Tumor angiogenesis, vascularization, and contrast-enhanced magnetic resonance imaging

Machine learning for semi-automated classification of glioblastoma, brain metastasis and central nervous system lymphoma using magnetic resonance advanced imaging

Background

Differentiating glioblastoma, brain metastasis, and central nervous system lymphoma (CNSL) on conventional magnetic resonance imaging (MRI) can present a diagnostic dilemma due to the potential for overlapping imaging features. We investigate whether machine learning evaluation of multimodal MRI can reliably differentiate these entities.

Methods

Preoperative brain MRI including diffusion weighted imaging (DWI), dynamic contrast enhanced (DCE), and dynamic susceptibility contrast (DSC) perfusion in patients with glioblastoma, lymphoma, or metastasis were retrospectively reviewed. Perfusion maps (rCBV, rCBF), permeability maps (K-trans, Kep, Vp, Ve), ADC, T1C+ and T2/FLAIR images were coregistered and two separate volumes of interest (VOIs) were obtained from the enhancing tumor and non-enhancing T2 hyperintense (NET2) regions. The tumor volumes obtained from these VOIs were utilized for supervised training of support vector classifier (SVC) and multilayer perceptron (MLP) models. Validation of the trained models was performed on unlabeled cases using the leave-one-subject-out method. Head-to-head and multiclass models were created. Accuracies of the multiclass models were compared against two human interpreters reviewing conventional and diffusion-weighted MR images.

Results

Twenty-six patients enrolled with histopathologically-proven glioblastoma (n=9), metastasis (n=9), and CNS lymphoma (n=8) were included. The trained multiclass ML models discriminated the three pathologic classes with a maximum accuracy of 69.2% accuracy (18 out of 26; kappa 0.540, P=0.01) using an MLP trained with the VpNET2 tumor volumes. Human readers achieved 65.4% (17 out of 26) and 80.8% (21 out of 26) accuracies, respectively. Using the MLP VpNET2 model as a computer-aided diagnosis (CADx) for cases in which the human reviewers disagreed with each other on the diagnosis resulted in correct diagnoses in 5 (19.2%) additional cases.

Conclusions

Our trained multiclass MLP using VpNET2 can differentiate glioblastoma, brain metastasis, and CNS lymphoma with modest diagnostic accuracy and provides approximately 19% increase in diagnostic yield when added to routine human interpretation.

Automation of pattern recognition analysis of dynamic contrast-enhanced MRI data to characterize intratumoral vascular heterogeneity

PURPOSE

To automate dynamic contrast-enhanced MRI (DCE-MRI) data analysis by unsupervised pattern recognition (PR) to enable spatial mapping of intratumoral vascular heterogeneity.

METHODS

Three steps were automated. First, the arrival time of the contrast agent at the tumor was determined, including a calculation of the precontrast signal. Second, four criteria-based algorithms for the slice-specific selection of number of patterns (NP) were validated using 109 tumor slices from subcutaneous flank tumors of five different tumor models. The criteria were: half area under the curve, standard deviation thresholding, percent signal enhancement, and signal-to-noise ratio (SNR). The performance of these criteria was assessed by comparing the calculated NP with the visually determined NP. Third, spatial assignment of single patterns and/or pattern mixtures was obtained by way of constrained nonnegative matrix factorization.

RESULTS

The determination of the contrast agent arrival time at the tumor slice was successfully automated. For the determination of NP, the SNR-based approach outperformed other selection criteria by agreeing >97% with visual assessment. The spatial localization of single patterns and pattern mixtures, the latter inferring tumor vascular heterogeneity at subpixel spatial resolution, was established successfully by automated assignment from DCE-MRI signal-versus-time curves.

CONCLUSION

The PR-based DCE-MRI analysis was successfully automated to spatially map intratumoral vascular heterogeneity. Magn Reson Med 79:1736-1744, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Vascular Reactivity Maps in Patients with Gliomas Using Breath-Holding BOLD fMRI

BACKGROUND AND PURPOSE

To evaluate whether breath-holding (BH) blood oxygenation level-dependent (BOLD) fMRI can quantify differences in vascular reactivity (VR), as there is a need for improved contrast mechanisms in gliomas.

METHODS

16 patients (gliomas, grade II = 5, III = 2, IV = 9) were evaluated using the BH paradigm: 4-second single deep breath followed by 16 seconds of BH and 40 seconds of regular breathing for five cycles. VR was defined as the difference in BOLD signal between the minimal signal seen at the end of the deep breath and maximal signal seen at the end of BH (peak-to-trough). VR was measured for every voxel and compared for gray versus white matter and tumor versus normal contralateral brain. VR maps were compared to the areas of enhancement and FLAIR/T2 abnormality.

RESULTS

VR was significantly lower in normal white matter than gray matter (P < .05) and in tumors compared to the normal, contralateral brain (P < 0.002). The area of abnormal VR (1103 ± 659 mm²) was significantly greater (P = .019) than the enhancement (543 ± 530 mm²), but significantly smaller (P = .0011) than the FLAIR abnormality (2363 ± 1232 mm²). However, the variability in the areas of gadolinium contrast enhancement versus VR abnormality indicates that the contrast mechanism elicited by BH (caused by abnormal arteriolar smooth muscles) appears to be fundamentally different from the contrast mechanism of gadolinium enhancement (caused by the presence of "leaky" gap junctions).

CONCLUSIONS

BH maps based on peak-to-trough can be used to characterize VR in brain tumors. VR maps in brain tumor patients appear to be caused by a different mechanism than gadolinium enhancement.

