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Detector-based spectral computed tomography is a novel dual-energy CT technology that employs two layers of detectors to simultaneously collect low- and high-energy data in all patients using standard CT protocols. In addition to the conventional polyenergetic images created for each patient, projection-space decomposition is used to generate spectral basis images (photoelectric and Compton scatter) for creating multiple spectral images, including material decomposition (iodine-only, virtual non-contrast, effective atomic number) and virtual monoenergetic images, on-demand according to clinical need. These images are useful in multiple clinical applications, including- improving vascular contrast, improving lesion conspicuity, decreasing artefacts, material characterisation and reducing radiation dose. In this article, we discuss the principles of this novel technology and also illustrate the common clinical applications.
• The top and bottom layers of dual-layer CT absorb low- and high-energy photons, respectively.
• Multiple spectral images are generated by projection-space decomposition.
• Spectral images can be generated in all patients scanned in this scanner.
Conventional computed tomography (CT) scanners utilising a single X-ray spectrum are able to discriminate several tissues based on their attenuation (Hounsfield units, HU), but the overlap in attenuation of many tissues limits discrimination. Tissues have a unique distribution of attenuation values as a function of X-ray energy and spectral CT (dual-energy/multi-energy CT) can further distinguish them by exploiting their energy-dependent attenuation properties. Data collected simultaneously from two energy levels can be used to determine the Compton scatter and photoelectric components of x-ray attenuation which have different energy dependencies. These components provide additional information about tissue density and atomic number that can be exploited to separate tissues with similar attenuation in a conventional image. Spectral CT technologies so far have been source-based, including dual-source, rapid kilovoltage (kVp) switching and dual-spin technologies .
Detector-based spectral CT (SDCT; Philips Healthcare, Cleveland, OH, USA) offers a novel approach to dual-energy imaging where the spectral separation occurs at the level of the detectors. In this article, we describe data acquisition and image generation with SDCT and the clinical applications of this technology.
SDCT utilises a single X-ray tube but has two layers of detectors; a top layer of an yttrium-based garnet scintillator and a bottom layer of gadolinium-oxysulphide (Fig. 1a). The top layer selectively absorbs low-energy photons while the high-energy photons penetrate this layer to reach the bottom layer where they are absorbed and converted into light. Attached to each layer, a photodiode converts light into an analogue electrical signal and an application-specific integrated circuit converts it into digital signals. The X-ray tube has a 120-KW generator with tube voltages of 80–140 kVp. Scanner collimation is up to 64 × 0.625 mm yielding a maximum z-coverage of 40 mm. The fastest gantry rotation time is 270 milliseconds. [2, 3].
SDCT uses projection-space decomposition to solve for the basis components. X-ray attenuation of any material can be expressed as a linear combination of photoelectric and Compton scatter coefficients and, by extension, the attenuation coefficients at low energy and high energy can be expressed as the attenuation contributions from a predefined pair of basis materials . More generally, the energy-dependent attenuation coefficient of a sample, μ(E), can be expressed as a linear combination of n basis functions f i (E) and weighting constants αi for i = 1, 2,…,n:
Empirically, a pair of basis functions can be derived where the function 1/E 3 closely approximates the photoelectric effect while f KN (E), defined as the Klien–Nishna function, closely approximates Compton scattering:
Solving for the weights α i and α 2 for each ray projection acquired at two different energies decomposes the raw data into photoelectric and Compton scatter basis components .
Several types of images are generated with SDCT, including conventional images and special spectral image types (Table (Table11).
These images are analogous to the images obtained from a single-energy scanner, and are utilised for routine diagnostic purposes. For every scan, the two detector layers are considered as a single detector and high- and low-energy data are combined. Filtered back projection or iterative reconstruction algorithms are used to reconstruct the combined raw data and create conventional images equivalent to those from a single-energy scanner. The image quality of these images have been shown comparable to images obtained from a single-energy scanner .
The basis pair raw data undergoes reconstruction to generate photoelectric and Compton scatter basis images, from which further additional material composition images are generated (Fig. 1b). Beam-hardening correction is inherent in this process, and image noise reduction is addressed in the reconstruction chain.
Therefore, from this ratio we can estimate the effective atomic number as a function of (x, y) (2). The effective atomic number provides a higher level of discrimination than attenuation in HU because it depicts the material make-up of each pixel [4, 5].
A major advantage of SDCT is elimination of the requirement to prospectively select patients who need dual energy CT. All other currently available commercial solutions—dual-source CT, rapid tube potential switching and dual-spin technology—require selection of a special dual-energy protocol prior to scanning . Spectral data are available for all patients imaged with SDCT without a change in clinical workflow, permitting evaluation of incidentally discovered findings and reduction of artifacts. The complete spatial and temporal alignment is ideal for cardiovascular studies and also allows projection-space decomposition with more accurate artefact correction and better noise removal in VMI compared to image space decomposition . All radiation dose-reduction strategies available with conventional CT can be employed with SDCT. There are no restrictions related to field of view, gantry rotation time or cross-scatter.
Sufficient spectral separation requires a tube potential of at least 120 kVp. Therefore, radiation dose reduction for smaller patients relies on lowering the tube current. Due to higher kVp, contrast is lowered in conventional images, but low-energy VMI may be helpful in such instances. The z coverage of the currently available scanner configuration is 4 cm, which makes some dynamic studies, such as myocardial perfusion, challenging.
There are several clinical applications of detector-based spectral CT, broadly divided into: (1) enhanced visualisation of contrast; (2) artefact reduction; (3) material decomposition; and (4) radiation dose reduction.
SDCT is a unique technology utilising dual-layer detectors which provides spectral data on demand for all patients. There are several applications of this technology in several organ systems. The major applications of this technology are to improve the vascular contrast using low-energy VMI images; decrease several artefacts using high-energy VMI images; material composition of several tissues and lesions using an iodine map, VNC and effective-z images; and radiation dose savings using VNC images instead of true non-contrast images as well as saving additional CT examinations for lesion characterisation.
Institutional research support from Philips Healthcare, not related to this paper.
Prabhakar Rajiah had received honoraria from Philips in the past (> 2 years).
Amar Dhanantwari is an employee of Philips.