Diffusion-weighted MR imaging in monitoring the effect of a vascular targeting agent on rhabdomyosarcoma in rats

Posttreatment imaging in head and neck cancer

Posttreatment imaging is done when a recurrent tumour is suspected, to confirm the presence of such a lesion and to determine its extent. The extent of a recurrent cancer is important information for determining the possibility of salvage therapy. Imaging may also be used to monitor tumour response and to try to detect recurrent or persistent disease before it becomes clinically evident, possibly with a better chance for successful salvage. This article reviews the expected imaging findings after treatment of head and neck squamous cell cancer, and how to differentiate these from persistent or recurrent cancer. The relative value of anatomical and biological imaging modalities, including newer techniques such as diffusion-weighted magnetic resonance imaging, is addressed. The imaging findings in treatment-induced complications, such as tissue necrosis, sometimes difficult to differentiate from cancer, are explained.

Technology insight: water diffusion MRI--a potential new biomarker of response to cancer therapy

Diffusion-weighted MRI (DW-MRI) is a functional imaging technique that displays information about the extent and direction of random water motion in tissues. Water movement in tissues is modified by interactions with hydrophobic cellular membranes, intracellular organelles and macromolecules. DW-MRI provides information on extracellular-space tortuosity, tissue cellularity and the integrity of cellular membranes. Images can be sensitive to large or small displacements of water, therefore, macroscopic water flows and microscopic water displacements in the extracellular space can be depicted. Preclinical and clinical data indicate a number of potential roles of DW-MRI in the characterization of malignancy, including determination of lesion aggressiveness and monitoring response to therapy. This Review outlines the biological basis of observations made on DW-MRI and describes how measurements are acquired and quantified, and discusses the interpretation of images and limitations of the technique. The strength of evidence for adoption of DW-MRI as a biomarker for the assessment of tumor response is presented.

[Imaging of response to treatment in oncology]

Imaging plays a crucial role in oncology to assist in the management of patients and selection of drug regimen. Recent advances in imaging techniques allowing to predict and evaluate response to treatments in oncology will be reviewed. The standard in the evaluation of response to treatment is based on the measurement of lesion size. Functional imaging assesses physiological or molecular processes that may be earlier indicators of early response to treatment. Dynamic imaging of tumor vascularization assesses the biodistribution of a contrast agent within tumoral tissues. Diffusion-weighted MR imaging can differentiate free water from water restricted by tissues, providing an assessment of tumor cellularity. MR spectroscopy assesses the relative quantity of specific chemical components within normal and tumoral tissues. 18 FDG PET imaging provides an assessment of the metabolic activity of tissues. FDG uptake is proportional to cellular proliferation and number of viable cells within a tumor. Results from studies assessing the role of these emerging imaging techniques remain preliminary and the medical community must determine their respective role in the routine evaluation of response to treatment in oncological patients.

Diffusion weighted imaging in small rodents using clinical MRI scanners

Diffusion weighted imaging (DWI) has emerged as a unique and powerful non-invasive magnetic resonance imaging (MRI) technique with a major potential impact on imaging-based diagnosis in a variety of clinical applications including oncology and tissue viability assessment. In light of increasing demand for applying this technique in preclinical investigations using small animals, we have explored the potentials of a clinical magnet for acquiring the DWI in rats and mice with either cerebral ischemia or solid tumors. Through technical adaptation and optimization, we have been able to perform a series of clinically relevant animal studies with conclusions based on DWI quantification. Focusing more on practical aspects and cross-referencing with the current literature, this paper is aimed to summarize our ongoing DWI studies on small rodents with stroke and tumors, and to provide protocols for researchers to replicate similar techniques in their own preclinical and clinical studies.