Comparison of Cerebral Blood Volume and Plasma Volume in Untreated Intracranial Tumors

Purpose

Plasma volume and blood volume are imaging-derived parameters that are often used to evaluation intracranial tumors. Physiologically, these parameters are directly related, but their two different methods of measurements, T1-dynamic contrast enhanced (DCE)- and T2-dynamic susceptibility contrast (DSC)-MR utilize different model assumptions and approaches. This poses the question of whether the interchangeable use of T1-DCE-MRI derived fractionated plasma volume (vp) and relative cerebral blood volume (rCBV) assessed using DSC-MRI, particularly in glioblastoma, is reliable, and if this relationship can be generalized to other types of brain tumors. Our goal was to examine the hypothetical correlation between these parameters in three most common intracranial tumor types.

Methods

Twenty-four newly diagnosed, treatment naïve brain tumor patients, who had undergone DCE- and DSC-MRI, were classified in three histologically proven groups: glioblastoma (n = 7), meningioma (n = 9), and intraparenchymal metastases (n = 8). The rCBV was obtained from DSC after normalization with the normal-appearing anatomically symmetrical contralateral white matter. Correlations between these parameters were evaluated using Pearson (r), Spearman's (ρ) and Kendall’s tau-b (τ_B) rank correlation coefficient.

Results

The Pearson, Spearman and Kendall’s correlation between vp with rCBV were r = 0.193, ρ = 0.253 and τ_B = 0.33 (p-_Pearson = 0.326, p-_Spearman= 0.814 and p-_Kendall= 0.823) in glioblastoma, r = -0.007, ρ = 0.051 and τ_B = 0.135 (p-_Pearson = 0.970, p-_Spearman= 0.765 and p-_Kendall= 0.358) in meningiomas, and r = 0.289, ρ = 0.228 and τ_B = 0.239 (p-_Pearson = 0.109, p-_Spearman= 0.210 and p-_Kendall= 0.095) in metastasis.

Conclusion

Results indicate that no correlation exists between vp with rCBV in glioblastomas, meningiomas and intraparenchymal metastatic lesions. Consequently, these parameters, as calculated in this study, should not be used interchangeably in either research or clinical practice.

The effects of applying breast compression in dynamic contrast material-enhanced MR imaging

PURPOSE

To evaluate the effects of breast compression on breast cancer masses, contrast material enhancement of glandular tissue, and quality of magnetic resonance (MR) images in the identification and characterization of breast lesions.

MATERIALS AND METHODS

This was a HIPAA-compliant, institutional review board-approved retrospective study, with waiver of informed consent. Images from 300 MR imaging examinations in 149 women (mean age ± standard deviation, 51.5 years ± 10.9; age range, 22-76 years) were evaluated. The women underwent diagnostic MR imaging (no compression) and MR-guided biopsy (with compression) between June 2008 and February 2013. Breast compression was expressed as a percentage relative to the noncompressed breast. Percentage enhancement difference was calculated between noncompressed- and compressed-breast images obtained in early and delayed contrast-enhanced phases. Breast density, lesion type (mass vs non-masslike enhancement [NMLE]), lesion size, percentage compression, and kinetic curve type were evaluated. Linear regression, receiver operating characteristic (ROC) curve analysis, and κ test were performed.

RESULTS

Mean percentage compression was 31.3% ± 9.2 (range, 5.8%-53.2%). Percentage enhancement was higher in noncompressed- versus compressed-breast studies in early (146% ± 66 vs 107% ± 42, respectively; P < .001) and delayed (158% ± 68 vs 107% ± 42, respectively; P = .1) phases. Among breast lesions, 12% (seven of 59) were significantly smaller when compressed, which led to underestimation of TNM classification (P < .001). Breast masses (n = 35) showed significantly higher early percentage enhancement (157% ± 71) than lesions with NMLE (n = 15, 120% ± 40; P = .02) and a percentage enhancement difference (47.5% ± 64 vs 17% ± 28, respectively; P = .023). Kinetic curve performance for identifying invasive cancer decreased after compression (area under ROC curve = 0.53 vs 0.71, respectively; P = .02). Breast compression resulted in complete loss of enhancement of nine of 210 lesions (4%).

CONCLUSION

Breast compression during biopsy affected breast lesion detection, lesion size, and dynamic contrast-enhanced MR imaging interpretation and performance. Limiting the application of breast compression is recommended, except when clinically necessary.

Non-invasive quantification of anti-angiogenic therapy by contrast-enhanced MRI in experimental pancreatic cancer

BACKGROUND

Currently, early changes of tumor vasculature after angiogenesis inhibition can only be evaluated by histopathology, a method not suitable in a clinical setting.

PURPOSE

To quantify effects of different angiogenesis inhibitors on the microvasculature of orthotopically implanted pancreatic cancers by contrast-enhanced magnetic resonance imaging (MRI) in order to establish a non-invasive technique for monitoring antiangiogenic cancer treatment.

MATERIAL AND METHODS

DSL-6A/C1 pancreatic cancers were implanted in the pancreas of 109 Lewis rats. Three weeks later, antiangiogenic treatment was initiated by administration of Bevacizumab (n = 38) or Suramin (n = 27) while the control group (n = 44) remained untreated. Dynamic MRI was performed 24 h, 1 week, and 4 weeks after treatment initiation. Fractional tumor plasma volume (fPV, %) and vascular permeability (K(PS), mL/min/100 cc) were calculated based on the MRI data by using a pharmacokinetic model.

RESULTS

Twenty-four hours after the initial dose, a significant decline in K(PS) was observed in the Bevacizumab group compared to the control and Suramin group (0.002 ± 0.008; 0.057 ± 0.046 and 0.064 ± 0.062 (mean ± SD); P < 0.05). At 1 week, fPV was significantly smaller in Bevacizumab and Suramin treated tumors compared to control tumors (6.25 ± 2.74, 7.47 ± 3.44, and 15.10 ± 9.97, respectively; P < 0.05). Differences in tumor volumes were first observed after 4 weeks of treatment with significantly larger control tumors (4380.3 ± 1590.6 vs. 869.6 ± 717.2 and 1676.5 ± 2524.1 mm(3); P < 0.05).

