A post-processing system uses these differential data to generate:

Transcription

1 Iterative reconstruction in single source dual-energy CT pulmonary angiography: Is sufficient to achieve a radiation dose as low as state-of-the-art single-energy CTPA? Poster No.: B-0300 Congress: ECR 2014 Type: Scientific Paper Authors: M. Ohana, M.-Y. Jeung, A. Labani, S. El-Ghannudi, C. Roy; Strasbourg/FR Keywords: Pulmonary vessels, Thorax, Radioprotection / Radiation dose, CT-Angiography, Technology assessment, Comparative studies, Embolism / Thrombosis DOI: /ecr2014/B-0300 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 22

2 Purpose Single source dual energy CT pulmonary angiography (DE-CTPA) induces an increase of radiation dose going up to 40% when compared to a single energy examination carried out on the same scanner. The main objective of this study was to nullify this increase by using iterative reconstruction (Adaptive Statistical Iterative Reconstruction - ASIR), and thus validate the diagnostic quality of DE CTPA made at the same radiation dose that state-of-the-art single energy CTPA. Dual Energy Computed Tomography (DECT) is based on a simultaneous acquisition at low and high kilovoltage (usually 80 and 140kV), with two different approaches on commercially available scanners (Fig. 1 on page 3): Dual Source technique (Somatom Definition FLASH, Siemens), using two separates couples of x-ray tube/detectors angled at 95 ; Single Source technique (Discovery CT750HD, GE Healthcare), using one x-ray tube with fast kilovoltage switching between 80 and 140kV every 0,5msec and detectors with fast response and very short afterglow. The main advantage of DECT is the material-based decomposition. The higher the molecular weight of a material, the greater the absorption differences between high and low kilovoltage are, due to the increased likelihood of a photoelectric effect at a lower kv. This is particularly marked for iodine. A post-processing system uses these differential data to generate: Monochromatic reconstructions, with real-time image reconstruction at a desired energy level, ranging from 40 to 140 kev. Lowering kev allows a dramatic increase in iodine contrast, as seen in Fig. 2 on page 4. Materiel density images, obtained through the decomposition of dual energy acquisitions using the attenuation spectrum of two basis materials, thus generating two sets of material-density images. In CT angiography, the winning pair for material-based decomposition is the iodine/water couple (Fig. 3 on page 5), from which iodine-density images corresponding to a cartography of contrast uptake and water-density images equivalent to an unenhanced acquisition can be obtained. Page 2 of 22

3 Low kev monochromatic reconstructions and iodine-density images are therefore particularly useful in CT pulmonary angiography, as they maximize iodine contrast and offer an appreciation of lung parenchymal perfusion. However, these advantages come at the expanse of an increased radiation dose: with dual source DECT, it is possible to set a different tension for each tube, i.e. fix higher ma for lower kv and vice-versa, so as to optimize the radiation dose delivered. In the literature, CTPA radiation dose ranges from 280 to 405mGy.cm. with single source DECT, it is currently impossible to set apart the tube current between high and low kv, or modulate it during the acquisition. Therefore, ma is fixed and identical for both kv, and cannot be lowered too much as the intensity of the low kv X-ray must remain sufficient. Dose Length Product for CTPA is reported going from 330 up to 470mGy.cm. In single source CTPA, these radiation doses were obtained using classical filtered back projection (FBP). Introduction of iterative reconstruction for dual energy CT is recent, and should allow as for single energy examinations an increase in image quality and in signal-to-noise ratio, therefore permitting to reduce radiation dose to attain the same quality level. We hypothesize that thanks to iterative reconstruction, we can cut the radiation dose of DE-CTPA to the one of a single energy examination, while maintaining at least the same image quality that a full dose FBP DE-CTPA. Images for this section: Page 3 of 22

