Assessment of hepatic perfusion parameters with dynamic MRI

Evaluation of quantitative portal venous, hepatic arterial, and total hepatic tissue blood flow using xenon CT in alcoholic liver cirrhosis: comparison with liver cirrhosis C

BACKGROUND/AIMS

Xenon computed tomography (Xe-CT) is a noninvasive method of quantifying and visualizing tissue blood flow (TBF). For the liver, Xe-CT allows separate measurement of hepatic arterial and portal venous TBF. The present study evaluated the usefulness of Xe-CT as a noninvasive diagnostic procedure for measuring hepatic TBF in alcoholic liver cirrhosis (AL-LC), compared with liver cirrhosis C (C-LC).

METHODS

Xenon computed tomography was performed on 12 patients with AL-LC and 17 patients with C-LC. The severity of LC was classified according to Child-Pugh classification. Correlations between hepatic TBF and Child-Pugh classification were examined. Correlations of hepatic TBF in Child-Pugh class A to C-LC and AL-LC were also examined.

RESULTS

The mean portal venous TBF (PVTBF) was significantly lower in AL-LC than in C-LC (p=0.0316). Similarly, the mean total hepatic TBF (THTBF) was significantly lower in AL-LC than in C-LC (p=0.0390). PVTBF displayed a significant negative correlation with Child-Pugh score (r=-0.396, p=0.0368).

CONCLUSIONS

Measurement of hepatic TBF using Xe-CT is useful as a noninvasive, objective method of assessing the state of the liver in chronic liver disease.

Perfusion-weighted MRI in evaluating the intranodular hemodynamic characteristics of dysplastic nodules and hepatocellular carcinomas in an experimental rat model

PURPOSE

To investigate the value of perfusion-weighted MRI in the evaluation of the intranodular hemodynamic characteristics of dysplastic nodules (DNs) and hepatocellular carcinomas (HCCs) in an experimental rat model.

MATERIALS AND METHODS

A total of 40 rats with chemically-induced DNs and HCCs were investigated. Single-slice gadolinium-enhanced perfusion-weighted MRI was performed to evaluate the nodules. Time to peak (Tp), maximal relative signal enhancement (REmax), and the initial slope of signal intensity (SI) vs. time curves of the nodules and cirrhotic liver were evaluated. Nodules precisely corresponding to MRI were examined histologically. Paired Student's t-tests were used to compare the difference between nodules and cirrhotic liver.

RESULTS

A total of 20 HCCs and 14 DNs were evaluated. HCCs showed a significantly higher REmax, shorter Tp, and higher slope than adjacent cirrhotic liver. The REmax and slope of DNs were significantly lower than adjacent cirrhotic liver parenchyma. Although the Tp of DNs was delayed two to three seconds compared to adjacent cirrhotic liver, there was no significant difference between them.

CONCLUSION

Perfusion-weighted MRI detected the intranodular hemodynamic characteristics of DNs and HCCs in an experimental rat model. DNs were hypovascular compared to cirrhotic liver, while HCCs were markedly hypervascular.

Transcatheter intraarterial perfusion: MR monitoring of chemoembolization for hepatocellular carcinoma--feasibility of initial clinical translation

PURPOSE

To prospectively test the hypothesis that intraprocedural transcatheter intraarterial perfusion (TRIP) magnetic resonance (MR) imaging can be used to successfully measure reductions in perfusion to the targeted hepatocellular carcinoma (HCC) and the adjacent surrounding liver tissue during MR-interventional radiology (IR)-monitored transcatheter arterial chemoembolization (TACE).

MATERIALS AND METHODS

This HIPAA-compliant prospective study was approved by the institutional review board. An MR-IR unit was used to perform TACE in 10 patients with HCC (seven male, three female; eight younger than 69 years, two older than 69 years). Intraprocedural reductions in tumor perfusion before and after TACE were monitored with TRIP MR imaging. Time-signal intensity curves were derived, and semiquantitative spatially resolved area under the time-signal intensity curve maps of tumor perfusion before and after TACE were produced. Mean perfusion values before and after TACE for liver tumors and adjacent liver tissue were compared by using a mixed-model analysis, with alpha = .05.

RESULTS

Perfusion reductions were measured successfully with TRIP MR imaging in 18 separate tumors during 13 treatment sessions. Perfusion maps showed significant perfusion reductions for tumors (P < .013) but not for adjacent nontumorous liver tissue (P = .21). For tumors, the mean perfusion value was 193 arbitrary units (AU) +/- 223 (standard deviation) before TACE and 45.3 AU +/- 91.9 after TACE, with a mean reduction in baseline perfusion of 74.6% +/- 24.8. For adjacent liver tissue, the mean perfusion value was 124 AU +/- 93.5 before TACE and 93.2 AU +/- 72.3 after TACE, with a mean reduction in baseline perfusion of 24.2% +/- 14.5.

CONCLUSION

TRIP MR imaging can be used to detect intraprocedural changes in perfusion to HCC and surrounding liver parenchyma during MR-IR-monitored TACE.

Pancreatic perfusion: noninvasive quantitative assessment with dynamic contrast-enhanced MR imaging without and with secretin stimulation in healthy volunteers--initial results

PURPOSE

To prospectively quantify pancreatic regional perfusion with dynamic contrast material-enhanced magnetic resonance (MR) imaging by using a one-compartment model and to assess perfusion changes during secretin stimulation in healthy volunteers.

MATERIALS AND METHODS

The study had institutional review board approval, and written informed consent was obtained. Ten healthy volunteers (five men, five women; mean age, 24.7 years +/- 1.9 [standard deviation]; range, 22-29 years) underwent MR imaging pancreatic perfusion studies performed twice without secretin and twice during secretin stimulation. Dynamic contrast-enhanced MR imaging consisted of saturation-recovery T1-weighted turbo-field-echo imaging with peripheral pulse triggering and respiratory tracking. A dose of 0.05 mmol gadodiamide per kilogram of body weight was injected at a rate of 3.5 mL/sec. Regional perfusion parameters were fitted with a one-compartment model. The analysis of variance test for repeated measurements was used to assess differences in pancreatic perfusion without and that with secretin administration.