CONCLUSION

Dynamic MRI can quantify antiangiogenic effects on tumor microvasculature before changes in tumor volumes are detectable. Thus, this technique is a reasonable addition to morphological MRI and may be applied as an alternative to histopathology.

Regional heterogeneity changes in DCE-MRI as response to isolated limb perfusion in experimental soft-tissue sarcomas

Experimental evidence supports an association between heterogeneity in tumor perfusion and response to chemotherapy/radiotherapy, disease progression and malignancy. Therefore, changes in tumor perfusion may be used to assess early effects of tumor treatment. However, evaluating changes in tumor perfusion during treatment is complicated by extensive changes in tumor type, size, shape and appearance. Therefore, this study assesses the regional heterogeneity of tumors by dynamic contrast-enhanced MRI (DCE-MRI) and evaluates changes in response to isolated limb perfusion (ILP) with tumor necrosis factor alpha and melphalan. Data were acquired in an experimental cancer model, using a macromolecular contrast medium, albumin-(Gd-DTPA)45. Small fragments of BN 175 (a soft-tissue sarcoma) were implanted in eight brown Norway rats. MRI of five drug-treated and three sham-treated rats was performed at baseline and 1 h after ILP intervention. Properly co-registered baseline and follow-up DCE-MRI were used to estimate the volume transfer constant (K(trans) ) pharmacokinetic maps. The regional heterogeneity was estimated in 16 tumor sectors and presented in cumulative map-volume histograms. On average, ILP-treated tumors showed a decrease in regional heterogeneity on the histograms. This study shows that heterogenic changes in regional tumor perfusion, estimated using DCE-MRI pharmacokinetic maps, can be measured and used to assess the short-term effects of a potentially curative treatment on the tumor microvasculature in an experimental soft-tissue sarcoma model.

3-T dynamic contrast-enhanced MRI of the breast: pharmacokinetic parameters versus conventional kinetic curve analysis

OBJECTIVE

The purpose of this article is to evaluate the incremental value of pharmacokinetic analysis of dynamic contrast-enhanced (DCE) MRI compared with conventional breast MRI (morphology plus kinetic curve type analysis) in characterizing breast lesions as malignant or benign.

SUBJECTS AND METHODS

Patients underwent 3D high-resolution T1-weighted contrast-enhanced MRI and DCE-MRI at 3 T and had pathology-proven diagnosis (95%) or more than 2 years of follow-up confirming lesion stability (5%). Lesions were identified using the high-spatial-resolution contrast-enhanced MRI. Morphologic features (margin, enhancement, and pattern) and conventional DCE-MRI results (kinetic curve types 1, 2, or 3) or pharmacokinetic parameters (forward volume transfer constant [K(trans)], reverse volume transfer constant [K(ep)], and the extravascular extracellular space volume per unit volume of tissue), were included in multivariate models for prediction of benign versus malignant diagnosis.

RESULTS

Ninety-five patients with 101 lesions were included: 52% of patients were premenopausal and 48% were postmenopausal. Sixty-eight lesions (67.3%) were malignant and 33 (32.7%) were benign. There was a significant association between K(trans) and K(ep) and the diagnosis of benign versus malignant (p < 0.001). The area under the curve for morphologic features (lesion margin and enhancement pattern) was 0.85, whereas inclusion of K(trans) or K(ep) in the model showed similar modest improvement in performance (area under the curve, 0.88-0.89). For DCE-MRI, both pharmacokinetic modeling and kinetic curve type analysis improved characterization of malignant and benign breast lesions. A diagnostic model including lesion morphology plus either pharmacokinetic parameters or kinetic curve assessment showed similar diagnostic performance in characterizing breast lesions.

CONCLUSION

The use of kinetic curve type assessment or pharmacokinetic modeling in conjunction with high-resolution 3D breast MRI appears to offer similar improvement in diagnostic performance. Although morphologic analysis alone provides good characterization of breast lesions on MRI as benign or malignant, analysis of the lesion perfusion on DCE-MRI using either kinetic curve shape assessment or a pharmacokinetic modeling approach improves diagnostic accuracy.

Hypoxic stress and cancer: imaging the axis of evil in tumor metastasis

Tumors emerge as a result of the sequential acquisition of genetic, epigenetic and somatic alterations promoting cell proliferation and survival. The maintenance and expansion of tumor cells rely on their ability to adapt to changes in their microenvironment, together with the acquisition of the ability to remodel their surroundings. Tumor cells interact with two types of interconnected microenvironments: the metabolic cell autonomous microenvironment and the nonautonomous cellular-molecular microenvironment comprising interactions between tumor cells and the surrounding stroma. Hypoxia is a central player in cancer progression, affecting not only tumor cell autonomous functions, such as cell division and invasion, resistance to therapy and genetic instability, but also nonautonomous processes, such as angiogenesis, lymphangiogenesis and inflammation, all contributing to metastasis. Closely related microenvironmental stressors affecting cancer progression include, in addition to hypoxia, elevated interstitial pressure and oxidative stress. Noninvasive imaging offers multiple means to monitor the tumor microenvironment and its consequences, and can thus assist in the understanding of the biological basis of hypoxia and microenvironmental stress in cancer progression, and in the development of strategies to monitor therapies targeted at stress-induced tumor progression.