4 Fig. 1: Dual Energy CT basic principles Page 4 of 22

5 Fig. 2: Monochromatic reconstructions Page 5 of 22

6 Fig. 3: Material-density images Page 6 of 22

7 Methods and materials This study was approved by our Institutional Review Board, and informed consent was obtained from all patients. Fifty adults referred to our department for CTPA in search of potential pulmonary embolism were prospectively included in this study between March and September All their examinations were carried out on a single source dual energy scanner (Discovery CT750 HD, GE Healthcare, Milwaukee, Wisconsin, USA), using a dual energy protocol with acquisitions parameters tweaked to target a radiation dose similar to a 100kV single energy CTPA, i.e. a DLP of 260mGy.cm. For this objective, fixed tube current was lowered to 275mA and iterative reconstruction was used, with an ASIR level set at 50%. Thirty patients from a previous prospective study who underwent classical FBP reconstructed DE-CTPA with full radiation dose (fixed tube current set at 375mA) on the same machine were used as the reference group. Apart from the tube current and the use of iterative reconstruction, all other acquisition parameters were exactly the same between the two groups: energies set at 80 and 140kV, 0.6s rotation time, automatic injection of 50mL of Iomeprol 400mg/ml (Iomeron, -1 Bracco, Milan, Italy) at 3.5ml.s, 50mL saline chaser, bolus-tracking technique with a ROI in the main pulmonary artery and a 60 HU threshold. Exclusion criteria were for both groups the classical contraindications for iodine contrast media injection (proven allergy, severe renal impairment with MDRD clearance <30-1 ml.min ), pregnancy and patient refusal. For each patient, height and weight were noted in order to calculate the Body Mass Index (BMI). All cases were thoroughly reviewed by two radiologists specialized in thoracic imaging using an Advantage Workstation in version 4.6 and the GSI Viewer software. Dynamic interpretation and analysis were conducted with special focus on: A qualitative analysis, with an evaluation of the overall image quality and examination diagnostic value using a qualitative 5 level scale (Table 1). Final Page 7 of 22

8 grade was given after a systematic dynamic analysis of the monochromatic reconstructions between 40 and 80 kev (concept of "spectral-surfing") and careful review of the iodine-density images in the axial, coronal and sagittal planes. Discrepant cases, defined by a difference greater than 2 points between the two readers' grades, were re-evaluated by a third expert. A quantitative analysis, with HU measurements including standard deviations obtained by a circular ROI averaging at least 5mm², made on 65keV monochromatic reconstructions. These systematic measures interested the main pulmonary artery, a principal pulmonary artery, a lobar branch (usually the left lower one) and an erector spinae muscle not in fatty atrophy. 1 Minimal or absent opacification Nondiagnostic examination 2 Weak opacification Large arteries visible but no definite diagnosis possible 3 Limited opacification Sufficient for a diagnosis 4 Good opacification To the peripheral pulmonary arteries up to the 4th division 5 Excellent opacification To the subsegmental pulmonary arteries beyond 5th division Table 1: Qualitative 5 level scale Qualitative results were analyzed by a Kruskal-Wallis nonparametric test and quantitative data were compared using a Student's t test. A p<0.05 was considered significant. Results Population Page 8 of 22

9 The population of the two groups shared overall comparable characteristics: age, sex-ratio and body mass index showed no statistically significant difference. These demographics are reported in Table 2. ASIR Group Reference Group p (Full dose with FBP) Age 64.8 ± ±18.6 0,9159 (yo) (30-92) (21-91) (Student's t test) BMI 25.6 ± ±4.6 0,5984 ( ) ( ) (Student's t test) 29M/21W 14M/16W Sex ratio (Kruskal-Wallis) Table 2: Demographics of the study Quantitative analysis HU enhancement weren't statistically different between both groups, whereas mean image noise was significantly lower in the ASIR group. However, signal to noise and contrast to noise ratios weren't significantly better in the ASIR group (Table 3). ASIR Group Reference Group p (Student's (Full dose with test) FBP) Pulmonary trunk Main pulmonary artery 365 ± ±136 ( ) ( ) 350 ± ±128 ( ) ( ) Literature reference t (50keV FBP) Page 9 of 22

10 Lower artery lobe 337 ±91 ( ) 388 ± ± * 28.6 ± ± ±7.5 ( ) Mean arterial 351 ±88 enhancement ( ) 394 ±131 Mean noise 22.1 ± ±6.9 ( ) ( ) Signal to 16.9 ±6 Noise ratio ( ) ( ) 14.9 ±6.9 (7.7-24) Contrast to 14.5 ± ±4.8 Noise ratio ( ) ( ) Table 3: Quantitative enhancement analysis Qualitative analysis There was no significant difference in the qualitative evaluation between both groups (p= Table 4). All the CTPA examinations were diagnostic (grade # 3), with an overall excellent image quality. ASIR Group Average 4.44 ±0.7 qualitative (3-5) score Table 4: Qualitative analysis Reference Group p Literature reference (Kruskal(Full dose with Wallis) FBP) (FBP) 4.6 ± ,3175 (4-5) Radiation dose Page 10 of 22

