Quantitative Imaging Parameters of Contrast-Enhanced Micro-Computed Tomography Correlate with Angiogenesis and Necrosis in a Subcutaneous C6 Glioma Model

The aim of this work was to systematically obtain quantitative imaging parameters with static and dynamic contrast-enhanced (CE) X-ray imaging techniques and to evaluate their correlation with histological biomarkers of angiogenesis in a subcutaneous C6 glioma model. Enhancement (E), iodine concentration (CI), and relative blood volume (rBV) were quantified from single- and dual-energy (SE and DE, respectively) micro-computed tomography (micro-CT) images, while rBV and volume transfer constant (Ktrans) were quantified from dynamic contrast-enhanced (DCE) planar images. CI and rBV allowed a better discernment of tumor regions from muscle than E in SE and DE images, while no significant differences were found for rBV and Ktrans in DCE images. An agreement was found in rBV for muscle quantified with the different imaging protocols, and in CI and E quantified with SE and DE protocols. Significant strong correlations (Pearson r > 0.7, p < 0.05) were found between a set of imaging parameters in SE images and histological biomarkers: E and CI in tumor periphery were associated with microvessel density (MVD) and necrosis, E and CI in the complete tumor with MVD, and rBV in the tumor periphery with MVD. In conclusion, quantitative imaging parameters obtained in SE micro-CT images could be used to characterize angiogenesis and necrosis in the subcutaneous C6 glioma model.

Susceptibility of iodine concentration map of dual-energy contrast-enhanced digital mammography for quantitative and tumor enhancement assessment

Background Dual-energy (DE) contrast-enhanced digital mammography (DE-CEDM) provides additional information on tumor angiogenesis. Purpose To investigate the susceptibility of reconstructing color-coded iodine concentration maps on the basis of quantitative calibrations of the iodine concentration and contrast-to-noise ratio (CNR) in DE-CEDM applications. Material and Methods A custom-made phantom filled with iodine concentrations in the range of 0.1-10 mg/cm² was used in calibrations. All DE images were acquired using the GE Senographe Essential system. From DE subtraction images, the image contrast and CNR were obtained, and the quantitative relationship between these two metrics and the iodine concentration at each phantom thickness was investigated. The quantitative CNR calibration curves were applied to reconstruct color-coded iodine maps on a pixel-by-pixel basis. Results Both the mean contrast and mean CNR increased linearly with the iodine concentration. The iodine concentration estimated from the iodine map reconstructed from quantitative CNR calibrations was highly consistent with the desired iodine concentration (R²= 0.989), and smaller relative errors (in the range of 3.0-19.5%) were observed with iodine concentrations not less than 1 mg/cm². Conclusion An iodine concentration map could be reconstructed based on the linear relationship between the CNR and iodine concentration. From the color-coded iodine concentration map, the contrast medium enhancement phenomenon could be further estimated quantitatively, and tumor enhancement patterns could be easily observed.

High Energy Resolution Hyperspectral X-Ray Imaging for Low-Dose Contrast-Enhanced Digital Mammography

Contrast-enhanced digital mammography (CEDM) is an alternative to conventional X-ray mammography for imaging dense breasts. However, conventional approaches to CEDM require a double exposure of the patient, implying higher dose, and risk of incorrect image registration due to motion artifacts. A novel approach is presented, based on hyperspectral imaging, where a detector combining positional and high-resolution spectral information (in this case based on Cadmium Telluride) is used. This allows simultaneous acquisition of the two images required for CEDM. The approach was tested on a custom breast-equivalent phantom containing iodinated contrast agent (Niopam 150®). Two algorithms were used to obtain images of the contrast agent distribution: K-edge subtraction (KES), providing images of the distribution of the contrast agent with the background structures removed, and a dual-energy (DE) algorithm, providing an iodine-equivalent image and a water-equivalent image. The high energy resolution of the detector allowed the selection of two close-by energies, maximising the signal in KES images, and enhancing the visibility of details with the low surface concentration of contrast agent. DE performed consistently better than KES in terms of contrast-to-noise ratio of the details; moreover, it allowed a correct reconstruction of the surface concentration of the contrast agent in the iodine image. Comparison with CEDM with a conventional detector proved the superior performance of hyperspectral CEDM in terms of the image quality/dose tradeoff.

