Introduction
Data acquisition hardware
Spectral decomposition
Image generation
Image type | Mechanism of generation | Clinical uses |
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Conventional (Polyenergetic/routine diagnostic) | Data from both layers considered as a single detector | Routine diagnostic use for all cases |
Iodine density (iodine map) | Material decomposition with pixels representing iodine | Visualisation and quantification of iodine in vessels and organs of interest |
Virtual non-contrast | Material decomposition and removal of iodine containing pixels | -Characterisation of lesions such as renal cysts/masses, adrenal nodules, lung nodules, etc. -Radiation dose saving by eliminating need for true non contrast |
Uric acid pair | Material decomposition; depiction of pixels containing uric acid | Urinary calculus characterisation |
Effective atomic number | Material decomposition; colour coding depending on atomic number | Tissue characterisation |
Virtual monoenergetic | Linear combination of basis pair images (40–200 keV) | -Low monoenergetic: enhanced vascular contrast -High monoenergetic: decreased artefacts |
Equivalent monoenergetic | Linear combination of basis pair, with attenuation values equivalent to conventional images | Higher image quality, with lower noise |
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Iodine density images- Iodine-containing pixels are assigned values equal to the concentration of the iodine in each pixel, expressed in milligrammes/ml (Fig. 2a). Pixels containing no iodine appear dark. Iodine-only images permit quantification of iodine in vessels and organs.
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Virtual non-contrast images- Images where the contributions from iodine are removed, resulting in non-contrast-enhanced HU values of the tissue contained within the pixels (Fig. 2b). The resulting image mimics a true non-contrast-enhanced (pre-contrast) image.
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Uric acid pair images- A uric acid image displays only uric acid pixels with original HU values while all others appear dark. The uric acid-removed image is a complement to the uric acid image These images are utilised in the evaluation of urinary calculi composition and gout.
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Z-effective images- are colour-coded based on the effective atomic number of tissues (Fig. 2c). The coefficients of the photoelectric and Compton scatter components (α1 and α2, respectively) computed during the spectral decomposition process are functions of the spatial distribution of tissue [i.e., α1(x, y) and α2(x, y)]. The ratio of these coefficients is proportional to the effective atomic number, Z:
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Virtual monoenergetic images (VMI) mimic images generated from application of a monoenergetic beam at a single kiloelectron voltage (keV) level. VMI are generated between 40 and 200 keV (Fig. 2d) by a linear combination of basis pair images. The attenuation, μ, at any energy can be determined from
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Equivalent virtual monoenergetic images mimic those generated by application of a monoenergetic beam at a single keV with attenuation values equivalent to conventional images but with lower artefacts and noise. For SDCT, the equivalent VMI levels are 70 keV for body, 66 keV for head and 64 keV for extremities [7].
Advantages
Limitations
Clinical utility
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Enhanced visualisation of vascular contrastThe attenuation of iodine is higher at lower energies due to the increased photoelectric attenuation at energies approaching the K-edge of iodine (33.2 keV). VMI reconstructions at lower energies (40–70 keV) can be used to enhance the visualisation of intravascular contrast, which can be clinically used in several settings [10, 11]. Suboptimal enhanced vascular studies are frequently encountered in imaging, either due to technical or patient factors. Usually, a suboptimal study requires a repeat study with additional bolus of contrast. However, with SDCT, due to the availability of low-energy VMIs on demand, suboptimal vascular studies can be salvaged, thus obviating the need for additional contrast injection or alternate imaging tests (Fig. 3). Although other currently available DECT scanners also have VMI capabilities, the dual-energy mode is not always switched on in these scanners and hence it may not always be available in a given patient. Vascular studies can be performed with low contrast dose, especially in patients with severe renal dysfunction and the contrast signal can be boosted by using VMI (Fig. 4). In addition, CT angiographic quality studies can be generated from routine contrast-enhanced CT scans, which may help in evaluation of incidentally seen vascular lesions. VMI, particularly at low energy or equivalent energy scans, can be used in improving the conspicuity of several lesions (Fig. 5). Hypervascular lesions may be more prominent at low-keV VMI due to improved iodine visualisation. Hypovascular lesions such as pancreatic adenocarcinomas are also well seen in low- or equivalent-keV VMI (Fig 5) [12].×××
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Artefact reductionSeveral artefacts are commonly seen in CT, including metallic artefacts, beam hardening and calcium blooming [13]. Metals seen in strategic locations can limit the diagnostic confidence by obscuring vital structures. If the presence of metal is known ahead of time, metal reconstruction algorithms can be used. Retrospectively generated high-energy VMI from SDCT can be used to reduce/eliminate incidentally encountered metallic artefacts, particularly metals with low atomic number such as stainless steel and aluminium (Fig 6). Artefacts from high atomic number metals such as embolisation coils are improved with iodine maps. Beam hardening artefacts which are caused by the polyenergetic nature of the x-ray energy can also be minimised/eliminated by using the high-energy VMI (Fig 7). Calcium blooming is an important artefact in vascular imaging, which causes overestimation of stenosis and inappropriate classification of the lesion. This can also be reduced by using high-energy VMI [13]. This ability of the SDCT to minimise artefacts in critical areas, particularly retrospectively, may improve the diagnostic confidence.××
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Material decompositionZ-effective images, iodine maps and calcium-uric acid images are currently used for the evaluation of urinary calculi, gout, lesion/plaque characterisation, perfusion of organs/tumour, tumour response to therapy, bone removal for CTA head, and virtual colonoscopy without bowel preparation. Increased utility may be possible with SDCT because of the opportunity to access spectral data for incidental findings. Characterisation of urinary calculi is important since management is different for uric acid and calcium calculi. Urinary calculi composition can be characterised by the z-effective images, and uric acid pair images (Fig 8) [14, 15]. Similarly, gout crystals can also be detected in joints. Iodine maps can be used to evaluate perfusion in several organs, including heart and lungs, the former in the diagnosis of myocardial ischemia and the latter in the evaluation of small pulmonary embolism (both acute and chronic) (Fig 9). Lesions involving the adrenal, kidney, liver and lung can be characterised with SDCT by a combination of VNC and iodine map, without the need for additional tests such as CT and MRI and possible additional radiation. For example, using VNC, the attenuation of an adrenal lesion from a contrast-enhanced image can be obtained and a lipid-rich adenoma can be diagnosed if attenuation is <10 HU [16]. A hyperattenuating renal lesion in contrast CT is likely to be a complicated cyst if there is high attenuation in VNC and no iodine uptake in the iodine map, and likely to be a solid tumour if there is no high attenuation in VNC, but there is iodine uptake (Fig 10). Some of these lesions usually require multi-phasic images for adequate characterisation.×××
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Radiation dose reductionThe attenuation values of the VNC images from SDCT have been shown to be similar to true unenhanced images in all tissues except for subcutaneous fat [17]. Hence, VNC images may eliminate the need for true non-contrast scans in multi-phasic studies, thus reducing radiation dose (Fig. 11). Radiation dose savings of up to 40% has been shown by eliminating the true non-contrast phase and up to 64% by eliminating the arterial phase [18, 19]. The ability to salvage suboptimal enhanced studies and characterise incidental findings (e.g., adrenal, renal lesions) may also obviate the need for an additional CT scan, thereby reducing radiation dose.×