RESULTS

Significant differences in perfusion parameters between the three pancreatic regions were observed (P < .05). During secretin stimulation, a significant difference was observed only between the body and the tail of the pancreas (P = .02). A significant increase (P = .003) in pancreatic perfusion was observed after secretin administration. Mean pancreatic perfusion was 184 mL/min/100 g of tissue +/- 71, 207 mL/min/100 g +/- 77, and 230 mL/min/100 g +/- 87 without secretin and 342 mL/min/100 g +/- 154, 338 mL/min/100 g +/- 156, and 373 mL/min/100 g +/- 176 after secretin stimulation in the head, body, and tail of the pancreas, respectively. Intraindividual variability was 21% without secretin stimulation and 46% with secretin stimulation.

CONCLUSION

Dynamic contrast-enhanced MR imaging enables noninvasive quantification of regional pancreatic perfusion in resting conditions and demonstrates the increase in pancreatic perfusion during secretin stimulation in healthy subjects.

Advanced liver fibrosis: diagnosis with 3D whole-liver perfusion MR imaging--initial experience

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. The purpose of this study was to prospectively evaluate sensitivity and specificity of various estimated perfusion parameters at three-dimensional (3D) perfusion magnetic resonance (MR) imaging of the liver in the diagnosis of advanced liver fibrosis (stage >or= 3), with histologic analysis, liver function tests, or MR imaging as the reference standard. Whole-liver 3D perfusion MR imaging was performed in 27 patients (17 men, 10 women; mean age, 55 years) after dynamic injection of 8-10 mL of gadopentetate dimeglumine. The following estimated perfusion parameters were measured with a dual-input single-compartment model: absolute arterial blood flow (F(a)), absolute portal venous blood flow (F(p)), absolute total liver blood flow (F(t)) (F(t) = F(a) + F(p)), arterial fraction (ART), portal venous fraction (PV), distribution volume (DV), and mean transit time (MTT) of gadopentetate dimeglumine. Patients were assigned to two groups (those with fibrosis stage <or= 2 and those with fibrosis stage >or= 3), and the nonparametric Mann-Whitney test was used to compare F(a), F(p), F(t), ART, PV, DV, and MTT between groups. Receiver operating characteristic curve analysis was used to assess the utility of perfusion estimates as predictors of advanced liver fibrosis. There were significant differences for all perfusion MR imaging-estimated parameters except F(p) and F(t). There was an increase in F(a), ART, DV, and MTT and a decrease in PV in patients with advanced fibrosis compared with those without advanced fibrosis. DV had the best performance, with an area under the receiver operating characteristic curve of 0.824, a sensitivity of 76.9% (95% confidence interval: 46.2%, 94.7%), and a specificity of 78.5% (95% confidence interval: 49.2%, 95.1%) in the prediction of advanced fibrosis.

Comparison of transcatheter intraarterial perfusion MR imaging and fluorescent microsphere perfusion measurements during transcatheter arterial embolization of rabbit liver tumors

PURPOSE

Transcatheter intraarterial perfusion (TRIP) magnetic resonance (MR) imaging is clinically used in the interventional MR imaging setting to verify distribution of injected embolic or chemoembolic material during liver-directed transcatheter therapies and to monitor reductions in perfusion. The accuracy of this technique remains unknown. In the present study, rabbit VX2 liver tumors were used to test the hypothesis that TRIP MR imaging accurately measures changes in tumor perfusion during transcatheter arterial embolization (TAE), with injection of fluorescent microspheres used as the gold-standard technique.

MATERIALS AND METHODS

Five New Zealand White rabbits were used for this study (two donor rabbits and three with VX2 liver tumors). In three rabbits with implanted VX2 liver tumors, catheters were superselectively placed under digital subtraction angiographic guidance into the left hepatic artery supplying the targeted tumor. Fluorescent microspheres were injected into each rabbit's left ventricle before and after TAE. TRIP MR images were obtained at baseline and after embolizations for all rabbits with intraarterial injections of 2.5% gadopentetate dimeglumine solution. Linear regression was used to compare relative reductions in tumor perfusion between TRIP MR imaging and fluorescent microspheres. Results were considered statistically significant at a P value less than .05.

RESULTS

There was good correlation between TRIP MR imaging and fluorescent microsphere measurements of reduction in tumor perfusion (r = 0.722, P < .012).

CONCLUSIONS

TRIP MR imaging provides accurate semiquantitative measurement of perfusion reduction during TAE in rabbit liver tumors.

A comparison of chemoembolization endpoints using angiographic versus transcatheter intraarterial perfusion/MR imaging monitoring

PURPOSE

Transcatheter arterial chemoembolization (TACE) is an established treatment for unresectable liver cancer. This study was conducted to test the hypothesis that angiographic endpoints during TACE are measurable and reproducible by comparing subjective angiographic versus objective magnetic resonance (MR) endpoints of TACE.

MATERIALS AND METHODS

The study included 12 consecutive patients who presented for TACE for surgically unresectable HCC or progressive hepatic metastases despite chemotherapy. All procedures were performed with a dedicated imaging system. Angiographic series before and after TACE were reviewed independently by three board-certified interventional radiologists. A subjective angiographic chemoembolization endpoint (SACE) classification scheme, modified from an established angiographic grading system in the cardiology literature, was designed to assist in reproducibly classifying angiographic endpoints. Reproducibility in SACE classification level was compared among operators, and MR imaging perfusion reduction was compared with SACE levels for each observer.

RESULTS

Twelve patients successfully underwent 15 separate TACE sessions. SACE levels ranged from I through IV. There was moderate agreement in SACE classification (kappa = 0.46 +/- 0.12). There was no correlation between SACE level and MR perfusion reduction (r = 0.16 for one operator and 0.02 for the other two).

CONCLUSIONS

Angiographic endpoints during TACE vary widely, have moderate reproducibility among operators, and do not correlate with functional MR imaging perfusion endpoints. Future research should aim to determine ideal angiographic and functional MR imaging endpoints for TACE according to outcome measures such as imaging response, pathologic response, and survival.