Correlation of MRI biomarkers with tumor necrosis in Hras5 tumor xenograft in athymic rats

Magnetic resonance imaging (MRI) can measure the effects of therapies targeting the tumor vasculature and has demonstrated that vascular-damaging agents (VDA) induce acute vascular shutdown in tumors in human and animal models. However, at subtherapeutic doses, blood flow may recover before the induction of significant levels of necrosis. We present the relationship between changes in MRI biomarkers and tumor necrosis. Multiple MRI measurements were taken at 4.7 T in athymic rats (n = 24) bearing 1.94 +/- 0.2-cm3 subcutaneous Hras5 tumors (ATCC 41000) before and 24 hours after clinically relevant doses of the VDA, ZD6126 (0-10 mg/kg, i.v.). We measured effective transverse relaxation rate (R2*), initial area under the gadolinium concentration-time curve (IAUGC(60/150)), equivalent enhancing fractions (EHF(60/150)), time constant (K(trans)), proportion of hypoperfused voxels as estimated from fit failures in K(trans) analysis, and signal intensity (SI) in T2-weighted MRI (T(2)W). ZD6126 treatment induced > 90% dose-dependent tumor necrosis at 10 mg/kg; correspondingly, SI changes were evident from T2W MRI. Although R2* did not correlate, other MRI biomarkers significantly correlated with necrosis at doses of > or = 5 mg/kg ZD6126. These data on Hras5 tumors suggest that the quantification of hypoperfused voxels might provide a useful biomarker of tumor necrosis.

Diffusion-weighted MRI in the body: applications and challenges in oncology

OBJECTIVE

In this article, we present the basic principles of diffusion-weighted imaging (DWI) that can aid radiologists in the qualitative and quantitative interpretation of DW images. However, a detailed discussion of the physics of DWI is beyond the scope of this article. A short discussion ensues on the technical aspects of performing DWI in the body. The emerging applications of DWI for tumor detection, tumor characterization, distinguishing tumor tissue from nontumor tissue, and monitoring and predicting treatment response are highlighted. The challenges to widespread adoption of the technique for cancer imaging in the body are discussed.

CONCLUSION

DWI derives its image contrast from differences in the motion of water molecules between tissues. Such imaging can be performed quickly without the need for the administration of exogenous contrast medium. The technique yields qualitative and quantitative information that reflects changes at a cellular level and provides unique insights about tumor cellularity and the integrity of cell membranes. Recent advances enable the technique to be widely applied for tumor evaluation in the abdomen and pelvis and have led to the development of whole-body DWI.

Body diffusion-weighted MR imaging using high b-value for malignant tumor screening: usefulness and necessity of referring to T2-weighted images and creating fusion images

RATIONALE AND OBJECTIVES

To evaluate the potential usefulness of high b-value body diffusion-weighted images (DWIs) as a screening tool in the depiction of abdominal malignant tumors.

MATERIALS AND METHODS

We selected 110 abdominal magnetic resonance examinations (1.5 T; 60 men; age range, 25-90 years) with and without malignant tumors (n = 37 and n = 73, respectively). Axial DWIs were obtained by single-shot spin-echo (SE) type echo planar imaging (EPI) sequence with inversion pulse (repetition time, 6,800 msec; echo time, 100 msec; T1, 150 msec; b value, 1,000 sec/mm(2)) without breath-holding. Two radiologists independently interpreted the DWIs, T2-weighted images (T2-WI), all three types of images including DWIs, T2-WIs, and fusion images at the same time (DWIs + T2-WIs + fusion) with 7-14 days' interval, and the diagnostic confidence for each patient was scored.

RESULTS

The area under the curve (AUC) of the composite receiver operating characteristic (ROC) curve of DWIs + T2-WIs + fusion (0.904) was significantly higher than those of DWIs (0.720; P < .001) and T2-WIs (0.822; P < .05). Both sensitivity and specificity were higher in DWIs + T2-WIs + fusion (89.5% and 81.9%, respectively) compared with those of DWIs (72.4% and 59.0%; P < .01 and P < .001, respectively).

CONCLUSIONS

Abdominal high b-value DWIs have a high sensitivity and specificity for malignant tumors when T2-WIs are referred and image fusion technique is employed, suggesting that it may potentially be a new screening tool.

Predicting response of colorectal hepatic metastasis: value of pretreatment apparent diffusion coefficients

OBJECTIVE

The purposes of this study were to determine whether the pretreatment apparent diffusion coefficients (ADCs) of hepatic metastatic lesions from colorectal cancer are predictive of response to chemotherapy and to compare the ADCs of metastatic lesions before and after chemotherapy.