Assessment of tumor angiogenesis: dynamic contrast-enhanced MRI with paramagnetic nanoparticles compared with Gd-DTPA in a rabbit Vx-2 tumor model

The purpose of this study was to evaluate the suitability of a macromolecular MRI contrast agent (paramagnetic nanoparticles, PNs) for the characterization of tumor angiogenesis. Our aim was to estimate the permeability of PNs in developing tumor vasculature and compare it with that of a low molecular weight contrast agent (Gd-DTPA) using dynamic contrast-enhanced MRI (DCE). Male New Zealand white rabbits (n = 5) underwent DCE MRI 12-14 days after Vx-2 tumor fragments were implanted into the left hind limb. Each contrast agent (PNs followed by Gd-DTPA) was evaluated using a DCE protocol and transendothelial transfer coefficient (K(i)) maps were calculated using a two-compartment model. Two regions of interest (ROIs) were located within the tumor core and hindlimb muscle and five ROIs were placed within the tumor rim. Comparisons were performed using repeated measures analysis of variance (ANOVA). The K(i) values estimated using PNs were significantly lower than those obtained for Gd-DTPA (p = 0.018). When PNs and Gd-DTPA data were analyzed separately, significant differences were identified among tumor rim ROIs for PNs (p < 0.0001), but not for Gd-DTPA data (p = 0.34). The mean K(i) for the tumor rim was significantly greater than that of either the core or the hindlimb muscle for both contrast agents (p < 0.05 for each comparison). In summary, the extravasation of Gd-DTPA was far greater than that of PNs, suggesting that PNs can reveal regional differences in tumor vascular permeability that are not otherwise apparent with clinical contrast agents such as Gd-DTPA. These results suggest that PNs show potential for the noninvasive delineation of tumor angiogenesis.

Visualizing vascular permeability and lymphatic drainage using labeled serum albumin

During the early stages of angiogenesis, following stimulation of endothelial cells by vascular endothelial growth factor (VEGF), the vascular wall is breached, allowing high molecular weight proteins to leak from the vessels to the interstitial space. This hallmark of angiogenesis results in deposition of a provisional matrix, elevation of the interstitial pressure and induction of interstitial convection. Albumin, the major plasma protein appears to be an innocent bystander that is significantly affected by these changes, and thus can be used as a biomarker for vascular permeability associated with angiogenesis. Traditionally, albumin leak in superficial organs was followed by colorimetry or morphometry with the use of albumin binding vital dyes. Over the last years, the introduction of tagged-albumin that can be detected by various imaging methods, such as magnetic resonance imaging and positron emission tomography, opened new possibilities for quantitative three dimension dynamic analysis of permeability in any organ. Using these tools it is now possible to follow not only vascular permeability, but also interstitial convection and lymphatic drain. Active uptake of tagged albumin by caveolae-mediated endocytosis opens the possibility for using labeled albumin for vital staining of cells and cell tracking. This approach was used for monitoring recruitment of perivascular stroma fibroblasts associated with tumor angiogenesis.

Vessel growth and function: depiction with contrast-enhanced MR imaging

Magnetic resonance (MR) imaging is a versatile noninvasive diagnostic tool that can be applied to the entire human body to revealing morphologic, functional, and metabolic information. The authors review how MR imaging can depict both the established and the developing vasculature with techniques involving intravenously administered contrast agents. In addition to macrovascular morphology and flow, MR imaging is able to exploit microvascular properties, including vessel size distribution, hyperpermeability, flow heterogeneity, and possibly also upregulation of endothelial biomarkers. For each MR method, the basic principles, potential acquisition and interpretation pitfalls, solutions, and applications are described. Furthermore, discussion includes current shortcomings and the impact of future developments (eg, higher magnetic field strength systems, targeted macromolecular contrast agents) on the visualization of blood vessel growth and function with contrast-enhanced MR imaging.

Use of H2(15)O-PET and DCE-MRI to measure tumor blood flow

Positron emission tomography (PET) with H2(15)O and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) provide noninvasive measurements of tumor blood flow. Both tools offer the ability to monitor the direct target of antiangiogenic treatment, and their use is increasingly being studied in trials evaluating such drugs. Antiangiogenic therapy offers great potential and, to an increasing extent, benefit for oncological patients in a variety of palliative and curative settings. Because this type of targeted therapy frequently results in consolidation of the tumor mass instead of regression, monitoring treatment response with the standard volumetric approach (Response Evaluation Criteria in Solid Tumors) leads to underestimation of the response rate. Monitoring direct targets of anticancer therapy might be superior to indirect size changes. In addition, measures of tumor blood flow contribute to a better understanding of tumor biology. This review shows that DCE-MRI and H2(15)O-PET provide reliable measures of tumor perfusion, provided that a certain level of standardization is applied. Heterogeneity in scan acquisition and data analysis complicates the interpretation of study results. Also, limitations inherent to both techniques must be considered when interpreting DCE-MRI and H2(15)O-PET results. This review focuses on the technical and physiological aspects of both techniques and aims to provide the essential information necessary to critically evaluate the use of DCE-MRI and H2(15)O-PET in an oncological setting.

Dynamic Contrast-Enhanced MRI Quantification of Synovium Microcirculation in Experimental Arthritis

OBJECTIVE

Our objective was to analyze MRI contrast-enhancement patterns in arthritic and nonarthritic knees and the relationship of those patterns with clinical, laboratory, and histologic synovium markers.

MATERIALS AND METHODS

Dynamic contrast-enhanced MRI was performed in nine arthritic and three nonarthritic knees of juvenile rabbits. A two-compartment pharmacokinetic model of signal intensity-time data was implemented to generate parametric maps of signal slope, maximal percentage of signal change, capillary permeability, leakage space volume, and time-to-peak. MRI values were compared with clinical, laboratory, and histologic markers for evaluation of synovial changes during the progression of arthritis.

