Fundamentals Of Dual Energy CT

PRESENTED BY :

Surakshya Koirala

Bsc.MIT 4^th Year

Roll no:124

Introduction

• One of the recent advances in CT imaging technology.
• DECT- CT that uses two photon spectra, also known as spectral CT .
• A new technique that allows differentiation of materials and tissues based on CT density values derived from two synchronous CT acquisitions at different tube potentials.

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Why is DECT in popularity?

• Conventional CT scanners provides morphologic imaging only, with little material-specific information in body imaging.
• Dual energy CT on the other hand utilizes the principle of differential attenuation of different tissue at varying energy levels , thereby enabling tissue characterization

SE-SSCT vs DECT

LIMITATIONS OF SE-SSCT

• No material differentiation

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• Less temporal resolution

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• Poor subtraction in Angiography and other studies

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• Small lesion detection

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• Noise and image quality in low kVp

History

• The concept of dual energy CT existed from the very beginning of the history of computed tomographic imaging.
• Alvarez and Macovski and Kalender et al also described the theoretical basis of dual energy CT scanning in the late 1970s and early 1980s.
• With the advent of newer multislice CT scanners having improved temporal resolution, dual energy CT has finally become a reality in clinical applications.
• First commercially available dual energy CT scanner came into use in 2006(Somatom Definition, Siemens, Erlangen, Germany).

History

• Over the time there have been newer modifications and advancements in the dual energy scanners.
• The DSDECT scanner was followed by the development of a single-detector, single-source DECT (SS-DECT) system with the capability for rapid alternation/fast switching between two kVp settings (Gemstone Spectral Imaging; GE Healthcare)
• In second generation of dual source dual energy scanners, the dual energy scan FOV was increased (from 26 to 33 cm); and the addition of a selective photon shield led to increased contrast to noise ratio and better and accurate material characterization.

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Principle

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• The principle of dual energy CT imaging is based on the differential absorption of energy at variable kVp settings.
• Principle of DECT is based on the differential absorption of energy(due to photoelectric absorption) of different materials at the K-edge of the materials at variable kVp settings.
• This exploits the property by using two x-ray spectra (suppose 80-100kVp and 120-140 kVp)such that the differential attenuation at the k-edge causes significant enhancement of one material compared to other structures.

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Principle

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• There is a significant spike in attenuation that results just beyond the energy of the K-edge, this peak is unique to every material and holds valuable information about the substance's composition.
• The different photoelectric energies and K-edges is the fundamental of dual-energy CT.
• Although most elements in the human body have very low K-edges (0.01-0.53 keV), elements like iodine and calcium have higher K-edges of 33.2 keV and 4.0 keV respectively, making them sufficiently larger than surrounding structures and are particularly important in the clinical setting .  

Principle

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• In clinical applications, the constituents of soft tissues have a different K-edge (variable from 0.01 to 0.53), away from that of iodine (33.2) or calcium (4); hence iodine or calcium can be distinguished from soft tissues at dual energy imaging.

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Principle

Fig: Principle of Dual energy CT

Principle

• In diagnostic imaging, once the datasets at 80 kVp and 140 kVp are generated, the attenuation of the enhanced structures containing iodine (vessels, highly perfused organs) are more on a 80 kVp image than on a 140 kVp image.
• This difference in attenuation varies between different organs; for example-highly vascular organs and vessels have higher difference than muscles.
• Post-processing softwares use this information to generate a virtual non-contrast image, or to calculate the material composition within a specific region of interest.

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Principle

Different CT attenuation of tissues at different energy levels, based on their constituents.

Principle

Principle

Principle

DECT Technologies

• Currently available dual energy CT scanners employ different technologies to obtain high and low-energy datasets:
• Dual source dual energy CT (DS-DECT)
• Single source dual energy CT (SS-DECT)
• Single source dual energy scanner with dual detector layers

Dual Energy Ratio

• The material characterization on DECT largely depends on their dual energy ratio.
• For calculation of dual energy ratio of a given material X, the CT attenuations at any given kVp (for example, 140 kV) at different material concentrations are assessed and arranged along a graph (A). Similar graph (B) is made for different material concentrations at another kVp (for example 80 kVp). The dual energy ratio of material X can be described as a ratio of the slope of the graph B to the slope of graph A.
• The dual-energy ratio is a material-specific parameter. It depends on the atomic number, not on the material density or concentration.

Dual Energy Ratio

• The difference between dual energy ratio of different materials depends on several factors such as:
• Difference in the atomic number of the materials
• Spectral separation between X-ray spectra at the two energies.