[An experimental organ model for magnetic resonance imaging]

The investigation of reperfusion of organs after an ischaemic phase is of great interest in transplantation medicine. This work presents an experimental organ model for the examination of isolated canine livers by means of magnetic resonance imaging in reperfusion experiments. The perfusion of the organs inside a perfusate container in the scanner's head coil was performed with approximately physiological conditions for 1.5 hours. The pumps for the perfusate circulation were installed outside the scanner's room, where oxygenation and heating of the perfusate was also performed. In the MR examinations, T1- and T2-weighted sequences could be used to display the morphology of the organs and the spatial perfusion distribution after administration of contrast media. The images revealed oedema development during reperfusion and an inhomogenous perfusion distribution. Thus, the model allows for the non-invasive investigation of morphology and perfusion distribution in isolated reperfused organs.

Liver tumors: monitoring embolization in rabbits with VX2 tumors--transcatheter intraarterial first-pass perfusion MR imaging

PURPOSE

To prospectively test the hypothesis that transcatheter intraarterial first-pass perfusion (TRIP) magnetic resonance (MR) imaging can depict serial reductions in rabbit liver tumor perfusion during transcatheter arterial embolization (TAE).

MATERIALS AND METHODS

All experiments had institutional animal care and use committee approval. In four rabbits implanted with eight VX2 liver tumors, catheters were positioned in the hepatic arteries with conventional angiographic guidance. After transfer to the MR imaging suite, serial TAE was performed, with approximately 0.5 million 40-120-microm embolic particles injected at each embolic stage. TRIP MR imaging was performed at baseline and after each subsequent embolic stage (10 minutes between stages). Serial TAE and TRIP MR imaging were repeated until stasis. The first-pass time course of signal enhancement was measured in both tumors and hepatic arteries. Tumor area under the curve (AUC) and maximum upslope (MUS) values, each normalized by arterial input, were measured to assess iterative perfusion reduction. Perfusion measurements across TAE stages were compared with paired t tests and linear regression.

RESULTS

AUC decreased from a pre-TAE baseline of 0.408 (95% confidence interval [CI]: 0.330, 0.486) to 0.065 (95% CI: 0.046, 0.085) (P<.001) after TAE. MUS decreased from a pre-TAE baseline of 0.151 (95% CI: 0.121, 0.181) to 0.027 (95% CI: 0.022, 0.031) (P<.001) after TAE. Reductions to AUC and MUS after each embolic stage were statistically significant (P<.006 for each group of paired comparisons). AUC strongly correlated with MUS (r=0.966, P<.001).

CONCLUSION

TRIP MR imaging can depict serial reductions in liver tumor perfusion during TAE. TRIP MR imaging offers the potential to target functional embolic end points during TAE.

Diffusion-weighted magnetic resonance imaging for the assessment of fibrosis in chronic hepatitis C

Liver biopsy is the gold standard for assessing fibrosis but has several limitations. We evaluated a noninvasive method, so-called diffusion-weighted magnetic resonance imaging (DWMRI), which measures the apparent diffusion coefficient (ADC) of water, for the diagnosis of liver fibrosis in patients with chronic hepatitis C virus (HCV). We analyzed 20 healthy volunteers and 54 patients with chronic HCV (METAVIR: F0, n = 1; F1, n = 30; F2, n = 8; F3, n = 5; and F4, n = 10) prospectively included. Patients with moderate-to-severe fibrosis (F2-F3-F4) had hepatic ADC values lower than those without or with mild fibrosis (F0-F1; mean: 1.10 +/- 0.11 versus 1.30 +/- 0.12 x 10(-3) mm2/s) and healthy volunteers (mean: 1.44 +/- 0.02 x 10(-3) mm2/s). In discriminating patients staged F3-F4, the areas under the receiving operating characteristic curves (AUCs) were 0.92 (+/-0.04) for magnetic resonance imaging (MRI), 0.92 (+/-0.05) for elastography, 0.79 (+/-0.08) for FibroTest, 0.87 (+/-0.06) for the aspartate aminotransferase to platelets ratio index (APRI), 0.86 (+/-0.06) for the Forns index, and 0.87 (+/-0.06) for hyaluronate. In these patients, the sensitivity, specificity, positive predictive value, and negative predictive value were 87%, 87%, 72%, and 94%, respectively, with an ADC cutoff level of 1.21 x 10(-3) mm2/s. In discriminating patients staged F2-F3-F4, the AUC values were 0.79 (+/-0.07) for MRI, 0.87 (+/-0.05) for elastography, 0.68 (+/-0.09) for FibroTest, 0.81 (+/-0.06) for APRI, 0.72 (+/-0.08) for the Forns index, and 0.77 (+/-0.06) for hyaluronate.

CONCLUSION

This preliminary study suggests that DWMRI compares favorably with other noninvasive tests for the presence of significant liver fibrosis.

New method for 3D parametric visualization of contrast-enhanced pulmonary perfusion MRI data

Three-dimensional (3D) dynamic contrast-enhanced magnetic resonance imaging (3D DCE-MRI) has been proposed for the assessment of regional perfusion. The aim of this work was the implementation of an algorithm for a 3D parametric visualization of lung perfusion using different cutting planes and volume rendering. Our implementation was based on 3D DCE-MRI data of the lungs of five patients and five healthy volunteers. Using the indicator dilution theory, the regional perfusion parameters, tissue blood flow, blood volume and mean transit time were calculated. Due to the required temporal resolution, the volume elements of dynamic MR data sets show a reduced spatial resolution in the z-direction. Therefore, perfusion parameter volumes were interpolated. Linear interpolation and a combination of linear and nearest-neighbor interpolation were evaluated. Additionally, ray tracing was applied for 3D visualization. The linear interpolation algorithm caused interpolation errors at the lung borders. Using the combined interpolation, visualization of perfusion information in arbitrary cutting planes and in 3D using volume rendering was possible. This facilitated the localization of perfusion deficits compared with the coronal orientated source data. The 3D visualization of perfusion parameters using a combined interpolation algorithm is feasible. Further studies are required to evaluate the additional benefit from the 3D visualization.

