Synopsis

Ultrafast dynamic contrast enhancement (DCE) is a MRI sequence that, when used standalone, can serially screen for breast cancer in 2 minutes. However, each acquisition generates thousands of 2D images in a 4D stack. Color intensity projections (CIP) images are 2 parameter color images that encode the time of arrival (ToA) of contrast agent in the hue (red, orange, yellow, green, cyan, blue) and the amount of contrast enhancement in the brightness. A CIP image of each ultrafast slice provides an informative summary to radiologists with the same sensitivity and specificity to malignancies as the ultrafast 4D stack.

Introduction

Methods

The 115 second ultrafast DCE sequence acquired 19 consecutive frames every 4.26 seconds with 152 slices per frame yielding a 4D stack with 2,888 2D images for each of water and fat. For each of the 89 patients in the study, two radiologists - with 8 and 4 years of experiences reading breast cancer MRIs respectively - recorded a BIRADS [DOrsiCJ2013] using only the CIP images first, and subsequently using both the CIP images and the 4D stack. No prior MRI images were used in the study. A total of 26 patients had malignancies. The p-value for the CIP and 4D sensitivities and specificities were calculated using the Fisher exact test [EmausMJ2015].

CIP summarizes a time series of greyscale images into a single color image by encoding the time of arrival (ToA) of contrast agent at each voxel in the hue (red, orange, yellow, green, cyan, blue) and the amount of contrast agent in the brightness. The time of arrival (ToA) of contrast agent for each voxel was defined as the frame which had the largest increase in signal from the previous frame (Figure 1). The enhancing of the aortic arch was defined as the zero frame and indicated with the red hue. The CIP images for the study were uploaded to clinical PACS so they could be read using the same PACS workstation as the clinical images but clearly labelled for research only.

Results

For reader 1, the CIP and 4D sensitives were 96% and 96% (p=1.0) with specificities of 59% and 70% (p=0.26) respectively. The statistics for reader 2 were 96% and 100% (p=1.0) with 62% and 71% (p=0.34) specificities, respectively. Therefore, there was no statistically significant difference between CIP and 4D stack, in terms of the sensitivity and specificity, when distinguishing between malignancies and benign tumors. The readers could not assign a BIRADS score to CIP in 1 and 5 patients respectively, primarily because of enhancing background [GiessCS2014]. These undefined BIRADS scores resulted in the slightly lower sensitivities and specificities provided above.

Figures 2, 3 and 4 provide examples of CIP for two patients.

Discussion

As the current study shows, CIP provides an effective summary of the thousands of images provided by each ultrafast acquisition. For each voxel, the encoding of the ToA in the hue combined with the amount of contrast agent seems to provide much of the information required by radiologist to detect malignancies while recognizing benign lesions. However, as with any processed version of the ultrafast 4D stack, the original 4D stack should always be reviewed for breast cancer screening with CIP only providing supplementary information.

While the current study was conducted without prior DCE images to aid the radiologists, with serial screening prior DCE images are readily available. Prior DCE images should significantly improve the specificity of the standalone ultrafast sequence found in the current study. The current full diagnostic protocols (FDP) for MRI screening of breast cancer has additional sequences to improve specificity when prior DCE images are not available [AbramoviciG2011]. The FDP typically includes both pre and post contrast high resolution T1 sequences, a high resolution T2 sequence, and/or a DWI sequence, and more recently, an ultrafast sequence.

While the ultrafast sequence runs in about 2 minutes, if used standalone for serial screening for breast cancer, inclusion of the localizers would bring the total acquisition time to about 4 minutes.

Conclusion

Acknowledgements

References

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