SUBJECTS AND METHODS

Twenty patients with potentially operable hepatic lesions larger than 1 cm in diameter metastatic from colorectal carcinoma were prospectively evaluated with diffusion-weighted imaging at three b values before and after chemotherapy. Quantitative ADC maps were calculated with images with b values of 0, 150, and 500 s/mm2 (ADC0-500) and with images with b values of 150 and 500 s/mm2 (ADC150-500). Regions of interest were drawn around metastatic lesions and randomly over liver. The mean ADC0-500 and mean ADC150-500 of metastatic lesions before and after chemotherapy were compared according to response defined by Response Evaluation Criteria in Solid Tumors criteria.

RESULTS

Twenty-five responding and 15 nonresponding metastatic lesions were evaluated. Nonresponding lesions had a significantly higher pretreatment mean ADC0-500 and mean ADC150-500 than did responding lesions (Mann-Whitney U test, p < 0.002). There was a linear regression relation (r2 = 0.34, p = 0.02) between percentage size reduction of metastatic lesions and pretreatment mean ADC150-500. After chemotherapy, responding lesions had a significant increase in mean ADC0-500 and ADC150-500 (Wilcoxon's signed rank, p = 0.025). No significant change was observed in nonresponding metastatic lesions (Wilcoxon's signed rank, p > 0.5) or in normal liver parenchyma (Wilcoxon's signed rank, p > 0.4).

CONCLUSION

High pretreatment mean ADC0-500 and mean ADC150-500 of colorectal hepatic metastatic lesions were predictive of poor response to chemotherapy. A significant increase in mean ADC0-500 and ADC150-500 was observed in metastatic lesions that responded to chemotherapy. These findings may have implications for development of individualized therapy.

High-b value diffusion-weighted MRI for detecting pancreatic adenocarcinoma: preliminary results

OBJECTIVE

The objective of our study was to evaluate the usefulness of high-b value diffusion-weighted MRI (DWI) in the detection of pancreatic adenocarcinoma.

SUBJECTS AND METHODS

Twenty-six patients with pancreatic adenocarcinoma were included in the study. Twenty-three other patients who were being followed up due to pancreatic diseases other than adenocarcinoma were included as control subjects. All patients and subjects underwent DWI, and the images were evaluated by three blinded radiologists.

RESULTS

Receiver operating characteristic (ROC) curve analysis yielded A(z) values (i.e., area under the ROC curve) of 0.998, 0.998, and 0.995 for the three radiologists. The mean sensitivity and specificity for the detection of pancreatic adenocarcinoma were 96.2% and 98.6%, respectively. The kappa values indicating interobserver agreement between different pairs of radiologists were in the category of excellent.

CONCLUSION

High-b value DWI allows the detection of pancreatic adenocarcinoma with a high sensitivity and specificity.

Extracranial applications of diffusion-weighted magnetic resonance imaging

Diffusion-weighted MRI has become more and more popular in the last couple of years. It is already an accepted diagnostic tool for patients with acute stroke, but is more difficult to use for extracranial applications due to technical challenges mostly related to motion sensitivity and susceptibility variations (e.g., respiration and air-tissue boundaries). However, thanks to the newer technical developments, applications of body DW-MRI are starting to emerge. In this review, we aim to provide an overview of the current status of the published data on DW-MRI in extracranial applications. A short introduction to the physical background of this promising technique is provided, followed by the current status, subdivided into three main topics, the functional evaluation, tissue characterization and therapy monitoring.

Evaluation of tumour necrosis during chemotherapy with diffusion-weighted MR imaging: preliminary results in osteosarcomas

BACKGROUND

During successful chemotherapy of osteosarcomas tumour size does not diminish significantly because the therapy has limited impact on the mineralized matrix of the tumour. Treatment response is considered successful if, histologically, more than 90% of tumour cells show necrosis.

OBJECTIVE

To determine if osteosarcomas change their water diffusion during preoperative chemotherapy in relation to the amount of tumour necrosis.