RESULTS

Parametric maps of capillary permeability and signal slope depicted significant differences between arthritic and nonarthritic knees. Arthritic knees showed increased capillary permeability (p = 0.006) and signal slope (p = 0.01) with time after onset of disease as opposed to nonarthritic knees (permeability, p = 0.65; slope, p = 0.56). Significant correlations were found between temporal changes in capillary permeability (p = 0.002), signal slope (p = 0.003), and serum concentrations of amyloid A. No relationship was noted between any MRI parameters and histologic scores. The discriminative power of MRI indexes varied according to the stage of arthritis: time-to-peak was most accurate for differentiation of presence versus absence of arthritis in early arthritis (day 1, p = 0.0002), and signal slope was most accurate in midterm arthritis (day 14, p = 0.001).

CONCLUSION

In vivo capillary permeability and signal slope have distinctive dynamic MRI properties. The accuracy of MRI parameters for diagnostic evaluation of experimental arthritis differs according to the stage of disease.

Antivascular cancer treatments: functional assessments by dynamic contrast-enhanced magnetic resonance imaging

New anticancer therapeutics that target tumor blood vessels promise improved efficacy and tolerability in humans. Early phase 1 drug trials have shown that the maximum tolerated dose may be inappropriate for more advanced clinical studies with efficacy endpoints. More advanced clinical trials have demonstrated that morphologic assessments of tumor response are of limited value for gauging the efficacy of treatment. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) can serve as pharmacodynamic indicator of biological activity for antivascular cancer drugs by helping to define the biologically active dose. DCE-MRI studies may also predict the efficacy of treatment on the basis of changes observed. If DCE-MRI is to be used for the selection of antivascular drugs that advance into efficacy trials, then it will be necessary to develop standardized approaches to measurement and robust analytic approaches with clear accepted endpoints specified prospectively that have biological validity. Such developments will be essential for multicenter trials in which it will be necessary to establish effective cross-site standardization of measurements and evaluation.

Simultaneous measurement of arterial input function and tumor pharmacokinetics in mice by dynamic contrast enhanced imaging: effects of transcytolemmal water exchange

A noninvasive technique for simultaneous measurement of the arterial input function (AIF) for gadodiamide (Omniscan) and its uptake in tumor was demonstrated in mice. Implantation of a tumor at a suitable location enabled its visualization in a cardiac short axis image. Sets of gated, low-resolution saturation recovery images were acquired from each of five tumor-bearing mice following intravenous administration of a bolus of contrast agent (CA). The AIF was extracted from the signal intensity changes in left ventricular blood using literature values of the CA relaxivity and a precontrast T1 map. The time-dependent 1H2O relaxation rate constant (R1 = 1/T1) in the tumor was modeled using the BOLus Enhanced Relaxation Overview (BOLERO) method in two modes regarding the equilibrium transcytolemmal water exchange system: 1) constraining it exclusively to the fast exchange limit (FXL) (the conventional assumption), and 2) allowing its transient departure from FXL and access to the fast exchange regime (FXR), thus designated FXL/FXR. The FXL/FXR analysis yielded better fittings than the FXL-constrained analysis for data from the tumor rims, whereas the results based on the two modes were indistinguishable for data from the tumor cores. For the tumor rims, the values of Ktrans (the rate constant for CA transfer from the vasculature to the interstitium) and ve (volume fraction of the tissue extracellular and extravascular space) returned from FXL/FXR analysis are consistently greater than those from the FXL-constrained analysis by a factor of 1.5 or more corresponding to a CA dose of 0.05 mmole/kg.

In vivo assessment of antiangiogenic activity of SU6668 in an experimental colon carcinoma model

PURPOSE

The purpose of this research was to assess in vivo by dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) the antiangiogenic effect of SU6668, an oral, small molecule inhibitor of the angiogenic receptor tyrosine kinases vascular endothelial growth factor receptor 2 (Flk-1/KDR), platelet-derived growth factor receptor, and fibroblast growth factor receptor 1.

EXPERIMENTAL DESIGN

A s.c. tumor model of HT29 human colon carcinoma in athymic mice was used. DCE-MRI with a macromolecular contrast agent was used to measure transendothelial permeability and fractional plasma volume, accepted surrogate markers of tumor angiogenesis. CD31 immunohistochemical staining was used for assessing microvessels density and vessels area. Experiments were performed after 24 h, and 3, 7, and 14 days of treatment.

RESULTS

DCE-MRI clearly detected the early effect (after 24 h of treatment) of SU6668 on tumor vasculature as a 51% and 26% decrease in the average vessel permeability measured in the tumor rim and core (respectively). A substantial decrease was also observed in average fractional plasma volume in the rim (59%) and core (35%) of the tumor. Histological results confirmed magnetic resonance imaging findings. After 3, 7, and 14 days of treatment, postcontrast magnetic resonant images presented a thin strip of strongly enhanced tissue at the tumor periphery; histology examination showed that this hyperenhanced ring corresponded to strongly vascularized tissue adjacent but external to the tumor. Histology also revealed a strong decrease in the thickness of peripheral viable tissue, with a greatly reduced vessel count. SU6668 greatly inhibited tumor growth, with 60% inhibition at 14 days of treatment.

CONCLUSIONS

DCE-MRI detected in vivo the antiangiogenic efficacy of SU6668.

In vivo mapping of spontaneous mammary tumors in transgenic mice using MRI and ultrasonography

PURPOSE

To compare T1- or T2-weighted magnetic resonance imaging (MRI) and ultrasonography (US) as tools for in vivo mapping of different tissue components in spontaneous tumors of transgenic mice.

MATERIALS AND METHODS

Human-like mammary adenocarcinomas from FVB/neuT transgenic mice were analyzed by T2-weighted and T1-weighted MRI at 4.7 Tesla and US and then, after excision, were paraffin-embedded for histologic analysis. The histologic samples were prepared taking care to obtain sections that spatially matched the MRI and US images as precisely as possible.

RESULTS

US can obtain basic information such as the size of developing tumors in experimental animals and can identify necrotic areas. T2-weighted MRI, especially if compared to T1-weighted MRI and/or US, allows advanced analysis of morphologic aspects, with high resolution in the differentiation of details of necrotic areas such as coagulation, liquefaction, biphasic splitting of cysts, and fibrotic and lipidic infiltration.