11 Radiation dose was commonsensically very significantly decreased in the ASIR group by 37% (p<0.0001), with our initial objective even surpassed as the final mean DLP was below the original target of 260mGy.cm (Table 5). ASIR Group Reference Group p (Student's (Full dose with test) FBP) DLP 244 ±33 (mgy.cm) ( ) Table 5: Mean radiation dose 388 ±100 <0.0001* Literature reference t (FBP) 412 ±34 ( ) Some examples to illustrate the utility of iterative reconstruction and the excellent image quality obtained at these low radiation doses are provided in Fig. 4 on page 11 and Fig. 5 on page 12. Images for this section: Page 11 of 22

12 Fig. 4: Utility of Iterative Reconstruction Page 12 of 22

13 Fig. 5: Examples of image quality Page 13 of 22

14 Conclusion Our initial objective is fulfilled: this study demonstrates the efficiency of iterative reconstruction to nullify the increase in radiation dose associated with the use of a dual energy technique. Therefore, one is able to realize DE CTPA of excellent diagnostic quality at a radiation dose (DLP = 244 mgy.cm) equivalent to that of a single energy examination carried out on the same scanner, without any significant difference in the arterial enhancement, the signal to noise and contrast to noise ratios or the qualitative aspect, and even with a lower mean image noise. Even though image rating was globally very good (4.44 out of 5) and all investigations were of diagnostic quality, there were still 5 examinations in the ASIR group that were rated only 3 out of 5 due to a limited opacification. Most of these unsatisfactory examinations could be related to a difficult patient (poor general condition, polypnea, pathological underlying lung parenchyma, obesity). We tried to determine if there were a relation between the image quality and the patient's morphotype using Pearson's correlation test: there was a strong positive relationship between image quality and signal to noise and contrast to noise ratios (#= 0.57 and 0.55 respectively - Fig. 6 on page 16); there was a strong negative relationship between body mass index and signal to noise and contrast to noise ratios (#= and respectively Fig. 7 on page 17); however, there was only a moderate negative relationship between body mass index and image quality (#= Fig. 8 on page 17). This low negative correlation between image quality and BMI is probably due to the lack of relationship between the whole body weight and the attenuation of the thorax. Indeed, apart from the extremes (BMI<18 or >40), the BMI is not necessarily a good reflection of the thorax diameter: it can overestimate it when fat deposits preferentially over the abdomen or the hip, or underestimate it in big-breasted women. Being far from perfect, it is still the most easily obtainable morphological information to routinely optimize the acquisition parameters. Page 14 of 22

15 We can propose the following compromises for DE CTPA with iterative reconstruction (Table 6): BMI Target DLP Fixed Tube Current (mgy.cm) (ma) < <BMI< <BMI< > Table 6: Proposed optimal acquisition parameters 375 This is eventually only an ersatz of automated tube current modulation, as the technique is not currently available on single source DECT. Study limitations Only 50 patients in the ASIR group, which is however enough to demonstrate the feasibility of the technique. Evaluation was made only on quantitative and qualitative levels, with no comparison of diagnostic accuracy to a reference standard. However, CTPA is now the standard of care for suspected PE in the emergency setting, and DE-CTPA is already well established as being more efficient than single energy examinations. Only one radiation dose (fixed tube current at 275mA with a target DLP at 260mGy.cm) was studied. Test of the other proposed intermediate doses is currently under way. Only one level of ASIR (50%) was studied. For CTA, a higher level of iteration is possible with ASIR pushed up to 70-80%, which could further increase the quality of the examination. The bolus-tracking triggering ROI was placed in the main pulmonary artery. Some authors recommend placing it in the right cardiac cavities, to avoid missing the bolus in hyperkinetic patients. We favored the main pulmonary artery to obtain an enhancement (even moderate) of the thoracic aorta, useful for differential diagnosis in the context of acute chest pain Page 15 of 22

