Precision, signal-to-noise ratio, and dose optimization of magnitude and phase arterial input functions in dynamic susceptibility contrast MRI

PURPOSE: To determine optimal conditions for precise measurement of arterial input function (AIFs) in dynamic susceptibility contrast (DSC) perfusion MRI. MATERIALS AND METHODS: Magnitude-based (DeltaR(2)*) and phase-based (Deltaphi) AIFs were numerically simulated for several doses and baseline MRI noise levels [SNR(I(0))]. Random noise (1000 realizations) was added to real/imaginary MRI signals (derived from an internal carotid AIF), and AIF signal, noise, and signal-to-noise ratio (SNR) were determined. The optimal dose was defined as the dose that maximizes mean AIF SNR over the first-pass (SNR(mean)), rather than SNR at the AIF peak (SNR(peak)) because, compared to SNR(peak), doses predicted by SNR(mean) reduced the AIF-induced variability in cerebral blood flow (CBF) by 24% to 40%. RESULTS: The AIF SNR is most influenced by choice of AIF signal, then optimal dosing, each with little penalty. Compared to DeltaR(2)*, Deltaphi signal has 4 to 80 times the SNR over all doses and time points, and approximately 10-fold SNR(mean) at respective optimal doses. Optimal doses induce 85% to 90% signal drop for the DeltaR(2)* method, and 70% to 75% for Deltaphi, with two-fold dose errors causing approximately 1.7-fold loss in SNR(mean). Increases in SNR(I(0)) proportionally increase AIF SNR, but at a cost. CONCLUSION: AIF SNR is affected most by signal type, then dosing, and lastly, SNR(I(0)).     

Partial volume effects on arterial input functions: shape and amplitude distortions and their correction

For quantification of perfusion values from a bolus-tracking MRI experiment, the measurement of an arterial input function (AIF) is necessary. Gradient-echo (GE) sequences are commonly used for this type of experiment because they offer a high signal-to-noise ratio (SNR) and the potential to quantify the concentration of contrast agent. Measurements of calibration curves for Gd-DTPA in human blood have shown a quadratic relation between the DeltaR(2)* and the concentration of contrast agent, and a linear relationship between phase changes and the concentration of contrast agent. However, for in vivo studies the spatial resolution is usually limited, which leads to partial volume effects. Partial volume effects result in a complex sum of signal arising from the tissue outside the vessel and a contrast agent concentration-dependent blood signal. Ignoring the presence of partial volume effects can lead to an overestimation or underestimation of the contrast agent concentration, depending on the experimental conditions. Correction for partial volume effects is feasible in arteries that are parallel to the main magnetic field by estimation and subtraction of the static signal of the surrounding tissue. Patient studies showed a large variation due to the AIF measurements, but it has also been shown that this influence can be minimized by correction for partial volume effects.     

Antiangiogenic tumor treatment: early noninvasive monitoring with USPIO-enhanced MR imaging in mice

PURPOSE: To prospectively investigate steady-state blood volume measurements for early quantitative monitoring of antiangiogenic treatment with ultrasmall superparamagnetic iron oxide (USPIO)-enhanced magnetic resonance (MR) imaging. MATERIALS AND METHODS: The institutional animal care committee approved all experiments. HT-1080 fibrosarcoma-bearing nude mice were injected with a thrombogenic vascular targeting agent (VTA) (11 nude mice, 20 tumors) or saline (12 nude mice, 20 tumors). USPIO-enhanced (SH U 555C) MR imaging was performed after the VTA was administered. USPIO-induced changes in tissue R2* (DeltaR2*) were measured with a T2-weighted dual-echo echo-planar imaging sequence, and the vascular volume fraction (VVF) was calculated. Parametric DeltaR2* maps were analyzed with respect to tumor perfusion patterns. Correlative histologic analysis was performed for grading of tissue thrombosis, and tissue perfusion was quantified with fluorescent microbeads. Unpaired Student t test and Spearman nonparametric correlation coefficient were used for statistical analysis. RESULTS: The DeltaR2* values were significantly (P < .001) reduced shortly after treatment initiation (mean DeltaR2*, 0.017 msec(-1) +/- 0.0014 [standard error] in control animals vs 0.005 msec(-1) +/- 0.0007 in animals that received VTA), which was also reflected by a decrease in the VVF (2.47% +/- 0.18 vs 0.41% +/- 0.48, P < .001). Histologic analysis revealed various degrees of tumor thrombosis after VTA treatment that correlated inversely with the DeltaR2* values (r = -0.83). Moreover, tumor perfusion measurements corroborated the MR results, indicating a significant reduction in tissue perfusion after VTA treatment (mean tissue fluorescence, 570.4 arbitrary units [au] per gram +/- 27 vs 161.7 au/g +/- 17; P < .05). CONCLUSION: USPIO-enhanced MR imaging enables early monitoring of antiangiogenic treatment of tumors.     