MATERIALS AND METHODS

Eight patients (age 11-19 years) with histologically proven limb osteosarcoma underwent T1-weighted, fat-suppressed T2-weighted and contrast-enhanced T1-weighted spin-echo imaging together with diffusion-weighted EPI sequences (b = 700) at 1.5 T before and after five cycles of standard chemotherapy. Tumour volume and apparent diffusion coefficient (ADC) maps were calculated before and after chemotherapy. The degree of tumour necrosis after chemotherapy was assessed using the histological Salzer-Kuntschik classification (grades 1-6).

RESULTS

During chemotherapy, the ADC values of osteosarcomas changed significantly. The ADC of untreated tumour was 2.1 +/- 0.4 x 10(-3) mm(2)/s (mean +/- SD) (95% CI 1.6-2.0). The ADC of chemotherapy-treated sarcomas was 2.5 +/- 0.4 x 10(-3) mm(2)/s (95% CI 1.8-2.2). Necrotic areas, which were confirmed by macroscopic examination, showed ADC values up to 2.7 x 10(-3) mm(2)/s. Four patients with little viable tumour tissue within the neoplasm (Salzer-Kuntschik grades 1-2) had an increase in ADC of 0.4 up to 0.7 x 10(-3) mm(2)/s. Four patients with larger areas of viable tumour (Salzer-Kuntschik grade 4) showed a lesser increase in ADC of 0.0 up to 0.3 x 10(-3) mm(2)/s. The differences in ADC values in tumour tissue before and after chemotherapy were highly significant (P = 0.01).

CONCLUSION

During chemotherapy of osteosarcomas, tumour ADC changes are related to the degree of tumour necrosis.

Head and neck cancer: how imaging predicts treatment outcome

Sophisticated imaging methods, such as computed tomography, magnetic resonance imaging and positron emission tomography, play an increasingly important role in the management of head and neck cancer. Pretreatment imaging findings have predictive value for patient outcome, independently from the currently used TNM classification, and may be used to tailor treatment to the individual patient. Based on per-treatment imaging, individualised replanning during radiotherapy may ameliorate tumour control rates and reduce toxic effects to normal tissues. Early posttreatment imaging studies contain important prognostic information, and allow selection of patients for further treatment or watchful waiting.

Diffusion-weighted magnetic resonance imaging and its application to cancer

Diffusion-weighted magnetic resonance imaging (DW-MRI) provides image contrast through measurement of the diffusion properties of water within tissues. Application of diffusion sensitising gradients to the MR pulse sequence allows water molecular displacement over distances of 1–20 μm to be recognised. Diffusion can be predominantly unidirectional (anisotropic) or not (isotropic). Combining images obtained with different amounts of diffusion weighting provides an apparent diffusion coefficient (ADC) map. In cancer imaging DW-MRI has been used to distinguish brain tumours from peritumoural oedema. It is also increasingly exploited to differentiate benign and malignant lesions in liver, breast and prostate where increased cellularity of malignant lesions restricts water motion in a reduced extracellular space. It is proving valuable in monitoring treatment where changes due to cell swelling and apoptosis are measurable as changes in ADC at an earlier stage than subsequent conventional radiological response indicators.

The response of RIF-1 fibrosarcomas to the vascular-disrupting agent ZD6126 assessed by in vivo and ex vivo 1H magnetic resonance spectroscopy

The response of radiation-induced fibrosarcoma 1 (RIF-1) tumors treated with the vascular-disrupting agent (VDA) ZD6126 was assessed by in vivo and ex vivo 1H magnetic resonance spectroscopy (MRS) methods. Tumors treated with 200 mg/kg ZD6126 showed a significant reduction in total choline (tCho) in vivo 24 hours after treatment, whereas control tumors showed a significant increase in tCho. This response was investigated further within both ex vivo unprocessed tumor tissues and tumor tissue metabolite extracts. Ex vivo high-resolution magic angle spinning (HRMAS) and 1H MRS of metabolite extracts revealed a significant reduction in phosphocholine and glycerophosphocholine in biopsies of ZD6126-treated tumors, confirming in vivo tCho response. ZD6126-induced reduction in choline compounds is consistent with a reduction in cell membrane turnover associated with necrosis and cell death following disruption of the tumor vasculature. In vivo tumor tissue water diffusion and lactate measurements showed no significant changes in response to ZD6126. Spin-spin relaxation times (T2) of water and metabolites also remained unchanged. Noninvasive 1H MRS measurement of tCho in vivo provides a potential biomarker of tumor response to VDAs in RIF-1 tumors.