Quantitative determination of Gd-DTPA concentration in T1-weighted MR renography studies

A method for calculating contrast agent concentration from MR signal intensity (SI) was developed and validated for T(1)-weighted MR renography (MRR) studies. This method is based on reference measurements of SI and relaxation time T(1) in a Gd-DTPA-doped water phantom. The same form of SI vs. T(1) dependence was observed in human tissues. Contrast concentrations calculated by the proposed method showed no bias between 0 and 1 mM, and agreed better with the reference values derived from direct T(1) measurements than the concentrations calculated using the relative signal method. Phantom-based conversion was used to determine the contrast concentrations in kidney tissues of nine patients who underwent dynamic Gd-DTPA-enhanced 3D MRR at 1.5T and (99m)Tc-DTPA radionuclide renography (RR). The concentrations of both contrast agents were found to be close in magnitude and showed similar uptake and washout behavior. As shown by Monte Carlo simulations, errors in concentration due to SI noise were below 10% for SNR = 20, while a 10% error in precontrast T(1) values resulted in a 12-17% error for concentrations between 0.1 and 1 mM. The proposed method is expected to be particularly useful for assessing regions with highly concentrated contrast.

Dopamine agonist cabergoline reduces hemoconcentration and ascites in hyperstimulated women undergoing assisted reproduction

CONTEXT

Ovarian hyperstimulation syndrome (OHSS) results from increased vascular permeability (VP) caused by ovarian hypersecretion of vascular endothelial growth factor (VEGF), which activates its receptor-2. In animals, the dopamine receptor 2 agonist cabergoline (Cb2) inactivates VEGF receptor-2 and prevents increased VP.

OBJECTIVE

Our objective was to test whether Cb2 reduces VP and prevents OHSS in humans.

DESIGN

We conducted a prospective, randomized, double-blind study on oocyte donors at risk of developing OHSS (>20 follicles, >12 mm developed, and >20 oocytes retrieved).

INTERVENTIONS

Cb2 0.5 mg/d (n = 37) or a placebo (n = 32) was administered from the day of human chorionic gonadotropin (d 0) until d 8. Ascites (a pocket of peritoneal fluid > 9 cm(2) in lithotomy position), hemoconcentration, and serum prolactin were recorded. Pharmacokinetic studies with magnetic resonance employing the transfer constant rate (K(trans), measure of permeability) and the extravascular extracellular space (upsilon(e), marker of cellular leakage) were performed to measure VP objectively.

RESULTS

Hematocrit (P < 0.01), hemoglobin (P = 0.003), and ascites (P = 0.005) were significantly lower on d 4 and 6 after treatment with Cb2 as compared with placebo. The incidence of moderate OHSS was 20.0 and 43.8%, respectively (P = 0.04). Magnetic resonance studies showed an increase in VP and extravascular leakage of fluid 5 d after human chorionic gonadotropin injection that was significantly prevented with Cb2 (K(trans) P = 0.04 and upsilon(e) P = 0.001, respectively).

CONCLUSIONS

Given that Cb2 is a well-established and safe medication, this study provides proof of concept for the use of dopamine agonists in the prevention of OHSS in women undergoing assisted reproduction.

Comparison of liver perfusion parameters studied with conventional extravascular and experimental intravascular CT contrast agents

RATIONALE AND OBJECTIVES

To compare liver perfusion parameters obtained by using an extravascular contrast agent and a blood-pool agent.

MATERIALS AND METHODS

Fifteen rabbits were imaged with a continuous 40-second single-slice computed tomography acquisition after a bolus injection of contrast agent (physiologic bolus duration 4-5 seconds, extravascular iohexol, n = 7; experimental nanoparticulated blood-pool agent WIN8883, n = 8). Time-density curves were generated for the aorta, portal vein, and liver. From the curves, arterial, portal, and total blood flows and hepatic perfusion index (HPI, arterial-to-total perfusion ratio) were determined by using two commonly applied fundamentally different analyzing methods: the single-compartment model and the peak gradient (PG) method. Also, the gamma variate fitting method was used.

RESULTS

By using the single-compartment model, the obtained HPI and total blood flow were 0.14 +/- 0.04 and 2.29 +/- 0.40 (mL/min/mL(tissue)) for WIN8883, and 0.15 +/- 0.06 (P = .54) and 4.60 +/- 1.14 (mL/min/mL(tissue)) (P = .0002) for iohexol, respectively. With the PG, HPI and total blood flow were 0.15 +/- 0.08 and 1.27 +/- 0.24 (mL/min/mL(tissue)) for WIN8883, and 0.20 +/- 0.06 (P = .12) and 2.11 +/- 0.25 (mL/min/mL(tissue)) (P = .00002) for iohexol, respectively. With the blood pool agent, similar contrast enhancement to the conventional agent was achieved with about 36% reduced dosage of iodine per body weight (mg I/kg).

CONCLUSIONS

HPI was found to be quite insensitive to different contrast agent types and analyzing methods. However, the arterial, portal and total liver blood flow values strongly depend on contrast agent type and modeling method.

Dynamic MRI signals in the second week of radiotherapy relate to treatment outcomes of hepatocellular carcinoma: a preliminary result

AIM

Radiotherapy (RT) has been used to treat hepatocellular carcinoma (HCC) in recent years. Despite its good local control, slow tumoral shrinkage and rapid recurrence compromise treatment outcomes. We evaluated the signal intensity of the hepatic parenchyma and tumours by using dynamic contrast enhanced magnetic resonance imaging (MRI) and correlated the findings with clinical outcomes. Nineteen patients with advanced HCC received 50 Gy in 25 fractions. They underwent a dynamic contrast-enhanced, turbo fast low-angle shot MR sequence at 1.5 T before therapy, at 2 weeks of therapy, and 1 month (week 9) later. Initial first-pass enhancement slopes (slope) and peak enhancement ratios (peak) were measured.

