Contrast medium dose optimization in the era of multi-energy CT

Abstract

With the increasing use of contrast-enhanced CT, optimizing the iodinated contrast medium (ICM) dose while maintaining diagnostically adequate image quality is essential to mitigate potential adverse effects on patients, the environment, and public health, as well as to reduce medical costs and address potential supply shortages. Multi-energy CT technologies including dual-energy CT and photon-counting detector CT enable data acquisition at multiple energy spectra, allowing for material characterization beyond the capabilities of conventional single-energy CT. Recent technical advancements and the growing adoption of these technologies in clinical practice have enhanced patient care across various diagnostic tasks. Among the spectral-based imaging options offered by multi-energy CT, virtual monoenergetic imaging holds significant promise for substantial ICM dose reduction due to the drastic improvement in iodine contrast at lower energy levels. This article aims to provide an overview of multi-energy CT technology and its utility for ICM dose optimization across various clinical indications, while also discussing current issues and related topics.

Keywords: Dual-energy CT; Iodinated contrast medium; Iodine k-edge; Low X-ray energy; Multi-energy CT; Photon-counting detector CT.

Fig. 1

A polyenergetic X-ray beam is composed of photons with a wide range of energies. The"kVp"defines the upper limit of the polychromatic X-ray beam energy. In conventional single-energy CT, only one kVp image dataset per scan is available for diagnostic evaluation

Fig. 2

In SECT, different materials such as iodine and calcium can exhibit the same attenuation, making them difficult to distinguish. With multi-energy CT, these materials can be distinguished and quantified based on their different attenuation properties at various energy levels

Fig. 3

The iodine contrast of polyenergetic images ranging from 80 to 140 kVp in single-energy CT and VMI ranging from 40 to 70 keV in multi-energy CT (dual-layer detector scanner). VMI levels below 60 keV yield higher iodine contrast than polyenergetic 80 kVp images

Fig. 4

A 40-year-old man with subarachnoid hemorrhage underwent cranial CTA for treatment planning using dual-layer detector scanner. An ICM dose of 25 mL (370 mgI/mL, diluted with 25 mL of normal saline) was injected over 10 s. Conventional 120-kVp imaging resulted in poor arterial enhancement. Arterial attenuation gradually increased as VMI energy decreased, with excellent visualization of intracranial arteries observed at 40 keV VMI. The short ICM injection duration also helped to minimize venous contamination at the cavernous sinus

Fig. 5

A 68-year-old woman with ovarian cancer and pulmonary embolism underwent two CTPA scans. The initial CTPA (left) was performed with a standard protocol (ICM 90 mL [370 mgI/mL]) using single-energy CT scanner. The follow-up CTPA (middle and right) was performed with a reduced ICM protocol (ICM 15 mL [370 mgI/mL]) using dual-layer detector scanner. In lower ICM dose protocol, a small pulmonary embolism (arrows) is barely visible at 120 kVp but easily detectable at VMI 40 keV, similar to the visibility in a standard ICM dose 120-kVp image

Fig. 6

A 66-year-old woman with breast cancer who underwent portal venous phase CT using dual-layer detector scanner. Incidental PEs (arrows), which may be missed on 120 kVp images due to suboptimal contrast enhancement in the pulmonary arteries, are confidently detectable on VMI 40 keV without the need for dedicated CTPA

Fig. 7

An 85-year-old man (BMI 23 kg/m²) presenting with chest pain underwent coronary CTA using a low ICM dose (19 mL, 370 mgI/mL) on dual-layer detector scanner. The low keV VMI reconstruction demonstrated superior iodine contrast and enhanced visualization of small and peripheral coronary arteries compared to conventional 120 kVp images. Image courtesy of Kumamoto Chuo Hospital

Fig. 8

A 70-year-old man (BMI 27 kg/m²) post-PCI underwent coronary CTA using dual-layer detector scanner with a low dose of ICM (21 mL, 350 mgI/mL). The low keV VMI provided sufficient coronary enhancement while preserving the visibility of in-stent patency. Image courtesy of Minamino Cardiovascular Hospital

Fig. 9

A 78-year-old man with a history of aortic replacement underwent follow-up CT scans at one-year intervals using both standard ICM dose (78 mL) and low ICM dose (25 mL) protocols on single-energy and dual-layer detector CT scanners, respectively. The standard image was reconstructed using conventional 120 kVp, while the low ICM dose images were reconstructed both with conventional 120 kVp and VMI at 40 keV. The reduction in arterial enhancement in the low ICM image was fully compensated by the VMI at 40 keV, resulting in even better arterial depiction compared to the standard ICM dose