High-B-value diffusion-weighted MRI in colorectal cancer

OBJECTIVE

The purpose of this article is to evaluate the usefulness of high-b-value diffusion-weighted MRI (DW-MRI) in the detection of colorectal adenocarcinoma.

CONCLUSION

High-b-value DW-MRI allows detection of colorectal adenocarcinoma with a high sensitivity and specificity.

Colorectal hepatic metastases: quantitative measurements using single-shot echo-planar diffusion-weighted MR imaging

The purpose of this study was to obtain quantitative measurements of the apparent diffusion coefficient (ADC1), flow insensitive apparent diffusion coefficient (ADC2) and perfusion fraction (F) of colorectal hepatic metastases using DWI and to compare these measurements with those obtained in liver parenchyma. Forty patients with 66 hepatic metastases from colorectal carcinoma were prospectively evaluated using DWI with three b values. Quantitative maps of the ADC1 (using b = 0, 150, 500 s/mm2 images), ADC2 (using b = 150, 500 s/mm2 images) and fractional variation (F) between ADC1 and ADC2, which reflects perfusion fraction, were calculated. The ADC1, ADC2 and F derived from metastases and liver parenchyma were compared. The mean ADC1 values of liver parenchyma and metastases were significantly higher than the mean ADC2 values (P < 0.0001, paired t-test). Colorectal metastases were found to have higher mean ADC1 and ADC2 values compared with liver (P < 0.0001, Mann-Whitney test). However, the estimated F was found to be lower in metastases compared to liver (P = 0.03, Mann-Whitney test). Colorectal hepatic metastases were characterised by higher ADC1 and ADC2 values, but lower F values compared to liver.

Diffusion Changes in a Tumor and Peritumoral Tissue After Stereotactic Irradiation for Brain Tumors

OBJECTIVE

Changes in apparent diffusion coefficient (ADC) in a tumor and peritumoral tissue after stereotactic irradiation (STI) were evaluated, and then the therapeutic efficacy of ADC measurement was assessed.

METHODS

In 20 tumors, diffusion-weighted imaging within 1 week before and 2-4 weeks after STI was performed. The normalized ADC (nADC) was measured. The nADCs in the tumor and peritumoral region before STI were compared with those after STI and the change in tumor nADC compared with the change in tumor size.

RESULTS

The nADC of the tumors was significantly higher 2-4 weeks after STI compared with that before STI. The nADC of the peritumoral regions 2-4 weeks after STI did not differ significantly from that before STI. A significant difference in the nADC at 2-4 weeks after STI was observed between the responder and nonresponder groups.

CONCLUSIONS

Changes in nADC as measured by diffusion-weighted imaging can predict response to STI.

Liver tumor model with implanted rhabdomyosarcoma in rats: MR imaging, microangiography, and histopathologic analysis

In compliance with institutional regulations for care and use of laboratory animals, the aim of this study was to establish and characterize a rodent liver tumor model to provide a platform for preclinical assessment of new diagnostic and therapeutic strategies. A rhabdomyosarcoma tumor was implanted in the right and left liver lobes of 20 rats, for a total of 40 tumors. T1- and T2-weighted magnetic resonance (MR) images, diffusion-weighted images, and dynamic susceptibility contrast agent-enhanced perfusion-weighted images were obtained up to 16 days after tumor implantation and were compared with postmortem three-dimensional computed tomographic (CT) images, digital microangiograms, and histopathologic findings. Fifteen tumors were examined with proton ((1)H) MR spectroscopy. All tumors grew, with a mean volume doubling time of 2.2 days +/- 0.9 (standard deviation) and a final size of 591 mm(3)+/- 124. The rhabdomyosarcoma tumor showed hypervascularity at MR imaging, three-dimensional CT, microangiography, and histologic analysis. On dynamic susceptibility contrast-enhanced perfusion-weighted images, the maximum signal intensity decrease differed in time and extent between the tumor and the liver, with a significantly (P < .001) higher relative blood volume, relative blood flow, and permeability value in the tumor than in the liver. With (1)H MR spectroscopy, the rhabdomyosarcoma tumor and the liver featured significant (P < .001) choline and lipid peaks, respectively. Implantation of a rhabdomyosarcoma tumor in the livers of rats is feasible and reproducible, and this animal model seems promising for future testing of new diagnostic and therapeutic strategies.