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Spectral Separation

• For efficient dual energy imaging the two incident X-ray beams should have sufficiently different energies.
• In reality, whenever X-ray is generated, it is emitted in the form of a continuous spectrum with photons having different energy levels (Bremsstrahlung) and a mean energy.
• The mean energy of an X-ray spectrum varies with the bombarding energy (kV).
• When two tubes with different kV are used, two spectra are generated with significant overlap between them (spectral overlap)
• The spectral overlap makes material characterization more difficult and also reduces the dose efficiency.
• Spectral separation indicates a situation when there is minimal overlap between the different energy spectra, i.e. less spectral overlap.

Spectral Separation

Adequate spectral separation can be achieved by adding separate filtration to one or more tubes in DS-DECT

Spectral Separation

• Adequate spectral separation can be achieved by several methods :

1. Adding separate filtration to one or more tubes in DS-DECT

2. Adding split filtration to SS-DECT

3. Sandwich (dual) detectors

4. Energy discriminating, photon counting detectors.

Selective Photon Shield

• Adding filtration to the higher energy tube in the second generation scanners, termed selective photon shield, has led to improved spectral separation .
• The use of added filtration for the high-kV tube can dramatically increase the DE contrast between clinically relevant materials (e.g., calcium and iodine) by decreasing spectral overlap.
• Selective photon shielding also improves the SNR efficiency and reduces dose to the patient.
• Tin is chosen as a filter component.

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Selective Photon Shield

Fig: with and without filter

DECT Technologies

Dual source DECT

• In the first generation dual source DECT scanner:
• two X-ray tubes, at 90 degrees angle to each other in the same gantry and operating at different kVp.
• The average kVp of the higher energy tube is 120 to 140 and that of the lower energy tube is 80 to 100.
• The tube with larger kVp (tube A) has a larger detector of FOV 50 cm, and the lower energy tube (B) has a smaller FOV detector (26 cm).

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• In the second generation DS-DECT scanner, the lower energy tube (tube A) is paired with a detector with FOV of 50 cm while the higher energy tube (tube B) is paired with a smaller detector with FOV of 33 cm.

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Dual source DECT

Fig: first generation and second generation dual source dual energy scanner

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Dual source DECT

• Higher energy tube has a selective proton shield made of tin filter.
• Two separate detector arrays lead to generation of two image datasets.
• Since both the tube kVp can be modified independently and additional selective photon shield can be used in DS-DECT scanners, it results in better spectral separation.
• The temporal resolution of a DS-DECT is one-quarter of the rotation time (approximately 75 ms), as one X-ray tube acquires data during 90 degree of rotation.
• However, the drawback of DS-DECT lies in term of temporal mis-registration and limited dual energy FOV.

Single source DECT

• SSDECT scanners use a single X-ray tube, which generates high and low energy X-ray spectra by rapid changing of the kVp settings (at an interval of 0.5 msec) in the same rotation.

Single Source DECT scanner

Single source DECT

• The SSDECT requires a very fast detector and data acquisition system with fast sampling capability.
• The detector arrays used in SS-DECT are made of cerium activated garnet(GE)

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Single source DECT

The advantage of SS-DECT over DS-DECT :

1. Better temporal registration between two datasets, as the images from high and low energy acquisition are acquired almost simultaneously.

2. Larger FOV of imaging (50 cm) and easier quantification of material density.

The disadvantage of SSDECT :

1. a poorer spectral separation (hence less accurate material characterization),compared to DS-DECT.

Technical approaches to �SS-DECT

• Sequential acquisition
• Rapid voltage switching
• Layer detectors and
• Energy resolving or quantum counting detectors

Sequential acquisition

• Achieved either as two subsequent helical scans or as a sequence with subsequent rotations at alternating tube voltages, usually 80 and 140 kVp and stepwise table feed.
• Disadvantage is rather long delay between both acquisitions, causing artifacts from cardiac or respiratory motion or changes in CM opacification.
• But, a viable option for clinical applications w/o CM such as metal artifact removal and kidney stone differentiation.
• This method uses an image-based dual-energy reconstruction algorithm; and radiation-lowering technique such as tube current modulation .

