitivity to visualize the continuity of blood flow in arterial stenoses, we investigated the diagnostic significance of PDI and the intermethod relationship for the measurement and classification of internal carotid artery (ICA) stenosis in comparison with both color Doppler flow imaging (CDFI) and angiography. METHODS: One hundred patients with a total of 128 ICA stenoses (50% to 69%, n = 37; 70% to 79%, n = 27; 80% to 99%, n = 64) and 12 ICA occlusions were consecutively investigated by means of PDI, CDFI, and intra-arterial angiography (n = 48). Reduction of the intrastenotic lumen was measured on longitudinal and transverse views of PDI and CDFI for the calculation of the degree of diameter and area stenosis, respectively. Angiographic stenosis was determined with the use of the North American Symptomatic Carotid Endarterectomy Trial (NASCET), European Carotid Surgery Trial (ECST), and common carotid (CC) methods. RESULTS: PDI provided significantly more excellent or good (92% versus 79%; P < .01) displays of the intrastenotic lumen than CDFI, particularly in complicated high-grade stenosis. While linear regression analysis demonstrated a high overall correlation between PDI and CDFI for diameter (r = .88; P < .001) and area stenosis (r = .79; P < .001), categorization of ICA stenosis revealed best agreement for 80% to 99% area stenoses. Since angiography frequently either underclassified (NASCET method) or overclassified (ECST, CC methods) the degree of ICA stenosis in comparison to both PDI and CDFI, the sonographic-angiographic correlation was only moderate (regression coefficients ranged from .62 to .70; P < .001). CONCLUSIONS: PDI further improves the assessment of ICA stenosis by providing better visualization of the stenotic vascular lumen than CDFI. Sonographic imaging of the stenotic plaque on both PDI and CDFI provided a direct measurement of the local degree of stenosis, while the angiographic grade of stenosis essentially depended on the method used for evaluation.