16 CONCLUSION This study demonstrates the efficiency of the iterative reconstruction to nullify the increase in radiation dose associated with the use of a dual energy technique. Therefore, single source dual energy CTPA of excellent diagnostic quality can be acquired at a radiation dose (DLP = 244 mgy.cm) equivalent to that of a single energy examination. Patient's morphotype directly impact the image quality, and while automated tube current modulation remains unavailable on single source DECT, we can advocate a manual adjustment of the dose delivered in correlation with the BMI. Images for this section: Fig. 6: SNR and CNR VS Image Quality Page 16 of 22

17 Fig. 7: SNR and CNR VS BMI Page 17 of 22

18 Fig. 8: Image Quality VS BMI Page 18 of 22

19 Personal information Dr Mickaël OHANA Radiologist Strasbourg University Hospital For any questions about this work: Follow me on Dr Mi-Young JEUNG Radiologist Strasbourg University Hospital Dr Aissam LABANI Radiology Fellow Strasbourg University Hospital Dr Soraya El-Ghannudi Cardiologist Strasbourg University Hospital Pr Catherine ROY Head of Radiology department Strasbourg University Hospital Images for this section: Page 19 of 22

20 Fig. 9: Aerial view of Strasbourg and the "Nouvel Hôpital Civil" Page 20 of 22

21 References Godoy MCB, Heller SL, Naidich DP & al. Dual-energy MDCT: Comparison of pulmonary artery enhancement on dedicated CT pulmonary angiography, routine and low contrast volume studies. European Journal of Radiology (2011). 79(2): p. e11-e17 Geyer LL, Scherr M, Körner M & al. Imaging of acute pulmonary embolism using a dual energy CT system with rapid kvp switching: Initial results. European Journal of Radiology (2011). In Press(Corrected Proof) Ferda J, Ferdová E, Mírka H & al. Pulmonary imaging using dual-energy CT, a role of the assessment of iodine and air distribution. European Journal of Radiology (2011). 77(2): p Thieme SF, Graute V, Nikolaou K & al. Dual Energy CT lung perfusion imaging-correlation with SPECT/CT. European Journal of Radiology (2012). 81(2): p Ho LM, Yoshizumi TT, Hurwitz LM & al. Dual Energy Versus Single Energy MDCT: Measurement of Radiation Dose Using Adult Abdominal Imaging Protocols. Academic Radiology (2009). 16(11): p Yuan R, Shuman WP, Earls JP & al. Reduced Iodine Load at CT Pulmonary Angiography with Dual-Energy Monochromatic Imaging: Comparison with Standard CT Pulmonary Angiography-A Prospective Randomized Trial. Radiology (2012). 262(1): p de Broucker T, Pontana F, Santangelo T & al. Single- and dual-source chest CT protocols: Levels of radiation dose in routine clinical practice. Diagnostic and Interventional Imaging (2012) 93:p Henzler T, Meyer M, Reichert M & al. Dual-energy CT angiography of the lungs: Comparison of test bolus and bolus tracking techniques for the determination of scan delay. European Journal of Radiology (2012). 81(1): p Hoey ETD, Gopalan D, Ganesh V & al. Dual-energy CT pulmonary angiography: a novel technique for assessing acute and chronic pulmonary thromboembolism. Clinical Radiology (2009). 64(4): p Kang M-J, Park CM, Lee C-H, Goo JM, and Lee HJ Dual-Energy CT: Clinical Applications in Various Pulmonary Diseases. Radiographics (2010). 30(3): p Viteri-Ramírez G, García-Lallana A, Simón-Yarza I & al. Low radiation and low-contrast dose pulmonary CT angiography: Comparison of 80 kvp/60 ml and 100 kvp/80 ml protocols. Clinical Radiology (2012). In press - corrected proof Engelkemier DR, Tadros A, and Karimi A Lower iodine load in routine contrast-enhanced CT: an alternative imaging strategy. J Comput Assist Tomogr (2012). 36(2): p Page 21 of 22

22 Kerl JM, Bauer RW, Renker M & al. Triphasic contrast injection improves evaluation of dual energy lung perfusion in pulmonary CT angiography. European Journal of Radiology (2011). 80(3): p. e483-e487 Page 22 of 22