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Rapid voltage switching

• Tube voltage is rapidly changed between 80 and 140 kVp,
• The two projection data sets are collected separately for subsequent use in a projection-based dual-energy reconstruction algorithm.
• Rotation speed of system must be reduced to account for the acquistion of these additional projections and rise and fall times of voltage modulation.
• The exposure time ratio is varied between the 80-kVp and the 140-kVp acquisitions to maximize the contrast-to-noise ratio.
• A detector with a fast response and a data acquisition system with a fast sampling capability are used to capture the alternating high- and low-energy data.

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• Fig: showing detector FOV of 50 cm with fast kv switching

Rapid voltage switching

• The advantages of dual-energy CT with fast kilovoltage switching are :
• good temporal registration between high- and low-energy datasets, which are obtained nearly simultaneously,
• the availability of the full 50-cm field of view for use in image analysis.
• Limited photon output at low voltages which results in high noise and necessity to choose a relatively high current and consequently high dose.
• However, because a single x-ray source is used, individual modification of the high- and low-energy x-ray beams is difficult and spectral overlap is increased.

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Dual energy CT with layered Detectors

• 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
• 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.

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• The top layer captures low energy data and the bottom layer captures high energy data; which are then used to reconstruct two separate image datasets.

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Dual energy CT with layered Detectors

• 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
• 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.
•  Multiple spectral images are generated by projection-space decomposition.

Quantum Counting detector

• Uses cadmium-based semiconductors (CdTe or CdZnTe)
• Single X-ray energy spectrum simultaneously acquired at a fixed tube voltage can be split into more than two photon energy bins
• Resolve the energy of each individual impacting photon
• Used to differentiate more than two photon energies and is very quantum efficient.
• In an optimized system, it has potential to not only eliminate electronic noise and misregistration between different energy bins, but also provide higher contrast-to-noise ratio, higher spatial resolution, higher radiation dose efficiency, and better spectral information

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Summary of currently available DECT techniques

Summary of currently available DECT techniques

Summary of currently available DECT techniques

Summary of currently available DECT techniques

Comparison between single source and dual source CT

Processing of data and Image Reconstruction in Dual Energy Imaging

The images generated from a DECT should have the combined morphologic data and material specific information. To obtain a material-specific image, the datasets can be processed in two ways:

• Image domain decomposition:
• Projection space decomposition

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Image domain decomposition

• Processing after reconstruction of low and high energy images.
• Used in DS-DECT and dual detector DECT
• Images constructed by linear or sigmoidal blending of the low and high energy datasets.
• Material specific images- obtained by calculating the difference in attenuation of varying materials between high and low energy image datasets.
• Color coded overlay image is generated by assigning color to the voxels containing the material and overlapping them on monochromatic images.

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Projection space decomposition

• Processing before the images are reconstructed from high and low energy sinograms .
• More robust processing which is preferred because of greater flexibility in material decomposition, and minimizing beam-hardening artifact
• The high and low energy datasets are calibrated and aligned in projection space to generate material density image and monochromatic images.
• Material based image is based on the theory of basis material decomposition.

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Projection space decomposition

• Stated as the attenuation coefficients of any material can be computed as a weighted sum of attenuation coefficient of two basis materials as long as the k edge of the material is not within the evaluated energy range.
• Projection-based algorithm used in the dual layer has potential advantage over image-based algorithm, particularly in beam-hardening correction at the expense of a higher noise level for material decomposition images

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Image display in DECT

Images generated in DECT can have two types of display:

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-Material density display

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-Monochromatic image display

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Material density display

• In DECT, the material density display can be iodine density display or water density display.
• Iodine density display, the enhanced organs containing iodine appear bright whereas unenhanced areas remain dark.
• The water density display is equivalent to virtual unenhanced images.

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Material density display

Material density display

Fig: Virtual monoenergetic images generated from a DS-DECT scanner

Monoenergetic image display

• Energy-specific display.
• Images are processed at any given kVp from the dual energy datasets, which resemble images physically acquired after scanning at that given kVp.
• Images are processed at any given kVp from the dual energy datasets, which resemble images physically acquired after scanning at that given kVp.
• However, the images at lower kVp (for example 40 kV) have more noise and higher kVp images have less contrast. The optimum image contrast to noise ratio is achieved at 70 kVp.

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Applications of DECT

• Renal applications

Renal calculi characterization

• Till now, single energy NCCT has remained the usual standard investigation for evaluating urinary tract calculi. However, single energy NCCT is unreliable for characterizing the type of calculi since calculi of different proportions of constituents can have overlapping CT attenuation values. When imaging is done on DECT scanners, the change in attenuation between high-and low-energy scans can be used to differentiate types of calculi.