Feasibility of dynamic susceptibility contrast perfusion MR imaging at 3T using a standard quadrature head coil and eight-channel phased-array coil with and without SENSE reconstruction

PURPOSE: To investigate changes in image and dynamic signal-to-noise ratios (SNRs) of the DeltaR2* curve, as well as magnetic susceptibility-induced artifacts between a standard quadrature head coil and an eight-channel phased-array coil with and without sensitivity-encoding (SENSE) at 3T, compared to the current clinical standard head coil acquisition at 1.5T. MATERIALS AND METHODS: Dynamic susceptibility contrast (DSC) perfusion MRI was performed on 80 brain tumor patients using a gradient-echo, echo-planar imaging (EPI) sequence. Image and dynamic SNR were compared between 1.5T and 3T field strengths, a quadrature and eight-channel phased-array coil, and a conventional vs. partially parallel EPI acquisition with SENSE reconstruction. The amount of geometric distortion and signal dropout was quantified and compared between conventional and SENSE EPI acquisitions within the same exam at 3T. RESULTS: An initial 2.6-fold elevation in dynamic SNR was observed in normal-appearing white matter when doubling the field strength (P < 0.001), with an additional 1.7-fold increase found when employing an eight-channel phased-array coil (P < 0.002). Compared to the standard 3T eight-channel coil acquisition, the implementation of SENSE reduced the number of voxels experiencing large anterior shifts in the phase-encode direction, lowered the volume of signal dropout by 2.0-11.5%, and allowed a 1.4-fold increase in slice coverage, while only decreasing the dynamic SNR by 22%. CONCLUSION: SENSE EPI at 3T yielded a significant improvement in dynamic SNR over the 1.5T acquisitions. A significant reduction in magnetic susceptibility-induced artifacts was achieved with SENSE EPI compared to the standard EPI eight-channel coil acquisition at 3T.     

In vivo R1-enhancement mapping of canine myocardium using ceMRI with Gd(ABE-DTTA) in an acute ischemia-reperfusion model

PURPOSE: To demonstrate the usefulness of normalized DeltaR1 (DeltaR1(n)) mapping in myocardial tissue following the administration of the contrast agent (CA) Gd(ABE-DTTA). MATERIALS AND METHODS: Ischemia-reperfusion experiments were carried out in 11 dogs. The method exploited the relatively long tissue lifetime of Gd(ABE-DTTA), and thus no fast R1 measurement technique was needed. Myocardial perfusion was determined with colored microspheres (MP). RESULTS: With varying extent of ischemia, impaired wall motion (WM) and lower DeltaR1(n) values were detected in the ischemic sectors, as opposed to the nonischemic sectors where normal WM and higher DeltaR1(n) were observed. Based on the DeltaR1(n), data from the myocardial perfusion assay and the DeltaR1(n) maps were compared in the ischemic sectors. A correlation analysis of these two parameters demonstrated a significant correlation (R = 0.694, P < 0.005), validating the DeltaR1(n)-mapping method for the quantitation of ischemia. Similarly, pairwise correlations were found for the MP, DeltaR1(n), and wall thickening (WT) values in the same areas. Based on the correlation between DeltaR1(n) and MP, DeltaR1(n) maps calculated with a pixel-by-pixel resolution can be converted to similarly high-resolution myocardial perfusion maps. CONCLUSION: These results suggest that the extent of the severity of ischemia can be quantitatively represented by DeltaR1(n) maps obtained in the presence of our CA.     