RESULTS

Initial signal intensities were not associated with RT outcomes. An increased slope and peak of the tumour at week 2 was associated with an improved local response (P<0.05). In the parenchyma, an increased slope at week 2 was associated with recurrence outside the radiation fields or with progression over distant sites (P<0.05). The differences in signal changes at week 2 during RT were not persistent at a statistically significant level at 1 month after RT.

CONCLUSION

Dynamic contrast-enhanced MRI signals may act as biomarkers for early prediction of responses to RT in patients with HCC. Signal intensities at week 2 are important in evaluating treatment outcomes.

An efficient method for calculating kinetic parameters in a dual-input single-compartment model

Quantitative measurement of hepatic perfusion has the potential to provide important information in the assessment and management of various liver diseases. The utility of hepatic perfusion characterization relies on the resolution of each component of its dual blood supply, i.e. the hepatic artery and portal vein. In this study, a linear equation was derived by integrating the differential equation describing the kinetic behaviour of contrast agent (CA) in a dual-input single-compartment model, from which the kinetic parameters can be easily obtained using the linear least-squares method. The usefulness of this method was investigated using computer simulations, in comparison with the non-linear least-squares (NLSQ) method. This method calculated the kinetic parameters faster than the NLSQ method by a factor of approximately 10, with almost the same accuracy as the NLSQ method. This method will be useful for analysing the kinetic behaviour of CA in the unique liver environment, especially by generating the functional images of kinetic parameters.

The prediction of radiation-induced liver dysfunction using a local dose and regional venous perfusion model

We have shown that high dose conformal radiation combined with chemotherapy appears to prolong the survival of patients with unresectable intrahepatic cancers. The ability to safely deliver higher doses is primarily limited by the development of radiation-induced liver disease, characterized by venous occlusion. In this study, we investigated whether portal venous perfusion measured prior to the end of radiation therapy (RT) together with dose could predict liver venous perfusion dysfunction after treatment. Ten patients with unresectable intrahepatic cancer participated in an IRB-approved computer tomography (CT) perfusion study. Hepatic arterial and portal vein perfusion distributions were estimated by using dynamic contrast enhanced CT and the single compartmental model. Scans were obtained at four time points: prior to treatment, after 15 and 30 fractions of 1.5 Gy treatments, and one month following the completion of RT. Multivariant linear regression was used to determine covariances among the first three time point measurements plus dose for prediction of the post RT measurement. The reduction in the regional venous perfusion one month following RT was predicted by the local accumulated dose and the change in the regional venous perfusion after -30 fractions (F=90.6,p <0.000 01). Each Gy produced an approximately 1.2% of reduction in the venous perfusion. This local dose and venous perfusion model has the potential to predict individual sensitivity to radiation. This is the first step toward developing a method to deliver higher and potentially more curative radiation doses to the patients who can safely receive these higher doses.

Assessment of Hepatic Perfusion in Transplanted Livers by Pharmacokinetic Analysis of Dynamic Magnetic Resonance Measurements

OBJECTIVE

The purpose of this study was to validate the assessment of hepatic perfusion by pharmacokinetic analysis of dynamic contrast-enhanced magnetic resonance image series.

MATERIALS AND METHODS

Dynamic measurements were performed with a saturation recovery turbo fast low angle shot (ie, FLASH) sequence over the course of approximately 4 minutes in 17 patients with transplanted livers. By pharmacokinetic analysis using an open 2-compartment model, we estimated and correlated an amplitude of signal enhancement, A, and the perfusion rate, kp, with invasive perfusion measurements from implanted thermo-diffusion probes (FTDP).

RESULTS

Data analysis for segment IV of the transplanted livers yielded a mean blood flow of 81 +/- 19 mL/min/100g and a mean perfusion rate of 13 +/- 6 minutes. There was a significant correlation between FTDP and kp (rS = 0.64, P = 0.01) but not with A.

CONCLUSIONS

Although our open 2-compartment model oversimplifies the complexity of hepatic perfusion, it allows a numerically robust estimation of regional blood flow per unit of blood volume. Thus, dynamic magnetic resonance imaging represents a noninvasive method to assess hepatic perfusion rate which can be visualized in color coded images.

Assessment of hemodynamics in precancerous lesion of hepatocellular carcinoma: Evaluation with MR perfusion

AIM: To investigate the hemodynamic changes in a precancerous lesion model of hepatocellular carcinoma (HCC).

METHODS: Hemodynamic changes in 18 Wistar rats were studied with non-invasive magnetic resonance (MR) perfusion. The changes induced by diethylnitrosamine (DEN) developed into liver nodular lesions due to hepatic cirrhosis during the progression of carcinogenesis. The MR perfusion data [positive enhancement integral (PEI)] were compared between the nodular lesions corresponding well with MR images and pathology and their surrounding hepatic parenchyma.

RESULTS: A total of 46 nodules were located by MR imaging and autopsy, including 22 dysplastic nodules (DN), 9 regenerative nodules (RN), 10 early HCCs and 5 overt HCCs. Among the 22 DNs, 6 were low-grade DN (LGDN) and 16 were high-grade DN (HGDN). The average PEI of RN, DN, early and overt HCC was 205.67 ± 31.17, 161.94 ± 20.74, 226.09 ± 34.83, 491.86 ± 44.61 respectively, and their liver parenchyma nearby was 204.84 ± 70.19. Comparison of the blood perfusion index between each RN and its surrounding hepatic parenchyma showed no statistically significant difference (P = 0.06). There were significant differences in DN (P = 0.02). During the late hepatic arterial phase, the perfusion curve in DN declined. DN had an iso-signal intensity at the early hepatic arterial phase and a low signal intensity at the portal venous phase. Of the 10 early HCCs, 4 demonstrated less blood perfusion and 6 displayed minimally increased blood flow compared to the surrounding parenchyma. Five HCCs showed significantly increased blood supply compared to the surrounding parenchyma (P = 0.02).

CONCLUSION: Non-invasive MR perfusion can detect changes in blood supply of precancerous lesions.

