Dual energy CT thorax: technique and applications
Dual energy CT explores the different attenuation properties of matter at two energy levels. Lower KvP is associated with increase photoelectric interactions especially in substance with higher atomic number such as iodine and calcium. The attenuation value at two different energy within a voxel can subsequently be analysis using material decomposition technique and post processing of images can provide additional diagnostic value compared to single energy conventional CT.
Imaging protocol and post processing
Currently available 1st and 2nd generation Dual energy scanner available include 64 slice DECT (Siemens Definition System) and 128 slice DECT (Siemens Flash Definition System).
Two x-ray source, tube A (140 KvP) and tube B (80 KvP or 100KvP) with an angular offset of 90 degrees. Tube A maintains full field of view, however, the FOV for tube B is limited to 26 and 30 cm (Siemens Definition System and Siemens Flash Definition System respectively). The radiation dose for DECT is equivalent or less than conventional CT technique.
The acquired images is then automatically reconstructed to three separate image sets: 80 kVp, 140 kVp and mixed 80:140 kVp image with weighting factor of 0.4 (40% image information from the 80 kVp image and 60% information from the 140 kVp image). The weighting factors can be adjusted, to achieve the desirable effect. The 80 kvP image have higher contrast attenuation but intrinsically lower SNR ratio and smaller FOV. The 140 KvP image have lower contrast attenuation better SNR and full FOV.
Material decomposition can be further performed on dedicated workstation to create different image setting including iodine map (virtual contrast image), iodine subtraction (virtual non-contrast image) and bone mask (bone and calcium subtraction). A further perfusion blood volume color coded image can be created by using a gray or color scale. This perfusion blood volume image reflect the lung perfusion at a single time point thus it is only a surrogate perfusion image.
- DECT aortogram- potential to eliminate need for pre contrast image.1
- Suboptimal contrast injection/triggering- 80 KvP image can be used to improve contrast resolution on either DECT aortogram or DECT pulmonary angiogram studies.2
- DECT can reduce volume of contrast injection as 80KvP image improves contrast resolution.2
- 80 KvP image may have potential to improve subsegmental pulmonary artery perfusion and distal pulmonary embolus detection.2
- Perfusion blood volume map can be used to identified the segmental or subsegmental areas of lung affected by pulmonary embolus. Review of lung window is paramount as other lung pathology- atelectasis, cardiac motion or strak artefact can all cause perfusion defects.3
- Perfusion of pulmonary nodules and nodes.
- Subtraction of overlying calcium and bone.
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