CONCLUSION

Of the three methods, T2-weighted MRI provides the most information about the anatomy of tumors. However, when distinctions between the different types of necrosis are not needed, US analysis is to be preferred for its practicality.

Dynamic contrast-enhanced MR imaging

Dynamic contrast-enhanced magnetic resonance imaging is a useful clinical tool in evaluation of soft tissue neoplasm and lymph nodes in head and neck. It is thought to be a useful predictor of response to radiotherapy for head and neck carcinoma and used to monitor the treatment and distinguish post-therapeutic changes from recurrent mass with greater confidence. It can be used to distinguish between normal and malignant tissue and to differentiate a malignant lymphoma from other lymph nodal enlargements. The technique utilizes relative differences in microvasculature and microcirculation between malignant and non-malignant tissue to achieve greater contrast in signal imaging following bolus contrast administration. This article explains the underlying principles and imaging techniques for this new diagnostic tool. The clinical applications and technical challenges are discussed. The future challenges and some contradictions in results are also outlined.

Perfusion MR imaging of extracranial tumor angiogenesis

Dynamic contrast-enhanced MRI (DCE-MRI) using small molecular weight gadolinium chelates enables noninvasive imaging characterization of tissue vascularity. Depending on the technique used, data reflecting tissue perfusion (blood flow, blood volume, mean transit time), microvessel permeability surface area product, and extracellular leakage space can be obtained. Insights into these physiological processes can be obtained from inspection of kinetic enhancement curves or by the application of complex compartmental modeling techniques. Combining morphologic and kinetic features can increase the accuracy of clinical diagnoses. Potential clinical applications include screening for malignant disease, lesion characterization, monitoring lesion response to treatment, and assessment of residual disease. Newer applications include prognostication, pharmacodynamic assessments of antivascular anticancer drugs, and predicting efficacy of treatment. For dynamic MRI to enter into widespread clinical practice, it will be necessary to develop standardized approaches to measurement and robust analysis approaches.

MRI estimation of the arterial input function in mice

RATIONAL AND OBJECTIVES

Dynamic contrast-enhanced (DCE) MRI offers the potential to provide quantitative maps of tumor perfusion parameters and is therefore expected to play an important role in the study of cancer in small animal models. Extraction of such information from DCE-MRI data requires a methodology for determination of the arterial input function (AIF) for the target tissues. An MRI based method for observation of the AIF in a mouse model is demonstrated in the present report.

MATERIALS AND METHODS

A series of short-axis cardiac images was acquired during the first pass of a bolus of Gadodiamide using a low-resolution, EEG-gated, saturation-recovery gradient echo imaging sequence. The AIF was then extracted from the observed signal intensity changes in the left ventricle (LV) blood pool.

RESULTS

The proposed technique provides sufficient temporal and spatial resolution to accurately characterize the AIF of Gadodiamide in mouse models. The AIF was observed in 4 mice and was found to be qualitatively similar to that previously observed in larger animals. However, significant inter-animal variability in the precise form of the AIF was evident.

CONCLUSIONS

The proposed method for determination of AIF in mice has been shown to be effective and reliable. The inter-animal variability observed in the present study suggests that the AIF should be measured in each animal undergoing analysis by DCE-MRI.

Structural, functional, and molecular MR imaging of the microvasculature

Magnetic resonance imaging (MRI) is widely applied for functional imaging of the microcirculation and for functional and structural studies of the microvasculature. The interest in the capabilities of MRI in noninvasively monitoring changes in vascular structure and function expanded over the past years, with specific efforts directed toward the development of novel imaging methods for quantification of angiogenesis. Molecular imaging approaches hold promise for further expansion of the ability to characterize the microvasculature. Exciting applications for MRI are emerging in the study of the biology of microvessels and in the evaluation of potential pharmaceutical modulators of vascular function and development, and preclinical MRI tools can serve for the design of mechanism-of-action-based noninvasive clinical methods for monitoring response to therapy. The aim of this review is to provide a current snapshot of recent developments in this rapidly evolving field.

Modulation of the pharmacokinetics of macromolecular contrast material by avidin chase: MRI, optical, and inductively coupled plasma mass spectrometry tracking of triply labeled albumin

The goal of this work was to develop an MRI method for mapping the clearance of interstitial macromolecular plasma proteins after their extravasation from permeable blood vessels. To that end, a well-defined window of exposure to elevated blood levels was generated by inducing rapid clearance of macromolecular contrast material from the blood. Experimental removal of the intravascular component allowed subsequent tracking of clearance from the interstitial compartment in the absence of further contrast extravasation. The contrast material was based on albumin triply labeled with biotin, fluorescent tag, and GdDTPA, allowing optical, inductively coupled plasma mass spectrometry (ICP-MS) and MRI detection. The biotin tag was used here for in vivo chasing of the contrast material from the blood by intravenous administration of avidin. Upon administration of avidin the contrast material disappeared from the blood vessels and was cleared by the liver and spleen as detected by MRI, fluorescence of blood samples and histological sections, and by ICP-MS. Nonbiotinylated fluorescent albumin was not affected by administration of avidin. Contrast material that extravasated from leaky blood vessels in a VEGF overexpressing tumor, prior to administration of avidin, was not cleared by the addition of avidin and showed continued interstitial convection. Thus, avidin-chase provides an effective tool for in vivo manipulation of the arterial input function by providing experimental control over the rate of clearance of the contrast material from the circulation.