Optimized and combinedT1 andB1 mapping technique for fast and accurateT1 quantification in contrast-enhanced abdominal MRI

Fast T(1) mapping techniques are a valuable means of quantitatively assessing the distribution and dynamics of intravenously or orally applied paramagnetic contrast agents (CAs) by noninvasive imaging. In this study a fast T(1) mapping technique based on the variable flip angle (VFA) approach was optimized for accurate T(1) quantification in abdominal contrast-enhanced (CE) MRI. Optimization methods were developed to maximize the signal-to-noise ratio (SNR) and ensure effective RF and gradient spoiling, as well as a steady state, for a defined T(1) range of 100-800 ms and a limited acquisition time. We corrected B(1) field inhomogeneities by performing an additional measurement using an optimized fast B(1) mapping technique. High-precision in vitro and abdominal in vivo T(1) maps were successfully generated at a voxel size of 2.8 x 2.8 x 15 mm(3) and a temporal resolution of 2.3 s per T(1) map on 1.5T and 3T MRI systems. The application of the proposed fast T(1) mapping technique in abdominal CE-MRI enables noninvasive quantification of abdominal tissue perfusion and vascular permeability, and offers the possibility of quantitatively assessing dilution, distribution, and mixing processes of labeled solutions or drugs in the gastrointestinal tract.

The use of perfusion CT for the evaluation of therapy combining AZD2171 with gefitinib in cancer patients

The purpose of this study was to determine the feasibility of dynamic contrast-enhanced perfusion CT (CTP) in evaluating the hemodynamic response of tumors in the chest and abdomen treated with a combination of AZD2171 and gefitinib. Thirteen patients were examined just before and every 4-6 weeks after starting therapy. Following intravenous injection of a contrast agent, dynamic image acquisition was obtained at the level of a selected tumor location. To calculate perfusion, the maximum-slope method was used. Pre-treatment average perfusion for extra-hepatic masses was 84 ml/min/100 g, for liver masses arterial perfusion was 25 ml/min/100 g, and a portal perfusion of 30 ml/min/100 g was found. After the administration of AZD2171 and gefitinib, in extra-hepatic masses an initial decrease in perfusion of 18% was followed by a plateau and in liver masses an initial decrease of 39% within the lesions and of 36% within a rim region surrounding the lesions was followed by a tendency to recovery of hepatic artery flow. In conclusion, CTP is feasible in showing changes of perfusion induced by anti-angiogenic therapy.

A practical approach for quantitative estimates of voxel-by-voxel liver perfusion using DCE imaging and a compartmental model

Voxel-by-voxel estimation of liver perfusion using nonlinear least-squares fits of dynamic contrast enhanced computed tomography or magnetic resonance imaging data to a compartmental model is a computational expensive process. In this report, a "linear" least-squares method for estimation of liver perfusion is described. Simulated data and the data from an example case of a patient with intrahepatic cancer are presented. Compared to the nonlinear method, the new method can improve computational speed by a factor of approximately 400, which makes it practical for use in clinical trials.

New MR techniques for the detection of liver metastases

It is well established that hepatic resection improves the long-term prognosis of many patients with liver metastases. However, incomplete resection does not prolong survival, so knowledge of the exact extent of intra-hepatic disease is crucially important in determining patient management and outcome. MR imaging is well recognised as one of the most sensitive methods for detecting metastases. Recent developments in gradient coil design, the use of body phased array coils and the availability of novel MR contrast agents have resulted in MR being recognised as the pre-operative standard in this group of patients. However, diagnostic efficacy is extremely dependent on the choice and optimisation of pulse sequences and the appropriate use of MR contrast agents. This article reviews current MR imaging techniques for the detection and characterisation of metastases and discusses the relative capability of different techniques for detecting small lesions.

Quantification of Gd-BOPTA uptake and biliary excretion from dynamic magnetic resonance imaging in rat livers: model validation with 153Gd-BOPTA

OBJECTIVES

We sought to develop and validate a pharmacokinetic model allowing description of the magnetic resonance (MR) signal intensity induced by the hepatobiliary contrast agent Gd-BOPTA and to quantify the overall Gd-BOPTA transport in rat liver.

MATERIALS AND METHODS

MR signal intensity was recorded during the perfusion of rat livers with Gd-DTPA, an extracellular contrast agent, and Gd-BOPTA, a hepatobiliary contrast agent. Similar experiments were conducted with Gd-labeled contrast agents for quantitative measurement in liver, bile and perfusate.

RESULTS

A complete 6-compartment, 8 parameter open model was first developed to describe the pharmacokinetics of the compound based on the radioactivity data analysis. Because perfusate and bile data were not available in MRI experiments, a reduced model (6-compartment, 5 parameters) was considered for the MRI data. The performance of the reduced model was tested using the radioactivity data. The reduced model successfully described the contrast agent amount in the liver and correctly predicted amounts in bile and perfusate.

CONCLUSIONS

Pharmacokinetic modeling of MR signal intensity induced by Gd-BOPTA permits quantification of Gd-BOPTA uptake and biliary excretion in rat livers.

Small-Bowel Perfusion Measurement: Feasibility with Single-Compartment Kinetic Model Applied to Dynamic Contrast-enhanced CT

This study was institutional review board approved and HIPAA compliant. Informed consent was obtained from all patients. The purpose of the study was to prospectively examine the feasibility of measuring small-bowel quantitative blood flow by using motion-corrected, contrast-enhanced computed tomographic (CT) images and a single-compartment kinetic model. Seven patients underwent abdominal CT in which 40 10-mm-thick sections were obtained at a single level. Small-bowel images were obtained every 3 seconds after contrast agent administration. Automated application of regions of interest yielded time-enhancement curves for the bowel wall and the aorta. A one-compartment model was applied to each set of time-enhancement curves for determination of the small-bowel volumetric blood flow F(V), volume of distribution V(D), and blood transit time tau. F(V) was also calculated by using the first-pass method and gamma variate analysis for model validation. The F(V) values obtained by using the single-compartment model (mean F(V), 0.47 min(-1)) showed excellent linear correlation with those obtained by using the first-pass method (Pearson r = 0.80) and gamma variate analysis (Pearson r = 0.97). Mean V(D) and tau values were 2.86 (unitless) and 4.28 seconds, respectively. A one-compartment kinetic model can be applied to motion-corrected, contrast-enhanced small-bowel CT images to quantify perfusion.