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Fig: Renal calculi characterization

Applications of DECT

Renal cyst

• On Iodine density images, the enhancing solid mass will appear bright; whereas high density cysts will be dark, since they do not have any iodine in them.
• In SS-DECT, it is not possible to calculate the HU value of the lesion, which is possible with DS-DECT.

Applications of DECT

CT urography

• CT urography is a routine imaging procedure in the evaluation of hematuria and multiple phase acquisitions (NCCT and delayed images) are performed for CT urography.

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• On DECT imaging, virtual NCCT generated from the dual energy datasets can prevent the need for an additional NCCT acquisition and thus reduce the radiation dose.

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Applications of DECT

Hepatic Mass Characterization

Characterization of hepatic mass lesions

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• Solid hepatic mass lesions are typically evaluated using a multiphase CECT.
• In scenario where detection of a hypervascular lesion on a delayed arterial phase becomes difficult ,the lesions can become more appreciable on a lower kVp image on a DECT scan.
• Non- enhancing lesions or cysts appear dark on iodine density maps.

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Applications of DECT

Fig: application of DECT in characterizing hepatic lesion

Applications of DECT

Cardiac applications

• DECT combines the benefits of superior resolution morphologic imaging in CAD coupled with detection of its hemodynamic significance in the form of ischemia/infarction, which is unique to this modality.
• Material differentiation based on dual energy ratio has been tried in the characterization of plaques.
• Although differentiating a calcified from a non-calcified plaque is possible, but the differentiation of various types of noncalcified plaques have not been successful.

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Applications of DECT

Application in Neuro Imaging

• Advantages of DECT scanning include avoidance of additional non-contrast scanning.
• The virtual NCCT images generated from the dual energy dataset helps detect hemorrhage and can save crucial time.
• CTA has been found to be the ideal tool for evaluating an acute intracranial hemorrhage.

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Radiation dose considerations

• As discussed by Henzler et al, multiple studies have shown that dual-source DECT does not lead to increased radiation dose compared with conventional single-energy multidetector computed tomography.
• The added benefit of DECT scanning lies in the generation of virtual unenhanced scans, which can obviate the need of another NCCT scanning and reduce radiation dose.
• Although DECT scanning involves imaging with two different X-ray beams, the dose is not doubled in comparison to the single energy scan.This is because the mAs are divided in the two sources.

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Radiation dose considerations

• With dual-source DECT, radiation dose reductions strategies include tube current modulation and the use of tin filters with the high-energy spectrum (to get rid of lower energy quanta and optimize the separation between the high- and low-energy spectra).
• For single-source DECT with rapid kilovolt switching, the radiation dose is usually higher than conventional monoenergetic CT, with ratios up to three times more radiation.
• When matched for image quality assessed by low-contrast detectability, the radiation dose remains roughly 22% and 14% higher with single-source DECT with rapid kilovolt switching compared with conventional monoenergetic CT (evaluated for head and body examinations)

Limitations

• First, the FOV of the smaller detector is significantly less in DS-DECT (33 cm in second generation DS-DECT) which leads to incomplete area coverage in obese patients.
• Second, the images generated from the lower energy tube have inherently high noise.
• Finally the large number of datasets generated leads to a problem in storage and archival.

IN TUTH ,

-Siemens Somatom Definition AS+ SSDECT

-Projection decomposition reconstruction.

-Specialized DECT application in syngo.via

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Conclusion

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• With its unique ability to differentiate basis materials by their atomic number, DECT has opened new perspectives in imaging.
• Used appropriately, it should not lead to an increase in radiation dose.
• Several image reconstruction algorithms have been developed to improve image contrast (i.e., by generating monoenergetic images), to characterize or subtract certain materials in the image, as well as to perform automatized volumetric measurements.

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References

1. Dual-Energy CT: Basic Principles, Technical Approaches, and Applications in Musculoskeletal Imaging (Part 1),Patrick Omoumi et. al, Thieme Medical Publisher.
2. Gupta AK, Chowdhury V, Khandelwal N. Diagnostic radiology: recent advances and applied physics in imaging. JP Medical Ltd; 2013 Jul 30.
3. Bushberg JT, Boone JM. The essential physics of medical imaging. Lippincott Williams & Wilkins; 2011 Dec 20
4. Detector-based spectral CT with a novel dual-layer technology: principles and applications, Negin Rassouli et al.,pubmed
5. Johnson TR. Dual-energy CT: general principles. American Journal of Roentgenology. 2012 Nov;199(5_supplement):S3-8.

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THANK YOU !!