Perfusion MRI with radial acquisition for arterial input function assessment.

Quantification of myocardial perfusion critically depends on accurate arterial input function (AIF) and tissue enhancement curves (TECs). Except at low doses, the AIF is inaccurate because of the long saturation recovery time (SRT) of the pulse sequence. The choice of dose and SRT involves a trade-off between the accuracy of the AIF and the signal-to-noise ratio (SNR) of the TEC. Recent methods to resolve this trade-off are based on the acquisition of two data sets: one to accurately estimate the AIF, and one to find the high-SNR TEC. With radial k-space sampling, a set of images with varied SRTs can be reconstructed from the same data set, allowing an accurate assessment of the AIF and TECs, and their conversion to contrast agent (CA) concentration. This study demonstrates the feasibility of using a radial acquisition for quantitative myocardial perfusion imaging.     

Multiple reference tissue method for contrast agent arterial input function estimation.

A precise contrast agent (CA) arterial input function (AIF) is important for accurate quantitative analysis of dynamic contrast-enhanced (DCE)-MRI. This paper proposes a method to estimate the AIF using the dynamic data from multiple reference tissues, assuming that their AIFs have the same shape, with a possible difference in bolus arrival time. By minimizing a cost function, one can simultaneously estimate the parameters and underlying AIF of the reference tissues. The method is computationally efficient and the estimated AIF is smooth and can have higher temporal resolution than the original data. Simulations suggest that this method can provide a reliable estimate of the AIF for DCE-MRI data with a moderate signal-to-noise ratio (SNR) and temporal resolution, and its performance increases significantly as the SNR and temporal resolution increase. As demonstrated by its clinical application, sufficient reference tissues can be easily obtained from normal tissues and subregions segmented from a tumor region of interest (ROI), which suggests this method can be generally applied to cancer-based DCE-MRI studies to estimate the AIF. This method is applicable to general kinetic models in DCE-MRI, as well as other CE imaging modalities.     

Is quantification of bolus tracking MRI reliable without deconvolution?

Bolus tracking data obtained with paramagnetic intravascular tracers are commonly analyzed and quantified by the direct measurement of properties of the tissue concentration-time curve (e.g., time to peak (TTP)). The measurement of these "summary parameters" is used as an accessible alternative approach to the complex deconvolution procedure, and provides indirect measures of perfusion. However, summary parameters do not take into account differences in arterial input functions (AIFs) or residue functions (R(t)) between patients or studies. Simulations were performed to assess the variability of summary parameters over a realistic range of AIFs and for differing R(t), to establish whether they can be used as reliable measures of tissue perfusion status. Results showed that the value of each summary parameter investigated is highly dependent upon both the AIF and R(t). The referencing of summary parameters to their corresponding value in the AIF or in normal tissue is a method commonly used to normalize results, but this approach did not lead to any measures that were independent of both the AIF and R(t) in this study. The results presented here show that the use of summary parameters requires considerable caution, since tissue or patient types can easily be incorrectly classified due to the effect of variations in patient AIF and R(t).     