Perfusion imaging of the liver: current challenges and future goals

Improved therapeutic options for hepatocellular carcinoma and metastatic disease place greater demands on diagnostic and surveillance tests for liver disease. Existing diagnostic imaging techniques provide limited evaluation of tissue characteristics beyond morphology; perfusion imaging of the liver has potential to improve this shortcoming. The ability to resolve hepatic arterial and portal venous components of blood flow on a global and regional basis constitutes the primary goal of liver perfusion imaging. Earlier detection of primary and metastatic hepatic malignancies and cirrhosis may be possible on the basis of relative increases in hepatic arterial blood flow associated with these diseases. To date, liver flow scintigraphy and flow quantification at Doppler ultrasonography have focused on characterization of global abnormalities. Computed tomography (CT) and magnetic resonance (MR) imaging can provide regional and global parameters, a critical goal for tumor surveillance. Several challenges remain: reduced radiation doses associated with CT perfusion imaging, improved spatial and temporal resolution at MR imaging, accurate quantification of tissue contrast material at MR imaging, and validation of parameters obtained from fitting enhancement curves to biokinetic models, applicable to all perfusion methods. Continued progress in this new field of liver imaging may have profound implications for large patient groups at risk for liver disease.

Comparison of the hepatic perfusion index measured with gadolinium-enhanced volumetric MRI in controls and in patients with colorectal cancer

The aim of the study was to adapt the methodology established for dynamic CT measurements of the hepatic perfusion index (HPI) to MRI, and to assess the potential role of MRI measurements of the HPI in detecting regional alterations in liver perfusion between patients with colorectal liver metastases and normal controls. The HPI was evaluated from serial T(1) volume acquisitions acquired over the course of a Gd-DTPA bolus injection. Time-course data from regions of interest in the liver, spleen and aorta were used to calculate the HPI; and HPI data from control subjects were compared with data from patients with known colorectal metastases. Significant differences were found between the relative portal perfusion and hepatic perfusion indices calculated for the patient and control groups (p<0.005). These results suggest that hepatic perfusion indices can be derived using MRI-based methods, and that these perfusion indices are sensitive to differences in liver perfusion associated with established metastatic liver disease on imaging. This technique may contribute to the early detection of liver metastases, allowing early surgical intervention and improved patient survival.

Quantification of perfusion of liver tissue and metastases using a multivessel model for replenishment kinetics of ultrasound contrast agents

Low-MI (mechanical index) ultrasound allows real-time observation of replenishment kinetics after destruction ("flash") of ultrasound contrast agents (USCA). We developed an examination protocol and a mathematical model to quantify perfusion of liver tissue and hepatic metastases. Using a modified multivessel model, we attempted a consistent, physiological description of microbubble replenishment in liver tissue. Perfusion parameters were calculated, separately for the arterial and portal venous phase of liver perfusion, using an i.v. bolus injection of 2 x 2.4 mL SonoVue. The model was evaluated for 10 examinations of liver metastases using flash/low-MI imaging. In contrast to the established, exponential model, the new model consistently describes the sigmoid replenishment of USCA measured in vivo, using flash/low-MI imaging. Parameters for blood volume, blood velocity and blood flow in liver tissue and metastases can be calculated during the arterial and the portal venous phase after a CA bolus injection. The median arterial perfusion in the examined liver metastases was more than 2.5 times higher than in normal liver tissue, whereas the median perfusion during the portal venous phase was more than five times higher in the liver tissue than that in metastases. Microbubble replenishment measured with flash/low-MI US techniques can be consistently analyzed using the multivessel model, even after a bolus injection of USCA. This allows for the quantification of perfusion of liver tissue and hepatic metastases and provides promising parameters of tissue viability and tumor characterization.

Gd-BOPTA Transport Into Rat Hepatocytes: Pharmacokinetic Analysis of Dynamic Magnetic Resonance Images Using a Hollow-Fiber Bioreactor

RATIONALE AND OBJECTIVES

To investigate the transport of the hepatobiliary magnetic resonance (MR) imaging contrast agent Gd-BOPTA into rat hepatocytes.

MATERIALS AND METHODS

In a MR-compatible hollow-fiber bioreactor containing hepatocytes, MR signal intensity was measured over time during the perfusion of Gd-BOPTA. For comparison, the perfusion of an extracellular contrast agent (Gd-DTPA) was also studied. A compartmental pharmacokinetic model was developed to describe dynamic signal intensity-time curves.

RESULTS

The dynamic signal intensity-time curves of the hepatocyte hollow-fiber bioreactor during Gd-BOPTA perfusion were adequately fitted by 2 compartmental models. Modeling permitted to discriminate between the behaviors of the extracellular contrast agent (Gd-DTPA) and the hepatobiliary contrast agent (Gd-BOPTA). It allowed the successfully quantification of the parameters involved in such differences. Gd-BOPTA uptake was saturable at high substrate concentrations.

CONCLUSIONS

The transport of Gd-BOPTA into rat hepatocytes was successfully described by compartmental analysis of the signal intensity recorded over time and supported the hypothesis of a transporter-mediated uptake.

Inflow correction of hepatic perfusion measurements usingT1-weighted, fast gradient-echo, contrast-enhanced MRI

Inflow effects were studied for T(1)-weighted, fast gradient-echo, contrast-enhanced MRI. This was done on the basis of realistic simulations (e.g., taking slice profiles into account) for unsteady flow. The area under the point spread function (PSF) was used to estimate the flow-related enhancement. A simple analytical model that accurately describes the inflow effects was derived and validated. This model was used to correct the experimental perfusion calibration curves (signal intensity vs. relaxation rate) for inflow effects. Hepatic perfusion measurements, performed on patients, were analyzed in terms of a dual-input, first-order linear model. It was shown that inflow causes incorrect perfusion input functions. The resulting estimated perfusion parameters displayed a systematic error of typically 30-40%. By performing two extra time-resolved flow measurements during the examination, one can correct the input functions.