Effect of vascular targeting agent in rat tumor model: dynamic contrast-enhanced versus diffusion-weighted MR imaging

PURPOSE

To compare dynamic contrast material-enhanced magnetic resonance (MR) imaging and diffusion-weighted MR imaging for noninvasive evaluation of early and late effects of a vascular targeting agent in a rat tumor model.

MATERIALS AND METHODS

The study protocol was approved by the local ethics committee for animal care and use. Thirteen rats with one rhabdomyosarcoma in each flank (26 tumors) underwent dynamic contrast-enhanced imaging and diffusion-weighted echo-planar imaging in a 1.5-T MR unit before intraperitoneal injection of combretastatin A4 phosphate and at early (1 and 6 hours) and later (2 and 9 days) follow-up examinations after the injection. Histopathologic examination was performed at each time point. The apparent diffusion coefficient (ADC) of each tumor was calculated separately on the basis of diffusion-weighted images obtained with low b gradient values (ADC(low); b = 0, 50, and 100 sec/mm(2)) and high b gradient values (ADC(high); b = 500, 750, and 1000 sec/mm(2)). The difference between ADC(low) and ADC(high) was used as a surrogate measure of tissue perfusion (ADC(low) - ADC(high) = ADC(perf)). From the dynamic contrast-enhanced MR images, the volume transfer constant k and the initial slope of the contrast enhancement-time curve were calculated. For statistical analyses, a paired two-tailed Student t test and linear regression analysis were used.

RESULTS

Early after administration of combretastatin, all perfusion-related parameters (k, initial slope, and ADC(perf)) decreased significantly (P < .001); at 9 days after combretastatin administration, they increased significantly (P < .001). Changes in ADC(perf) were correlated with changes in k (R(2) = 0.46, P < .001) and the initial slope (R(2) = 0.67, P < .001).

CONCLUSION

Both dynamic contrast-enhanced MR imaging and diffusion-weighted MR imaging allow monitoring of perfusion changes induced by vascular targeting agents in tumors. Diffusion-weighted imaging provides additional information about intratumoral cell viability versus necrosis after administration of combretastatin.

Diffusion-weighted magnetic resonance imaging allows noninvasive in vivo monitoring of the effects of combretastatin a-4 phosphate after repeated administration

The noninvasive assessment of anticancer treatment efficacy is very important for the improvement of therapeutic window. The purpose of the present study was to evaluate the antitumoral effects of the vascular targeting agent, combretastatin A-4 phosphate (CA-4-P), at selected time points after repeated intraperitoneal drug administrations (25 mg/kg), using diffusion-weighted magnetic resonance imaging (DW-MRI). The experiments were performed during an overall follow-up period of 3 weeks on WAG/Rij rats with subcutaneously growing rhabdomyosarcomas. Each animal served as its own baseline. The DW-MRI studies were quantified by calculating the apparent diffusion coefficient (ADC) for different low and high b-values to separate the effects on tumor vasculature and cellular integrity. The changes in ADC as well as the extent of necrosis development (proportional to the tumor volume), measured on the MR images, were of comparable magnitude after each treatment. All ADC values showed a significant decrease at 6 hours, followed by a significant increase at 2 days for various CA-4-P administrations. DW-MRI allowed us to monitor both reduction in perfusion and changes in the extent of tumor necrosis after CA-4-P injection. Repeated CA-4-P administration retains efficacy in rat rhabdomyosarcomas, with similar findings after each drug administration.