A novel approach to background subtraction in contrast-enhanced dual-energy digital mammography with commercially available mammography devices: Polychromaticity correction

PURPOSE

Contrast-enhanced digital mammography is an image subtraction technique that is able to improve the detectability of lesions in dense breasts. One of the main sources of error, when the technique is performed by means of commercial mammography devices, is represented by the intrinsic polychromaticity of the x-ray beams. The aim of the work is to propose an iterative procedure, which only assumes the knowledge of a small set of universal quantities, to take into account the polychromaticity and correct the subtraction results accordingly.

METHODS

In order to verify the procedure, it has been applied to an analytical simulation of a target containing a contrast medium and to actual radiographs of a breast phantom containing cavities filled with a solution of the same medium.

RESULTS

The reconstructed densities of contrast medium were compared, showing very good agreement between the theoretical predictions and the experimental results already after the first iteration. Furthermore, the convergence of the iterative procedure was studied, showing that only a small number of iterations is necessary to reach limiting values.

CONCLUSIONS

The proposed procedure represents an efficient solution to the polychromaticity issue, qualifying therefore as a viable alternative to inverse-map functions.

Clinical study of contrast-enhanced digital mammography and the evaluation of blood and lymphatic microvessel density

OBJECTIVE

To correlate image parameters in contrast-enhanced digital mammography (CEDM) with blood and lymphatic microvessel density (MVD).

METHODS

18 Breast Imaging-Reporting and Data System (BI-RADS)-4 to BI-RADS-5 patients were subjected to CEDM. Craniocaudal views were acquired, two views (low and high energy) before iodine contrast medium (CM) injection and four views (high energy) 1-5 min afterwards. Processing included registration and two subtraction modalities, traditional single-energy temporal (high-energy) and "dual-energy temporal with a matrix", proposed to improve lesion conspicuity. Images were calibrated into iodine thickness, and iodine uptake, contrast, time-intensity and time-contrast kinetic curves were quantified. Image indicators were compared with MVD evaluated by anti-CD105 and anti-podoplanin (D2-40) immunohistochemistry.

RESULTS

11 lesions were cancerous and 7 were benign. CEDM subtraction strongly increased conspicuity of lesions enhanced by iodine uptake. A strong correlation was observed between lymphatic vessels and blood vessels; all benign lesions had <30 blood microvessels per field, and all cancers had more than this value. MVD showed no correlation with iodine uptake, nor with contrast. The most frequent curve was early uptake followed by plateau for uptake and contrast in benign and malignant lesions. The positive-predictive value of uptake dynamics was 73% and that of contrast was 64%.

CONCLUSION

CEDM increased lesion visibility and showed additional features compared with conventional mammography. Lack of correlation between image parameters and MVD is probably due to tumour tissue heterogeneity, mammography projective nature and/or dependence of extracellular iodine irrigation on tissue composition.

ADVANCES IN KNOWLEDGE

Quantitative analysis of CEDM images was performed. Image parameters and MVD showed no correlation. Probably, this is indication of the complex dependence of CM perfusion on tumour microenvironment.

Optimization of a dual-energy contrast-enhanced technique for a photon-counting digital breast tomosynthesis system: I. A theoretical model

PURPOSE

Dual-energy (DE) iodine contrast-enhanced x-ray imaging of the breast has been shown to identify cancers that would otherwise be mammographically occult. In this article, theoretical modeling was performed to obtain optimally enhanced iodine images for a photon-counting digital breast tomosynthesis (DBT) system using a DE acquisition technique.

METHODS

In the system examined, the breast is scanned with a multislit prepatient collimator aligned with a multidetector camera. Each detector collects a projection image at a unique angle during the scan. Low-energy (LE) and high-energy (HE) projection images are acquired simultaneously in a single scan by covering alternate collimator slits with Sn and Cu filters, respectively. Sn filters ranging from 0.08 to 0.22 mm thickness and Cu filters from 0.11 to 0.27 mm thickness were investigated. A tube voltage of 49 kV was selected. Tomographic images, hereafter referred to as DBT images, were reconstructed using a shift-and-add algorithm. Iodine-enhanced DBT images were acquired by performing a weighted logarithmic subtraction of the HE and LE DBT images, The DE technique was evaluated for 20-80 mm thick breasts. Weighting factors, w(t) that optimally cancel breast tissue were computed. Signal-difference-to-noise ratios (SDNRs) between iodine-enhanced and nonenhanced breast tissue normalized to the square root of the mean glandular dose (MGD) were computed as a function of the fraction of the MGD allocated to the HE images. Peak SDNR/ mean square root of MGD and optimal dose allocations were identified. SDNR/ mean square root of MGD and dose allocations were computed for several practical feasible system configurations (i.e., determined by the number of collimator slits covered by Sn and Cu). A practicalsystem configuration an d Sn-Cu filterpair that accounts for the trade-off between SDNR, tube-output, and MGD were selected.