Glomerular filtration rate: Assessment with dynamic contrast-enhanced MRI and a cortical-compartment model in the rabbit kidney

PURPOSE

To describe the use of MRI and a cortical-compartment model to measure the glomerular filtration rate (GFR), and compare the results with those obtained with the Patlak-Rutland model.

MATERIALS AND METHODS

Dynamic MRI of rabbit kidneys was performed during and after injection of gadoterate dimeglumine. The enhancement curves in the aorta and the kidney were analyzed with the cortical-compartment and Patlak-Rutland models to assess the GFR.

RESULTS

A substantial correlation was observed between the GFR measured with MRI using the cortical-compartment model and the plasma clearance of 51Cr-EDTA (r=0.821, P=0.004). No significant correlation was observed between the 51Cr-EDTA clearance (r=0.628, P=0.052) and the GFR obtained with the Patlak-Rutland model in regions of interest (ROIs) encompassing the renal cortex and medulla. A Bland and Altman analysis showed that GFR(cortical) (compartment) agreed better with the 51Cr-EDTA clearance compared to GFR(Patlak) when ROIs were limited to the cortex. However, the GFR values obtained by MRI were lower than the plasma clearance of 51Cr-EDTA.

CONCLUSION

MRI with a cortical-compartment model provides more accurate assessments of glomerular filtration than the Patlak-Rutland model.

Calculation of the renal perfusion and glomerular filtration rate from the renal impulse response obtained with MRI

The aim of this study was to assess the importance of deconvolution for the calculation of renal perfusion and glomerular filtration rate (GFR) on the basis of concentration-time curves as measured with perfusion MRI. Six rabbits were scanned dynamically after injection of a gadolinium chelate. Concentration-time curves were generated by manually drawing regions of interest in the aorta and the renal cortex. To remove the dependency on the arterial input function, a regularized structured total least-squares deconvolution algorithm was used to calculate the renal impulse response. This curve was fitted by the sum of two gamma variate functions, corresponding to the passage of the contrast agent in the glomeruli and the proximal convoluted tubules. Tracer kinetics models were applied to these two functions to obtain the renal perfusion and GFR. For comparison, these two parameters were also calculated on the basis of the renal concentration-time curve before deconvolution. The renal perfusion values correlated well (r = 0.9, P = 0.014) with the values calculated by a validated upslope method. The GFR values correlated well (r = 0.9, P = 0.014) with the values obtained from the clearance of (51)Cr-EDTA. A comparison of the values obtained with and without deconvolution demonstrated the necessity of deconvolution.

Hepatic flow parameters measured with MR imaging and Doppler US: correlations with degree of cirrhosis and portal hypertension

PURPOSE

To determine the correlations between hemodynamic parameters of hepatic flow measured with magnetic resonance (MR) imaging and Doppler ultrasonography (US) and the severity of cirrhosis and portal hypertension.

MATERIALS AND METHODS

Forty-six patients referred for measurements of portal venous pressure (three with normal liver, 12 with chronic hepatitis, and 31 with cirrhosis [10 with Child-Pugh class A cirrhosis; 13 with class B cirrhosis; and eight with class C cirrhosis]) were included in the study. Apparent liver perfusion, apparent arterial and portal perfusion, portal fraction, distribution volume, and mean transit time were measured with dynamic contrast material-enhanced MR imaging. Portal velocity, portal flow, congestion index, right hepatic artery resistance index, and modified hepatic index were measured with Doppler US. Results in patients with cirrhosis and those without cirrhosis were compared with the Wilcoxon rank sum test. Correlations were assessed with Spearman rank correlation coefficients.

RESULTS

With MR imaging, all flow parameters except distribution volume were significantly different between patients with and those without cirrhosis (P <.05). There was a significant correlation between all flow parameters measured with MR imaging and portal pressure (P <.02). Apparent arterial (P =.024) and portal (P <.001) perfusion, portal fraction (P <.001), and mean transit time (P =.004) were correlated with Child-Pugh class. Flow parameters measured with Doppler US did not differ significantly between patients with and those without cirrhosis. Only right hepatic arterial resistance (P <.007) and portal flow (P <.043) were weakly (r < 0.7) correlated with portal pressure. No Doppler US parameter was correlated with Child-Pugh class.

CONCLUSION

Hepatic flow parameters measured with MR imaging correlate with the severity of cirrhosis and portal hypertension. Doppler US parameters are only weakly correlated with portal pressure.

Capillarization of the sinusoids in liver fibrosis: noninvasive assessment with contrast-enhanced MRI in the rabbit

Sinusoidal capillarization induces microcirculatory changes in liver cirrhosis and fibrosis. The purpose of this study was to assess whether contrast-enhanced MRI can be used to demonstrate the effects of sinusoidal capillarization in liver fibrosis. Dynamic MRI after injection of a low-molecular-weight contrast agent of 0.56 kDa (Gd-DOTA), and two high-molecular-weight contrast agents of 6.47 kDa and 52 kDa (P792 and P717) was performed in rabbits with liver fibrosis induced by cholesterol and diethylstilbestrol. The hepatic distribution volume accessible to the high-molecular-weight agents decreased in the rabbits with liver fibrosis (P792: 7.8% +/- 1.7% vs. 10.1% +/- 1.8% in normal rabbits, P =.038; P717: 6.2% +/- 2.1% vs. 9.7% +/- 1.6% in normal rabbits, P =.007), whereas the hepatic mean transit time (MTT) of the low-molecular-weight agent was increased (15.9 +/- 8.0 s vs. 8.8 +/- 2.6 s in normal rabbits, P =.015). In rabbits with liver fibrosis, the clearance of indocyanine green (ICG) was correlated with the volume accessible to the high-molecular-weight agents (P792: r = 0.810, P =.015; P717: r = 0.857, P =.007). The collagen content of the liver was inversely correlated with the distribution volume of P717 (r = -.833, P =.010) and with the ICG clearance (r = -.810, P =.015). It was concluded that the microcirculatory changes induced by sinusoidal capillarization in liver fibrosis can be demonstrated noninvasively with MRI.