Magnetic resonance brain perfusion imaging with voxel-specific arterial input functions

PURPOSE: To propose an automatic method for estimating voxel-specific arterial input functions (AIFs) in dynamic contrast brain perfusion imaging. MATERIALS AND METHODS: Voxel-specific AIFs were estimated blindly using the theory of homomorphic transformations and complex cepstrum analysis. Wiener filtering was used in the subsequent deconvolution. The method was verified using simulated data and evaluated in 10 healthy adults. RESULTS: Computer simulations accurately estimated differently shaped, normalized AIFs. Simple Wiener filtering resulted in underestimation of flow values. Preliminary in vivo results showed comparable cerebral flow value ratios between gray matter (GM) and white matter (WM) when using blindly estimated voxel-specific AIFs or a single manually selected AIF. Significant differences (P < or = 0.0125) in mean transit time (MTT) and time-to-peak (TTP) in GM compared to WM was seen with the new method. CONCLUSION: Initial results suggest that the proposed method can replace the tedious and difficult task of manually selecting an AIF, while simultaneously providing better differentiation between time-dependent hemodynamic parameters.     

Correction of arterial input function in dynamic contrast-enhanced MRI of the liver

Purpose:

To develop a postprocessing method to correct saturation of arterial input function (AIF) in T1‐weighted dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) for quantification of hepatic perfusion.

Materials and Methods:

The saturated AIF is corrected by parameterizing the first pass of the AIF as a smooth function with a single peak and minimizing a least‐squares error in fitting the liver DCE‐MRI data to a dual‐input single‐compartment model. Sensitivities of the method to the degree of saturation in the AIF first‐pass peak and the image contrast‐to‐noise ratio were assessed. The method was also evaluated by correlating portal venous perfusion with an independent overall liver function measurement.

Results:

The proposed method corrects the distorted AIF with a saturation ratio up to 0.45. The corrected AIF improved hepatic arterial perfusion by ?23.4% and portal venous perfusion by 26.9% in a study of 12 patients with liver cancers. The correlation between the mean voxelwise portal venous perfusion and overall liver function measurement was improved by using the corrected AIFs (R² = 0.67) compared with the saturated AIFs (R² = 0.39).

Conclusion:

The method is robust for correcting AIF distortion and has the potential to improve quantification of hepatic perfusion for assessment of liver tissue response to treatment in patients with hepatic cancers. J. Magn. Reson. Imaging 2012;36:411–421. © 2012 Wiley Periodicals, Inc.

     

Conversion of arterial input functions for dual pharmacokinetic modeling using Gd‐DTPA/MRI and 18F‐FDG/PET

Reaching the full potential of magnetic resonance imaging (MRI)‐positron emission tomography (PET) dual modality systems requires new methodologies in quantitative image analyses. In this study, methods are proposed to convert an arterial input function (AIF) derived from gadolinium‐diethylenetriaminepentaacetic acid (Gd‐DTPA) in MRI, into a ¹⁸F‐fluorodeoxyglucose (¹⁸F‐FDG) AIF in PET, and vice versa. The AIFs from both modalities were obtained from manual blood sampling in a F98‐Fisher glioblastoma rat model. They were well fitted by a convolution of a rectangular function with a biexponential clearance function. The parameters of the biexponential AIF model were found statistically different between MRI and PET. Pharmacokinetic MRI parameters such as the volume transfer constant (K^trans), the extravascular–extracellular volume fraction (ν_e), and the blood volume fraction (ν_p) calculated with the Gd‐DTPA AIF and the Gd‐DTPA AIF converted from ¹⁸F‐FDG AIF normalized with or without blood sample were not statistically different. Similarly, the tumor metabolic rates of glucose (TMRGlc) calculated with ¹⁸F‐FDG AIF and with ¹⁸F‐FDG AIF obtained from Gd‐DTPA AIF were also found not statistically different. In conclusion, only one accurate AIF would be needed for dual MRI‐PET pharmacokinetic modeling in small animal models. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Comparison of quantitative dynamic susceptibility-contrast MRI perfusion estimates obtained using different contrast-agent administration schemes at 3 T