RESULTS

w(t) depends on the Sn-Cu filter combination used, as well as on the breast thickness; to optimally cancel 0% with 50% glandular breast tissue, w(t) values were found to range from 0.46 to 0.72 for all breast thicknesses and Sn-Cu filter pairs studied. The optimal w(t) values needed to cancel all possible breast tissue glandularites vary by less than 1% for 20 mm thick breasts and 18% for 80 mm breasts. The system configuration where one collimator slit covered by Sn is alternated with two collimator slits covered by Cu delivers SDNR/ mean square root of MGD nearest to the peak value. A reasonable compromise is a 0.16 mm Sn-0.23 mm Cu filter pair, resulting in SDNR values between 1.64 and 0.61 and MGD between 0.70 and 0.53 mGy for 20-80 mm thick breasts at the maximum tube current.

CONCLUSIONS

A DE acquisition technique for a photon-counting DBT imaging system has been developed and optimized.

Phantom study to evaluate contrast-medium-enhanced digital subtraction mammography with a full-field indirect-detection system

This phantom study simulates contrast-medium-enhanced digital subtraction mammography (CEDM) and compares subtracted image quality and total mean glandular dose for two alternative spectral combinations available in a GE Senographe DS mammography unit. The first choice takes advantage of large iodine attenuation at low photon energies and uses traditionally available spectra (anode/filter combinations Mo/Mo at 25 kV and Rh/Rh at 40 kV, "Mo25-Rh40"). The second choice, selected from a previous analytical optimization, includes harder spectra obtained by adding external filtration to traditional beams (Rh/Rh at 34 kV and Rh/Rh+5 mm of Al at 45 kV, "Rh34-Rh45H"). Individual images of a custom-made phantom containing tubes of various diameters filled with water- or iodine-based contrast agent were acquired with both spectral combinations. The total breast entrance air kerma, considering subtraction of two images, was limited to 8.76 mGy (1 R). The results were compared to predictions obtained through an analytical formalism that assumes noise of stochastic origin. Individual images were evaluated and subtracted under five combinations of temporal and dual-energy modalities. Signal variance analysis in individual raw images showed important contributions of nonstochastic origin, associated with the software applied to raw images, the curved geometry, and strong attenuation of the phantom cylindrical iodine-filled tubes, causing experimental SNR to vary from 2.2 to 0.8 times the predictions from low to high values of SNR. Iodine contrast in the subtracted images was found to be mainly defined by the spectra, independent of exposure, and linearly dependent on the iodine mass thickness. The highest contrast was obtained with the combined dual-energy temporal subtraction with Rh34-Rh45H, its value was 7% larger than the highest value measured with Mo25-Rh40. As expected, temporal modalities (single and dual energy, any spectral choice) led to higher contrast-over-noise ratio (CNR) than nontemporal dual-energy subtraction, the latter being negligibly small with Mo25-Rh40. CNR for 4 mg iodine/cm2 imaged temporally in a dual-energy fashion with Rh34-Rh45H (iodine imaged at high energy) is about 1.7 times the optimum for Mo25-Rh40 (iodine imaged at low energy). Iodine thicknesses needed to fulfill Rose's criterion were 0.78 +/- 0.02 mg iodine/cm2 for Mo25-Rh40 and 0.54 +/- 0.17 mg iodine/cm2 for Rh34-Rh45H, both lower than the proposed biological concentration of iodine in breast tumors after contrast medium administration. Although similar dose levels were obtained with both spectral choices under dual-energy (temporal and nontemporal) subtraction, the dose obtained in single-energy temporal subtraction with the Mo25 spectrum was 1.2 mGy lower than the dose from the modality offering the highest CNR. In all results considered, the spectral choice Mo25-Rh40 was found to represent an interesting alternative to the use of high-energy hardened spectra for CEDM, particularly when performing dynamic studies of the contrast-agent uptake in breast lesions.