Assessment of an advanced image-based technique to calculate virtual monoenergetic computed tomographic images from a dual-energy examination to improve contrast-to-noise ratio in examinations using iodinated contrast media.
BACKGROUND
Following the trend of low-radiation dose computed tomographic (CT) imaging, concerns regarding the detectability of low-contrast lesions have been growing. The goal of this research was to evaluate whether a new image-based algorithm (Mono+) for virtual monoenergetic imaging with a dual-energy CT scanner can improve the contrast-to-noise ratio (CNR) and conspicuity of these low-contrast objects when using iodinated contrast media.


METHODS
Four circular phantoms of different diameter (10-40 cm) with an iodine insert at the center were scanned at a fixed radiation dose with different single- (80, 100, 120 kV) and dual-energy protocols (80/140 kV, 80/140 Sn kV, 100/140 Sn kV) using a dual-source CT system. In addition, an anthropomorphic abdominal phantom with different low-contrast lesions was scanned with the settings previously mentioned but also at only a half and a quarter of the initial dose. Dual-energy data were processed, and virtual monoenergetic images (range, 40-190 keV) were generated. Beside the established technique, a newly developed prototype algorithm to calculate monoenergetic images (Mono+) was used. To avoid noise increase at lower calculated energies, which is a known drawback of virtual monoenergetic images at low kilo electron-volt, a regional spatial frequency-based recombination of the high signal at lower energies and the superior noise properties at medium energies is performed to optimize CNR in case of Mono+ images. The CNR and low-contrast detectability were evaluated.


RESULTS
For all phantom sizes, the Mono+ technique provided increasing iodine CNR with decreasing kilo electron-volt, with the optimum CNR obtained at the lowest energy level of 40 keV. For all investigated phantom sizes, CNR of Mono+ images at low kilo electron-volt was superior to the CNR in single-energy images at an equivalent radiation dose and even higher than the CNR obtained with 80-kV protocols. In case of the anthropomorphic phantom, low-contrast detectability in monoenergetic images was, for all settings, similar to the circular phantoms, best for the voltage combination 80/140 Sn kV, irrespective of the dose level. For all dual-energy voltage combinations, the Mono+ algorithm led to superior results compared with single-energy imaging.


CONCLUSIONS
With regard to optimized iodine CNR, it is more efficient to perform dual-energy scans and compute virtual monoenergetic images at 40 keV using the Mono+ technique than to perform low kilovolt scans. Given the improved CNR, the Mono+ algorithm could be very useful in improving both detection and differential diagnosis of abdominal lesions, specifically low-contrast lesions, as well as in other anatomical regions where improved iodine CNR is beneficial.
From the *Computed Tomography, Imaging & Therapy Division, Healthcare Sector, Siemens Medical Solutions, Malvern, PA; †Computed Tomography, Imaging & Therapy Division, Healthcare Sector, Siemens AG, Forchheim; ‡Institute of Diagnostic Radiology, Eberhard-Karls-Universität Tübingen, Tübingen; and §Institute of Medical Physics, Friedrich-Alexander-Universität Erlangen, Erlangen, Germany.
Katharine L
KL
Grant
Thomas G
TG
Flohr
Bernhard
B
Krauss
Martin
M
Sedlmair
Christoph
C
Thomas
Bernhard
B
Schmidt
D000465:Algorithms; D003287:Contrast Media; D007455:Iodine; D019047:Phantoms, Imaging; D011857:Radiographic Image Interpretation, Computer-Assisted; D059629:Signal-To-Noise Ratio; D014057:Tomography, X-Ray Computed / Q000379:methods*
D003287:Contrast Media; D007455:Iodine