Absolute cerebral perfusion parameters were obtained by dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) carried out using different contrast-agent administration protocols. Sixteen healthy volunteers underwent three separate DSC-MRI examinations each, receiving single-dose (0.1 mmol/kg b.w.) gadobutrol, double-dose gadobutrol and single-dose gadobenate-dimeglumine on different occasions. DSC-MRI was performed using single-shot gradient-echo echo-planar imaging at 3 T. The arterial input functions (AIFs) were averages (4-9 pixels) of arterial curves from middle cerebral artery branches, automatically identified according to standard criteria. Absolute estimates of cerebral blood volume (CBV), cerebral blood flow (CBF) and mean transit time (MTT) were calculated without corrections for non-linear contrast-agent (CA) response in blood or for different T2* relaxivities in tissue and artery. Perfusion estimates obtained using single and double dose of gadobutrol correlated moderately well, while the relationship between estimates obtained using gadobutrol and gadobenate-dimeglumine showed generally lower correlation. The observed degree of CBV and CBF overestimation, compared with literature values, was most likely caused by different T2* relaxivities in blood and tissue in combination with partial-volume effects. The present results showed increased absolute values of CBV and CBF at higher dose, not predicted by the assumption of a quadratic response to contrast-agent concentration in blood. This indicates that the signal components of measured AIFs were not purely of arterial origin and that arterial signal components were more effectively extinguished at higher CA dose. This study also indicates that it may not be completely straightforward to compare absolute perfusion estimates obtained with different CA administration routines.     

Comparison of contrast agents with high molarity and with weak protein binding in cerebral perfusion imaging at 3 T

PURPOSE: To examine and compare properties of high-molarity contrast agent gadobutrol (Gadovist) and weakly protein-binding agent gadobenate-dimeglumine (MultiHance in dynamic susceptibility contrast (DSC) perfusion imaging at 3 T. MATERIALS AND METHODS: Sixteen healthy volunteers underwent three separate examinations with contrast agent doses of 0.1 and 0.2 mmol/kg body weight (bw) gadobutrol and 0.1 mmol/kg bw gadobenate-dimeglumine. Maps of relative regional cerebral blood volume (rCBV) and blood flow (rCBF) were calculated using deconvolution based on singular value decomposition. Signal and concentration time curves, the concentration-to-noise ratio (SNR(c)), and gray matter (GM)-to-white matter (WM) rCBV and rCBF contrast and ratios were evaluated in a region of interest (ROI)-based analysis. Image quality of calculated parametric maps was assessed in direct visual comparison and with respect to suitability for diagnostic purposes. RESULTS: The contrast agents displayed very similar results in the 0.1 mmol/kg examinations, both with respect to the quantitative evaluation parameters and in the qualitative assessment of the calculated parametric maps. Maps from 0.2 mmol/kg examinations were rated as being superior in quality, but with respect to diagnostic suitability all contrast agents and doses yielded images of sufficient quality. CONCLUSION: At 3 T, a gadobutrol or gadobenate-dimeglumine dose of 0.1 mmol/kg is sufficient for DSC magnetic resonance imaging (MRI) perfusion assessment. At the used small injection volumes, the tissue concentration curve was determined only by the gadolinium (Gd) dosage in mmol/kg, and the T2* relaxation effects of the two agents can be considered to be nearly identical in the applied gradient-echo (GRE) sequence.     

Comparison of errors associated with single- and multi-bolus injection protocols in low-temporal-resolution dynamic contrast-enhanced tracer kinetic analysis.

Accurate sampling of the arterial input function (AIF) in low-temporal-resolution quantitative dynamic contrast-enhanced MRI (DCE-MRI) studies is crucial for accurate and reproducible parameter estimation. However, when conventional AIFs are sampled at low temporal resolution, they introduce an unpredictable degree of error. An alternative double contrast agent (CA) bolus injection protocol designed to compensate for temporal mis-sampling of the AIF and tissue uptake curve was simulated in addition to a commonly used single CA bolus injection protocol. A range of tissue uptake curves for each AIF form were generated using a distributed parameter model, and Monte Carlo simulation studies were performed over a range of offset times (to mimic temporal mis-sampling), temporal resolutions and SNR in order to compare the performance of both AIF forms in compartmental modeling. Insufficient data sampling of the single bolus AIF at temporal resolutions in excess of 9 s leads to large errors, which can be reduced by employing an additional, appropriately administered, second CA bolus injection.     

