Abstract

Article Series “Dual-Energy CT: What about Radiation Dose?” (1/3) – Introduction

posted by U. Joseph Schoepf, M.D. | May 11, 2011

This article is the first part of the three-article series on “Dual Energy CT: What about Radiation Dose?” that outlines the potential of dual-energy applications with a special focus on radiation dose.

Introduction

Dual Energy Computed Tomography (DECT) is one of the most exciting and promising developments in radiology in recent history. The ability to enhance the resolution of CT images using different x-ray spectra was demonstrated in the 1970s, but was clinically implemented only for the purpose of emission tomography.
Early diagnostic CT approaches exploring multiple energy applications involved two subsequent scans at different tube voltages over the same anatomical position.
However, these approaches, besides the high computational efforts, failed to seamlessly align the imaged anatomy and to capture the same phase of contrast enhancement. Only since the introduction of dual-source CT (DSCT) in 2006 have multiple energy image acquisition methods achieved clinical significance and widespread application for diagnostic CT imaging.

Since then, various methods for acquiring DECT data have been proposed for use with recent generations of advanced multi detector-row CT systems:

simultaneously applying two x-ray tubes and two corresponding detectors at different kV and mA settings, as with dual-source CT;
ultrafast kV switching based on single-source CT;
compartmentalization of detected x-ray photons into energy bins by double layer detectors of a single-source CT scanner operating at constant kV and mA settings;
or the principle of a counting detector system which allows dual-energy or spectral CT imaging.

The former methods all aim at acquiring multiple energy data of a given anatomic area simultaneously, at the same time interval during the gantry rotation. DSCT and double layer technology derive their information from one set of data, acquired at identical time points. With ultrafast kV switching there is a small delay of a few ms between readings; however, the data is fully interlaced as full images are based on several hundred readings during which the system switched back and forth multiple times.

Another, conceptually different, approach revisits the above described earlier concepts of generating dual energy data, which involved scanning patients twice, by combining image data from two consecutive 270° rotations across the same anatomic region, one at low kV followed by one at high kV. This latter method accordingly appears less suitable for imaging organs that change position during scan acquisition, foremost the heart, or patients with limited compliance (trauma, pediatric patients). Also, with such an approach, quantitative dual-energy evaluation at applications involving contrast administration is limited, since contrast concentration changes between two acquisitions which are 270° apart.

Over the last 5 years, the use of DECT has been evaluated for a variety of clinical applications. Studies, overwhelmingly based on dual-source CT technology (>130 peer reviewed articles), have shown substantial clinical benefits e.g. in

the identification of renal stones [1-4],
detection and characterization of liver and kidney neoplasms [5, 6],
characterization of pulmonary nodules [7] (Figure 1),
assessment of myocardial perfusion [8-12] (Figure 2),
visualization of lung perfusion and ventilation [13-16] (Figure 3),
and the automatic removal of bone from angiographic data sets [17-19].

Figure 1: Adenosine stress DECT coronary CT study of a 64-year-old man shows reversible ischemia of the left lateral wall and a papillary muscle (A and B). The diagnosis was confirmed by stress SPECT (C and D).

Figure 2: Coronal DECT images of a 51 year-old man with lung cancer. (A) represents 120 kV images, (B) shows a calculated virtual non contrast image (VNC); (C) shows the color-coded iodine distribution within the tumor.

Figure 3: 120 kV image in sagittal orientation demonstrate a small isolated segmental pulmonary embolism of the right lower lobe (A). DECT iodine map demonstrates the corresponding wedge shaped perfusion defect (B).

However, this exciting advancement has coincided with a growing public awareness regarding the use of ionizing radiation for medical imaging, and increasing concerns over total radiation dose received by the population from imaging tests. Accordingly, successful clinical implementation of multiple-energy imaging will require convincing evidence that these techniques do not come at the price of higher patient radiation exposure. Dual-source CT and dual-energy CT both carry the word “dual” in their name; likely on this semantic background, concern has been raised, whether this technology involves increased (as in “double”) radiation exposure as compared to single-source CT, single-energy investigations.

Purpose of this article series

The purpose of this article series is to review the available literature regarding the radiation dose associated with DECT imaging applications in comparison with traditional single-energy CT techniques, keeping in mind that dual-energy CT scans provide clinically useful material-specific information in addition to the mere structural, anatomic information of conventional single-energy scans.

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