Fig. 10

An 86-year-old woman with severe aortic valve stenosis and renal dysfunction underwent pre-TAVR CT using a two-step ICM injection protocol on dual-layer detector scanner. The first step involved ECG-gated scanning for aortic root with 14 mL of ICM, followed by non-gated scanning of the entire body with 15 mL of ICM. Using 40 keV VMI provided sufficient contrast enhancement for the pre-TAVR CT with a total of only 29 mL of ICM

Fig. 11

A 91-year-old woman with toe gangrene underwent runoff CTA using dual-layer detector scanner. A total of 25 mL of ICM (370 mgI/mL, diluted with 50 mL normal saline) was injected over 25 s. Conventional imaging at 120 kVp demonstrated insufficient arterial opacity, while VMI at 40 keV showed improved visibility of the lower extremity arteries

Fig. 12

A 78-year-old man with hepatocellular carcinoma underwent multiphase hepatic CT with both standard (600 mgI/kg) and low (300 mgI/kg) ICM dose protocols on single-energy and dual-layer detector CT scanners, respectively, at 6 months interval. Compared with the standard protocol, better lesion conspicuity was attained at 40 keV VMI even with a 50% reduction in ICM dose without increasing radiation dose (size specific dose estimate: 15.4 vs. 14.7 mGy)

Fig. 13

An 83-year-old man with pancreatic ductal adenocarcinoma in the pancreatic head and hepatic metastasis underwent multiphase pancreas CT (upper row: pancreatic phase; lower row: portal venous phase) using 33 mL of ICM (200 mgI/kg) on dual-layer detector scanner. In the 40 keV VMI, both the pancreatic and hepatic lesions are clearly delineated, in contrast to the conventional 120 kVp images where both lesions are barely visible due to insufficient contrast enhancement

Fig. 14

A 72-year-old man with esophageal cancer underwent scanning with both a standard 120 kVp protocol (iodine dose, 37 g; SSDE, 19.2 mGy) and a low ICM dose CT protocol (iodine dose, 12.6 g; SSDE, 16.3 mGy) on single-energy and dual-layer detector CT scanners, respectively, with a 12-month interval. Compared with the 120 kVp images, an equivalent or higher iodine contrast was attained at VMI 40 keV, despite a 66% reduction in iodine dose

Fig. 15

A 40-year-old man underwent abdominal CT with a low ICM dose protocol (350 mgI/kg) on a PCD-CT scanner. Compared with the VMI 70 keV images, iodine contrast of VMI 50 keV is considerably improved while preserving diagnostically acceptable image noise level. Image courtesy of Medical Scanning Tokyo

Fig. 16

A 68-year-old woman with bilateral total hip replacement who underwent contrast-enhance CT using dual-layer detector scanner. Compared to conventional 120 kVp (a), VMI at 200 keV without MAR (b) shows limited artifact reduction and poor visualization of iodinated structures. In contrast, MAR-processed images (d–f) show significant reduction in metallic artifacts. The combination of 200 keV with MAR (e) is suitable for evaluating bone structures but offers no advantage for soft tissue assessment compared to 120 kVp with MAR (d). The best visualization of iodinated structure such as arteries (arrows) is obtained at VMI 40 keV with MAR (f)

Fig. 17

Stretched curved MPR of the right coronary artery in an 86-year-old man who underwent coronary CTA using dual-layer detector scanner with a low ICM dose (18 mL, 370 mgI/mL). The images were reconstructed with VMI at 90 and 40 keV. At the same window setting (WL/WW: 200/1000 HU), blooming artifacts from calcified plaques lead to an overestimation of luminal stenosis in the 40 keV VMI compared to the 90 keV VMI (red). However, mixed plaques with positive remodeling (yellow) are better visualized in the 40 keV VMI due to decreased attenuation of pericoronary fat and increased luminal attenuation. By optimizing the window setting for 40 keV, the degree of visual blooming artifacts from calcified plaques remains consistent with those at 90 keV, while luminal brightness, sharpness, and depiction of mixed plaques with positive remodeling are superior in the 40 keV images compared to the 90 keV images