Indirect detection of lung perfusion using susceptibility-based hyperpolarized gas imaging

PURPOSE: To address the problem of inadequate signal-to-noise ratio (SNR) encountered in lung perfusion magnetic resonance imaging (MRI) by developing an indirect detection based on the strong hyperpolarized (HP) gas signal. MATERIALS AND METHODS: Our model is based on detecting the effects of gadolinium (Gd) flowing through lung capillaries by recording the phase of the nearby alveolar HP gas. In a HP gas 3He phantom we imaged gas phases before and after removing tubes containing paramagnetic solution away from the phantom. We also imaged HP gas phases in pig lungs before and after injection of Gd. Finally, parenchymal spin phase in excised lungs was measured as a function of Gd concentration. RESULTS: In the phantom, the differential phase map displayed a pattern characteristic of a susceptibility-induced dipole field, showing the possibility of an indirect detection. In vivo, the differential phase map showed homogeneous appearance, as expected for uniform perfusion in healthy lungs. Ex vivo, the parenchymal spin phases were shown to depend linearly on Gd concentration. CONCLUSION: Our method should allow indirect perfusion (Q) and direct ventilation (V) to be assessed simultaneously, thus allowing for diagnosis of V/Q mismatches. The linear dependency of parenchymal spin phase vs. Gd concentration may allow for quantification of lung perfusion.     

Cortical depth-dependent gradient-echo and spin-echo BOLD fMRI at 9.4T.

To examine cortical depth-related spatial specificity and signal changes in gradient-echo (GE) and spin-echo (SE) blood oxygenation level-dependent (BOLD) fMRI signals, a well-established cat visual stimulation model was used at 9.4T. The GE BOLD signal percent change is the highest at the surface of the cortex containing pial vessels, and decreases as cortical depth increases. In contrast, the SE BOLD signal is more specific to parenchyma, showing the highest signal change in the middle cortical areas. The stimulation-induced DeltaR2* to DeltaR2 ratio is dependent on the vessel size, which is related to basal susceptibility effects. The averaged ratio of DeltaR2* to DeltaR2 in all active regions, including large vessels, is 3.3 +/- 0.5 (N = 6). The averaged ratio of DeltaR2* to DeltaR2 is 8.8 +/- 1.7 (N = 4) on the surface of the cortex with large pial draining vessels, and decreases to 1.9 +/- 0.1 on the middle cortical areas with parenchymal microvessels. DeltaR2*/DeltaR2 is closely related to basal susceptibility effects and can be used to differentiate tissue from vessel regions.     

Evaluation of signal formation in local arterial input function measurements of dynamic susceptibility contrast MRI

Correct arterial input function (AIF) measurements in dynamic susceptibility contrast‐MRI are crucial for quantification of the hemodynamic parameters. Often a single global AIF is selected near a large brain‐feeding artery. Alternatively, local AIF measurements aim for voxel‐specific AIFs from smaller arteries. Because local AIFs are measured higher in the arterial‐tree, it is assumed that these will reflect the true input of the microvasculature much better. However, do the measured local AIFs reflect the true concentration‐time curves of small arteries? To answer this question, in vivo data were used to evaluate local AIF candidates selected based on two different types of angiograms. For interpretation purposes, a 3D numerical model that simulated partial‐volume effects in local AIF measurements was created and the simulated local AIFs were compared to the ground truth. The findings are 2‐fold. First, the in vivo data showed that the shape‐characteristics of local AIFs are similar to the shape‐characteristics of gray matter concentration‐time curves. Second, these findings are supported by the simulations showing broadening of the measured local AIFs compared to the ground truth. These findings are suggesting that local AIF measurements do not necessarily reflect the true concentration‐time curve in small arteries. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Cerebral blood flow estimation in vivo using local tissue reference functions

Purpose

To evaluate the use of bolus signals obtained from tissue as reference functions (or local reference functions [LRFs]) rather than arterial input functions (AIFs) when deriving cross‐calibrated cerebral blood flow (CBF_CC) estimates via deconvolution.

