A systematic approach towards the objective evaluation of low-contrast performance in MDCT: Combination of a full-reference image fidelity metric and a software phantom

Objectives

To assess the feasibility of an objective approach for the evaluation of low-contrast detectability in multidetector computed-tomography (MDCT) by combining a virtual phantom containing simulated lesions with an image quality metric.

Materials and methods

A low-contrast phantom containing hypodense spheric lesions (−20 HU) was scanned on a 64-slice MDCT scanner at 4 different dose levels (25, 50, 100, 200 mAs). In addition, virtual round hypodense low-contrast lesions (20 HU object contrast) based on real CT data were inserted into the lesion-free section of the datasets. The sliding-thin-slab algorithm was applied to the image data with an increasing slice-thickness from 1 to 15 slices. For each dataset containing simulated lesions a lesion-free counterpart was reconstructed and post-processed in the same manner. The low-contrast performance of all datasets containing virtual lesions was determined using a full-reference image quality metric (modified multiscale structural similarity index, MS-SSIM*). The results were validated against a reader-study of the real lesions.

Results

For all dose levels and lesion sizes there was no statistically significant difference between the low-contrast performance as determined by the image quality metric when compared to the reader study (p < 0.05). The intraclass correlation coefficient was 0.72, 0.82, 0.90 and 0.84 for lesion diameters of 4 mm, 5 mm, 8 mm and 10 mm, respectively. The use of the sliding-thin-slab algorithm improves lesion detectability by a factor ranging from 1.15 to 2.69 when compared with the original axial slice (0.625 mm).

Conclusion

The combination of a virtual phantom and a full-reference image quality metric enables a systematic, automated and objective evaluation of low-contrast detectability in MDCT datasets and correlates well with the judgment of human readers.     

A versatile flow phantom for intravoxel incoherent motion MRI

Although there have been many advancements in cancer research, much is still unknown about the heterogeneous tumor microenvironment. Diffusion-weighted MRI has proven to be a viable and versatile microstructural probe. Diffusion-weighted sequences specifically sensitive to intravoxel incoherent motion (IVIM) have seen a recent resurgence of interest as they promise to provide a valuable window on the vascular microenvironment. To understand, test, and optimize IVIM-sensitive approaches, a complex flow phantom was constructed to mimic certain characteristics of the tumor microenvironment such as tortuous microvasculature, heterogeneous vascular permeability, and interstitial fluid pressure buildup. Results using this phantom on a clinical scanner platform confirmed IVIM sensitivity to microscopic flow effects. Biexponential fitting of signal decay curves enabled quantitative extraction of perfusion fraction, IVIM-related pseudodiffusivity, and tissue diffusivity. Parametric maps were also generated, illustrating the potential utility of IVIM-sensitive imaging in clinical settings. The flow phantom proved to be an effective test-bed for validating and optimizing the IVIM-MRI technique to provide surrogate markers for microvascular properties.     

A comparison between the electronic magnification (EM) and true magnification (TM) of breast phantom images using a CDMAM phantom

Purpose

To provide a comparison between the image quality of electronically magnified (EM) and geometric, or true, magnification (TM) mammographic images.

Materials and methods

One Computed Radiography (CR), one Digital Radiography (DR) and two screen–film (S–F) imaging systems were investigated. A Contrast-Detail Mammography (CDMAM) phantom was used as a test object. Three contact images and three sets of TM images with a magnification factor of 1.8 were taken on all systems. Software was used to zoom the contact images by a factor of 1.8 to produce EM images. Two observers evaluated all of the images. An Image Quality Figure and contrast detail curve were used to analyze the observer data and Mann–Whitney U-tests were performed to determine the statistical significance of the results.

Results

No significant differences were found between soft copy and hard copy for any imaging modality. No significant difference in contrast detail detectability (CDD) was seen between EM images from the two digital systems and TM images on S–F systems. The results for the DR EM images and S–F TM images also showed no differences. The CDD of DR TM images was significantly better than both EM and S–F TM images.

Conclusion

Digitally zoomed images offer the same level of CDD as S–F TM images, and so may be viably used in their place. DR systems offer greater CDD than conventional S–F images, when comparing the TM images. This implies that doses can be greatly reduced for TM views using DR systems, while maintaining acceptable image quality.     

一个基于基本概念和原理性错误的"超声波骨密度仪标准体模"

指出和分析了鄢铃等研制的所谓"超声波骨密度仪校准体模"的一系列基本概念和原理性错误,对其实用价值提出了否定性意见.     

Quantitation of in vitro coronary artery calcium using ultrafast computed tomography

Ultrafast computed tomography (UFCT) has the potential to quantify coronary hydroxyapatite (HAP). However, no definitive studies validating this technique are available. We constructed a human chest phantom model with coronary arteries represented by cylindrical holes containing: (1) calcium chloride solutions, (2) a block of HAP immersed in paraffin (without partial volume effect), and (3) HAP granules embedded in a gelatin matrix (with partial volume effect). We scanned this model to determine the relationship between measured CT number per voxel and density of the calcium per voxel. The relationships between CT number and concentration of calcium chloride was linear (r = 0.992 to 0.999). Using a commercially available standard bone mineral phantom, we were able to estimate the concentration of HAP to an accuracy from 94 to 97% when partial volume effects were absent. However, when partial volume effects were present, two methods of estimating HAP produced significant errors (1 to 384%, and 17 to 52%). We conclude that significant partial voluming errors degrade the accuracy of HAP quantitation and that further evaluation and corrections are needed before such quantitation is clinically applied. © 1994 Wiley-Liss,Inc..     

A novel MRI-compatible brain ventricle phantom for validation of segmentation and volumetry methods

Purpose:

To create a standardized, MRI‐compatible, life‐sized phantom of the brain ventricles to evaluate ventricle segmentation methods using T₁‐weighted MRI. An objective phantom is needed to test the many different segmentation programs currently used to measure ventricle volumes in patients with Alzheimer's disease.

Materials and Methods:

A ventricle model was constructed from polycarbonate using a digital mesh of the ventricles created from the 3 Tesla (T) MRI of a subject with Alzheimer's disease. The ventricle was placed in a brain mold and surrounded with material composed of 2% agar in water, 0.01% NaCl and 0.0375 mM gadopentetate dimeglumine to match the signal intensity properties of brain tissue in 3T T₁‐weighted MRI. The 3T T₁‐weighted images of the phantom were acquired and ventricle segmentation software was used to measure ventricle volume.

Results:

The images acquired of the phantom successfully replicated in vivo signal intensity differences between the ventricle and surrounding tissue in T₁‐weighted images and were robust to segmentation. The ventricle volume was quantified to 99% accuracy at 1‐mm voxel size.

Conclusion:

The phantom represents a simple, realistic and objective method to test the accuracy of lateral ventricle segmentation methods and we project it can be extended to other anatomical structures. J. Magn. Reson. Imaging 2012;36:476–482. © 2012 Wiley Periodicals, Inc.