Newer MR imaging techniques for head and neck

Dynamic and functional imaging techniques are being developed to improve the evaluation of various pathologic processes of the head and neck region. These techniques include dynamic contrast-enhanced MR imaging for evaluating soft tissue masses and cervical lymph nodes, the use of ultrasmall superparamagnetic iron oxide contrast agent, and functional techniques such as in vivo and in vitro MR spectroscopy of head and neck cancer and lymph nodes and apparent diffusion coefficient mapping of parotid glands. These techniques can help to differentiate nonmalignant tissue from malignant tumors and lymph nodes and can aid in differentiating residual malignancies from postradiation changes. From methodological development, they are making the critical transition to preclinical and clinical validating methods and eventually to widespread clinical tools.

Osteogenic and Ewing sarcomas: estimation of necrotic fraction during induction chemotherapy with dynamic contrast-enhanced MR imaging

Dynamic contrast material-enhanced magnetic resonance (MR) images of primary osteogenic sarcoma (n = 19) and Ewing sarcoma (n = 10) were reviewed in 29 patients undergoing induction chemotherapy before surgery. Histogram distributions containing the initial slope and pharmacokinetic model parameters from individual voxels within each tumor were fitted for each patient. The histogram analysis of initial slope from the tumor correlated well with percentage necrosis as determined at pathologic examination (r = 0.60, P <.001), as did a two-compartment pharmacokinetic model (r = 0.64, P <.001). Both methods predicted tumors with clinically important degrees of necrosis (ie, > or =90%) in a large majority of cases. The ability to determine response to induction chemotherapy by means of noninvasive monitoring of necrotic fraction with perfusion MR imaging methods may provide useful prognostic information and help surgical planning.

In vivo mapping of fractional plasma volume (fpv) and endothelial transfer coefficient (Kps) in solid tumors using a macromolecular contrast agent: correlation with histology and ultrastructure

Contrast-enhanced MRI, immunostaining and electron microscopy were used to detect areas of intense angiogenesis in experimental tumors. This work was also aimed at evaluating the possible effect of the surrounding tissues on tumor microvasculature and at studying the penetration of macromolecules in avascular areas. Human colon carcinoma cells were implanted in subcutaneous tissue of nude mice. Dynamic T(1)-weigthed 3D pulse sequences were acquired before and after administration of Gd-DTPA-albumin to obtain parametric maps of fractional plasma volume (fpv) and transendothelial permeability (Kps). The maps suggested that tumor can be subdivided into 4 zones located in the peripheral rim (zones I-II) or in the core (zones III-IV) of the tumor itself. Significant differences (p<0.001) were found in the values of Kps and fpv of zones I-II with respect to zones III-IV. In the peripheral rim, permeability was significantly higher (p<0.01) in the muscle-peripheral region (zone I) with respect to the skin-peripheral region (zone II). In areas with high Kps, histological and ultrastructural examination revealed clusters of newly formed vessels and signs of intense permeability. Numerous vascular vesicular organs were visible in these areas. In the tumoral core, analysis of the microcirculatory parameters revealed regions with mild permeability (zone III) and regions with negligible permeability (zone IV). These 2 zones were discriminated by the average value of Kps (p<0.05), while their fpv was not significantly different. Upon histological examination, the tumoral core exhibited necrotic areas; CD31 immunocytochemistry exhibited that it was diffusely hypovascularized with large avascular areas. Upon ultrastructural examination, capillaries were rarely visible and exhibited signs of endothelial cell damage. The results suggest that segmentation based on microvascular parameters detects in vivo zones characterized by immunocytochemical and ultrastructural aspects of intense angiogenesis. The finding that a certain amount of contrast agent penetrates in the tumoral core suggests that high oncotic and hydrostatic pressure only partially hinders the penetration of macromolecules.

Tumor vascular architecture and function evaluated by non-invasive susceptibility MRI methods and immunohistochemistry

PURPOSE

To investigate the physiological origins responsible for the varying blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI) responses to carbogen (95% O(2)/5% CO(2)) breathing observed with different tumor types.

MATERIALS AND METHODS

Susceptibility contrast-enhanced MRI using the exogenous blood pool contrast agent NC100150 to determine blood volume and vessel size, and immunohistochemical-derived morphometric parameters, were determined in GH3 prolactinomas and RIF-1 fibrosarcomas, both grown in mice, which exhibited very different BOLD responses to carbogen.

RESULTS

Administration of NC100150 increased the R(2)* and R(2) rates of both tumor types, and indicated a significant four-fold larger blood volume in the GH3 tumor. The ratio deltaR(2)*/deltaR(2) showed that the capillaries in the GH3 were two-fold larger than those in the RIF-1, in agreement with morphometric analysis. Carbogen breathing induced a significant 25% decrease in R(2)* in the GH3 prolactinoma, whereas the response in the RIF-1 fibrosarcoma was negligible.

CONCLUSION

Low blood volume and small vessel size (and hence reduced hematocrit) are two reasons for the lack of R(2)* change in the RIF-1 with carbogen breathing. BOLD MRI is sensitive to erythrocyte-perfused vessels, whereas exogenous contrast agents interrogate the total perfused vascular volume. BOLD MRI, coupled with a carbogen challenge, provides information on functional, hemodynamic tumor vasculature.

Dynamic contrast-enhanced MRI in clinical oncology: current status and future directions

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is performed after the administration of intravenous contrast medium to noninvasively access tumor vascular characteristics. DCE-MRI techniques utilizing low-molecular-weight contrast media have successfully made the transition from methodological development to preclinical and clinical validation and are now rapidly becoming mainstream clinical tools. DCE-MRI using macromolecular contrast medium (MMCM) can also assay microvascular characteristics of human tumor xenografts. MMCM approval for human use will occur soon. The success of both techniques depends on their ability to demonstrate quantitative differences of contrast medium behavior in a variety of tissues. Evidence is mounting that kinetic parameters correlate with immunohistochemical surrogates of tumor angiogenesis, including microvessel density, and with pathologic tumor grade. DCE-MRI is being applied to monitor the clinical effectiveness of a variety of treatments, including antiangiogenic drugs. Kinetic parameter changes following treatment have correlated with histopathological outcome and patient survival. This article reviews the current clinical status of low-molecular-weight DCE-MRI and reviews the potential of MMCM techniques for evaluating human tumors. Ongoing challenges faced by DCE-MRI as clinical and research tools will be explored.

