Improved sensitivity of computed tomography towards iodine and gold nanoparticle contrast agents via iterative reconstruction methods

Abstract

Computed tomography is a widely used medical imaging technique that has high spatial and temporal resolution. Its weakness is its low sensitivity towards contrast media. Iterative reconstruction techniques (ITER) have recently become available, which provide reduced image noise compared with traditional filtered back-projection methods (FBP), which may allow the sensitivity of CT to be improved, however this effect has not been studied in detail. We scanned phantoms containing either an iodine contrast agent or gold nanoparticles. We used a range of tube voltages and currents. We performed reconstruction with FBP, ITER and a novel, iterative, modal-based reconstruction (IMR) algorithm. We found that noise decreased in an algorithm dependent manner (FBP > ITER > IMR) for every scan and that no differences were observed in attenuation rates of the agents. The contrast to noise ratio (CNR) of iodine was highest at 80 kV, whilst the CNR for gold was highest at 140 kV. The CNR of IMR images was almost tenfold higher than that of FBP images. Similar trends were found in dual energy images formed using these algorithms. In conclusion, IMR-based reconstruction techniques will allow contrast agents to be detected with greater sensitivity, and may allow lower contrast agent doses to be used.

Figure 1. Effect of x-ray tube current and reconstruction algorithm on the attenuation rate of gold nanoparticles.

(A–C) Data from scans performed at 80, 120 and 140 kV, respectively. (D) Comparison of the attenuation rate of gold nanoparticles at different X-ray tube voltages when scans are performed at 350 mA.

Figure 2. CT images of phantoms.

(A) The gold nanoparticle (gold-NP) phantom and (B) the iopamidol phantom acquired at 140 kV and 350 mA reconstructed with different algorithms.

Figure 3. Effect of x-ray tube current, voltage and reconstruction algorithm on image noise.

(A–C) Data from scans performed at 80, 120 and 140 kV, respectively. (D) Comparison of the noise of scans acquired at different X-ray tube voltages when scans are performed at 350 mA.

Figure 4. Effect of x-ray tube current and reconstruction algorithm on CNR rate at 140 kV.

(A) Data for gold nanoparticles. (B) Data for iopamidol. CT images of the (C) gold nanoparticle phantom and (D) iopamidol phantom reconstructed with different algorithms when scanned at 140 kV and 350 mA displayed with a narrow Hounsfield unit window.

Figure 5. Effect of reconstruction algorithm in dual energy CT.

(A) Dual energy CT ratiometric maps formed from 80 and 140 kV images acquired at 350 mA using different reconstruction protocols. The color bar represents the attenuation at 140 kV divided by the attenuation at 80 kV (with both attenuations adjusted by 1080 HU). (B) Effect of X-ray tube current and reconstruction algorithm on noise in dual energy CT ratiometric maps in 95 mM samples.