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Lung SURESubtraction in Everyday Practice - Dr R. Bull

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Post contrast iodine maps were introduced as part of dual-energy imaging over 10 years ago but these have never become part of routine practice in most centres for the investigation of pulmonary thromboembolic disease.

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Lung SURESubtraction in Everyday Practice - Dr R. Bull

  1. 1. VISIONS26 | 49 These dual-energy iodine maps allow assessment of the concentration of iodine in the lung parenchyma at the time of scanning and can act as a surrogate of lung perfu- sion providing additional information regarding respira- tory physiology not usually obtainable on conventional anatomical CTPA images. The generation of these iodine maps using dual energy relies on the difference in attenu- ation of enhanced lung parenchyma at the 2 energies used (Fig. 1). Lung SURESubtraction in Everyday Practice Post contrast iodine maps were introduced as part of dual-energy imaging over 10 years ago but these have never become part of routine practice in most centres for the investigation of pulmonary thromboembolic disease. Dr R. Bull 1) R. Bull 1) Royal Bournemouth Hospital, UK CTEU150108 SUBTRACTION COMPUTED TOMOGRAPHY CTPA, Lungs, Subtraction As dual energy imaging utilizes the post contrast differ- ence in attenuation of iodine between 2 separate kVp images, the amount of signal generated is rather small meaning that noise levels have to be low in order to generate images with sufficient signal to noise ratio. This usually means that radiation doses have to be relatively high to generate reasonable quality iodine maps (typi- cally 5mSv). SURESUBTRACTION – IODINE MAPS WITHOUT THE RADIATION PENALTY This is a novel technique whereby signal generated from a pre contrast image (typically at 100 kVp) is subtracted from the same anatomical area scanned post contrast. Figure 1: Iodine map generation with dual energy1 Figure 2: Iodine map generation with SURESubtraction1 0 0 5 10 15 100 200 300 400 500 mg/ml iodine Enhancement(HU) 100 kVp 140 kVp Dual Energy 0 0 5 10 15 100 200 300 400 500 mg/ml iodine Enhancement(HU) 100 kVp 140 kVp Subtraction 0 0 5 10 15 100 200 300 400 500 mg/ml iodine Enhancement(HU) 100 kVp 140 kVp Dual Energy 0 0 5 10 15 100 200 300 400 500 mg/ml iodine Enhancement(HU) 100 kVp 140 kVp Subtraction As can be seen in Fig. 2, this results in much higher signal generation compared with dual energy.This allows radia- tion doses to be reduced to very low levels on both pre and post contrast scans whilst maintaining a high signal to noise ratio. In combination with the ultra-dose efficient PUREViSION detector this allows iodine maps to be used routinely in the investigation of thromboembolic disease at extremely low radiation doses. Although conceptu- ally simple, the key to the success of SURESubtraction is advanced non-rigid registration allowing compensation for slight patient movement or differences in breathing between the pre and post contrast scans. SURESubtraction also has the advantage that there is no restriction on field of view, allowing iodine maps to be obtained even in large patients (dual tube systems are restricted due to the limited field of view of the‘B’tube). SURESUBTRACTION DOES NOT SLOW WORKFLOW In order to perform a SURESubtraction scan, the radiog- rapher simply selects the appropriate protocol which automatically covers the same anatomical area on the pre and post contrast scans and automatically sets an appropriate modulated radiation dose for each run based on desired image quality (more noise can be
  2. 2. ©2015 TOSHIBA MEDICAL SYSTEMS50 | VISIONS26 tolerated on the pre contrast scan thus the radiation dose is set to be lower). On the newest version 7 soft- ware lung iodine maps are generated automatically in 3 planes and are sent to PACS together with the conven- tional anatomical CTPA images. SURESUBTRACTION ALLOWS MORE SENSITIVE DETECTION OF PULMONARY EMBOLI The addition of functional images of lung iodine concen- tration to conventional post contrast anatomical images allows detection of sub-segmental emboli that would be otherwise missed. SURESUBTRACTION ALLOWS BETTER ASSESSMENT OF CHRONIC THROMBOEMBOLIC DISEASE Anatomical images are able to assess resolution of central thrombus within the larger pulmonary vessels but are unable to accurately assess clot burden at the level of the small vessels which can be highly significant functionally (Figs. 4a, b, c). SURESUBTRACTION ALLOWS DETECTION OF SUBTLE LUNG DISEASE Conditions such as small airways disease cause air trap- ping and reduced lung attenuation. Although this can be very significant functionally, this is easy to miss on conventional anatomical images as the difference in lung attenuation can be small (Fig. 5a). As air trapping leads to hypoxic pulmonary vasoconstriction, changes in blood flow and thus iodine concentration are much more obvious (Fig. 5b). SURESUBTRACTION INCREASES ROBUSTNESS Despite apparent optimal timing, conventional CTPA studies are commonly not diagnostic due to inadequate vessel opacification. This is particularly common in Figure 3: Sub-segmental emboli causing peripheral wedge shaped iodine defect and subtle cutoff in contrast of sub-segmental pulmonary artery (arrows) Figure 4c: SURESubtraction iodine map at time of follow up demonstrates multiple wedge shaped defects (arrows) indicating persistent high clot burden consistent with persistent severe dyspnea and pulmonary hypertension. Figure 4b : Extensive organized thrombus removed from pulmonary arterial tree at surgery with resolution of dyspnea and pulmonary hypertension Figure 4a: 25 year old man. Axial post contrast images demonstrating improvement in central clot burden 3 months after acute presentation with Pulmonary Embolism (arrows)
  3. 3. VISIONS26 | 51 Figure 5a: Subtle geographical area of reduced attenuation in left upper lobe indicating small airways disease (arrows) Figure 6a: Septic patient, 38 weeks pregnant, HR 120. History of asthma. Suboptimal pulmonary arterial opacification due to very high cardiac output Figure 5b: corresponding iodine map demonstrating obvious wedge shaped defect in same area due to reduced blood flow (arrows) Figure 6b: Iodine map demonstrating minor heterogeneity of lung perfusion secondary to asthma but no pulmonary emboli. Total radiation dose 0.7 mSv (CTPA + SURESubtraction iodine map) patients with high cardiac outputs due to e.g. sepsis or pregnancy due to contrast dilution and very rapid transit of contrast through the central pulmonary arteries leading to ‘late’ scan triggering. The addition of iodine maps allows an assessment of lung perfusion to be made even in the cases of poor central vascular opacification. In the setting of suboptimal pulmonary atrial opacification with normal lung iodine maps, pul- monary embolus can be excluded at low radiation dose (Fig. 6a, b). CONCLUSION For many years lung iodine perfusion maps generated using dual energy have been a promising technique but have had limited acceptance in routine practice due to the potential radiation penalty and/or need for specialized equipment. Lung SURESubtraction allows gen- eration of these iodine maps at very low radiation doses using standard pre and post contrast images allowing the potential for this technique to be adopted in routine clinical practice. Reference 1. W.J. van der Woude, “Superiority of Lung Subtraction above Dual Energy”, VISIONS Vol. 23, p. 49, 2014. CTEU150108
  • RachelHo13

    Feb. 9, 2016
  • BurcuHz

    Feb. 3, 2016

Post contrast iodine maps were introduced as part of dual-energy imaging over 10 years ago but these have never become part of routine practice in most centres for the investigation of pulmonary thromboembolic disease.

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