Magnetic resonance of vascular anomalies

More than half of the patients with vascular anomalies referred to the Vascular Anomalies Clinic at Children's Hospital, Boston, have been misdiagnosed. A major consequence of misdiagnosis is inappropriate treatment, including deferral of necessary treatment and inappropriate use of pharmacotherapy, radiation, surgery, and embolotherapy. Hemangiomas and vascular malformations are distinct categories with completely different biologic and clinical behavior, therapeutic requirements, and imaging features. This article reviews the biologic classification of vascular anomalies and corresponding MR imaging features, and presents a simplified guide to diagnosis.

Evaluation of the microangioarchitecture of tumors by use of monochromatic x-rays

RATIONALE AND OBJECTIVES

The purpose was to compare differences in the depiction of small vessels in tumors seen on microangiograms from a conventional soft x-ray system with those from a synchrotron radiation system and to evaluate the microangioarchitecture of these tumors and the growth of neovascularization.

METHODS

VX2 carcinomas transplanted to the auricles of 15 rabbits randomized into three groups were investigated after 1, 3, and 7 days. Five normal rabbits were the controls. Barium sulfate, to which sufficient gelatin had been added, was injected into the auricular artery. Microangiograms of auricle specimens were obtained with both a conventional soft x-ray system and a synchrotron radiation system.

RESULTS

The conventional x-ray system could detect vessels with diameters of approximately 100 microm, whereas the monochromatic synchrotron radiation system could detect small vessels with diameters of less than 25 microm. On day 1, there was moderate vascularization and flexure vessels were present in the transplantation area. On day 3, dilated vessels were present in the peripheral areas of the tumors and tortuous vessels in the central areas. On day 7, hypovascular areas had increased in the central area.

CONCLUSIONS

The synchrotron radiation system confirmed the growth of neovascularization in the tumors. This system should provide a useful tool for evaluating the microangioarchitecture of tumors.

Antiangiogenesis therapy. Current and future agents

Approaches to cancer therapy for most tumors in adults and children have changed little in 50 years: surgery, radiation, and chemotherapy are standard for many solid tumors. When the concept of angiogenesis in cancer biology was introduced in the 1970s, there was little recognition of the therapeutic potential of attacking a tumor's blood supply. Advances in understanding the molecular processes that regulate tumor blood supply and novel agents that can interfere with them have generated a great deal of scientific interest and excitement. This article reviews the current understanding of angiogenesis and its role in cancer then discusses new therapeutic options in animals and humans, with a focus on pediatric tumors and the potential for treating them.

Vascular differences detected by MRI for metastatic versus nonmetastatic breast and prostate cancer xenografts

Several studies have linked vascular density, identified in histologic sections, to "metastatic risk." Functional information of the vasculature, not readily available from histologic sections, can be obtained with contrast-enhanced MRI to exploit for therapy or metastasis prevention. Our aims were to determine if human breast and prostate cancer xenografts preselected for differences in invasive and metastatic characteristics established correspondingly different vascular volume and permeability, quantified here with noninvasive MRI of the intravascular contrast agent albumin-GdDTPA. Tumor vascular volume and permeability of human breast and prostate cancer xenografts were characterized using MRI. Parallel studies confirmed the invasive behavior of these cell lines. Vascular endothelial growth factor (VEGF) expression in the cell lines was measured using ELISA and Western blots. Metastasis to the lungs was evaluated with spontaneous as well as experimental assay. Metastatic tumors formed vasculature with significantly higher permeability or vascular volume (P<.05, two-sided unpaired t test). The permeability profile matched VEGF expression. Within tumors, regions of high vascular volume usually exhibited low permeability whereas regions of low vascular volume exhibited high permeability. We observed that although invasion was necessary, without adequate vascularization it was not sufficient for metastasis to occur.

Imaging of cardiac and paracardiac masses

Magnetic resonance imaging (MRI) and computed tomography (CT) are important imaging modalities for the noninvasive characterization of cardiac and paracardiac masses. They are, in general, superior to other modalities (e.g., echocardiography) in their ability to delineate the exact location and the extent of the lesion and to demonstrate the effects of the lesion on surrounding structures. MRI and CT may also be helpful in suggesting a specific diagnosis, because some tumors have rather characteristic locations and appearances. In addition, both modalities can be extremely helpful in both treatment planning and posttreatment follow-up because they are noninvasive, reproducible, and enable detection of residual or recurrent mass.

Applications of magnetic resonance in model systems: tumor biology and physiology

A solid tumor presents a unique challenge as a system in which the dynamics of the relationship between vascularization, the physiological environment and metabolism are continually changing with growth and following treatment. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) studies have demonstrated quantifiable linkages between the physiological environment, angiogenesis, vascularization and metabolism of tumors. The dynamics between these parameters continually change with tumor aggressiveness, tumor growth and during therapy and each of these can be monitored longitudinally, quantitatively and non-invasively with MRI and MRS. An important aspect of MRI and MRS studies is that techniques and findings are easily translated between systems. Hence, pre-clinical studies using cultured cells or experimental animals have a high connectivity to potential clinical utility. In the following review, leaders in the field of MR studies of basic tumor physiology using pre-clinical models have contributed individual sections according to their expertise and outlook. The following review is a cogent and timely overview of the current capabilities and state-of-the-art of MRI and MRS as applied to experimental cancers. A companion review deals with the application of MR methods to anticancer therapy.