Materials and Methods

AIF and white matter (WM) LRF CBF_CC maps (cross‐calibrated so that normal WM was 23.7 mL/minute/100 g) derived using singular value decomposition (SVD) were examined in 28 ischemic stroke patients. Median CBF_CC estimates from normal gray matter (GM) and ischemic tissue were obtained.

Results

AIF and LRF median CBF_CC estimates resembled one another for all 28 patients (average paired CBF_CC difference 0.4 ± 1.7 mL/minute/100 g and –0.4 ± 1.4 mL/minute/100 g in GM and ischemic tissue, respectively). Wilcoxon signed‐rank comparisons of patient median CBF_CC measurements revealed no statistically significant differences between using AIFs and LRFs (P > 0.05).

Conclusion

If CBF is quantified using a patient‐specific cross‐calibration factor, then LRF CBF estimates are at least as accurate as those from AIFs. Therefore, until AIF quantification is achievable in vivo, perfusion protocols tailored for LRFs would simplify the methodology and provide more reliable perfusion information. J. Magn. Reson. Imaging 2009;29:183–188. © 2008 Wiley‐Liss, Inc.     

Reproducibility assessment of a multiple reference tissue method for quantitative dynamic contrast enhanced–MRI analysis

Bone metastases of 16 prostate cancer patients were scanned twice 1 week apart by dynamic contrast enhanced (DCE) –MRI at 2‐s resolution using a two‐dimensional gradient‐echo pulse sequence. With a multiple reference tissue method (MRTM), the local tissue arterial input function (AIF) was estimated using the contrast agent enhancement data from tumor subregions and muscle. The 32 individual AIFs estimated by the MRTM, which had considerable intra‐patient and inter‐patient variability, were similar to directly measured AIFs in the literature and using the MRTM AIFs in a pharmacokinetic model to derive estimated individual cardiac outputs provided physiologically reasonable results. The MRTM individual AIFs gave better fits with smaller sum of squared errors and equally reproducible estimate of kinetic parameters compared with a previous reported population AIF measured from remote arteries. The individual MRTM AIFs were also used to obtain a mean local tissue AIF for the unique population of this study, which further improved the reproducibility of the estimated kinetic parameters. The MRTM can be applied to DCE‐MRI studies of bone metastases from prostate cancers to provide an AIF from which reproducible quantitative DCE‐MRI parameters can be derived, thus help standardize DCE‐MRI studies in cancer patients. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Calculation of cerebral perfusion parameters using regional arterial input functions identified by factor analysis

PURPOSE: To calculate regional cerebral blood volume (rCBV), regional cerebral blood flow (rCBF), and regional mean transit time (rMTT) accurately, an arterial input function (AIF) is required. In this study we identified a number of AIFs using factor analysis of dynamic studies (FADS), and performed the cerebral perfusion calculation pixel by pixel using the AIF that was located geometrically closest to a certain voxel. MATERIALS AND METHODS: To verify the robustness of the method, simulated images were generated in which dispersion or delay was added in some arteries and in the corresponding cerebral gray matter (GM), white matter (WM), and ischemic tissue. Thereafter, AIFs were determined using the FADS method and simulations were performed using different signal-to-noise ratios (SNRs). Simulations were also carried out using an AIF from a single pixel that was manually selected. In vivo results were obtained from normal volunteers and patients. RESULTS: The FADS method reduced the underestimation of rCBF due to dispersion or delay that often occurs when only one AIF represents the entire brain. CONCLUSION: This study indicates that the use of FADS and the nearest-AIF method is preferable to manual selection of one single AIF.