A Breath of Fresh Air

Pulmonary embolism (PE) is a common form of venous thromboembolism in the middle-aged and older adult population (1Silverstein M.D. Heit J.A. Mohr D.N. et al.Trends in the incidence of deep vein thrombosis and pulmonary embolism: a 25-year population-based study.Arch Intern Med. 1998; 158: 585-593Crossref PubMed Scopus (2210) Google Scholar). PE can be rapidly fatal especially in the frail and elderly (2Horlander K.T. Mannino D.M. Leeper K.V. Pulmonary embolism mortality in the United States, 1979–1998: an analysis using multiple-cause mortality data.Arch Intern Med. 2003; 163: 1711-1717Crossref PubMed Scopus (489) Google Scholar). Those who survive are at risk of serious long-term complications including recurrent thromboembolism and chronic thromboembolic pulmonary hypertension. Treatment often includes chronic anticoagulation, which in itself is not without significant risk. Fast and reliable diagnosis is, therefore, essential. The clinical presentation of acute PE, however, is often nonspecific. Even after the introduction of D-dimer testing to screen out low risk patients, the majority of patients suspected of PE undergo imaging to establish the diagnosis (3van Belle A. Büller H.R. Huisman M.V. et al.Effectiveness of managing suspected pulmonary embolism using an algorithm combining clinical probability, D-dimer testing, and computed tomography.JAMA. 2006; 295: 172-179Crossref PubMed Scopus (893) Google Scholar). Pulmonary artery computed tomography angiography (PA-CTA) is now the well-established gold standard imaging test for the diagnosis of PE (4Remy-Jardin M. Remy J. Deschildre F. et al.Diagnosis of pulmonary embolism with spiral CT: comparison with pulmonary angiography and scintigraphy.Radiology. 1996; 200: 699-706Crossref PubMed Scopus (587) Google Scholar). An optimal quality PA-CTA study will provide peak enhancement of the pulmonary arteries while reducing artifact related to cardiac and respiratory motion. Modern scanners provide submillimeter resolution with near-isotropic images that lend themselves to excellent two- and three-dimensional reconstructions and give us the ability to detect thrombi below sixth-order pulmonary artery branches (5Schoepf U.J. Costello P. CT angiography for diagnosis of pulmonary embolism: state of the Art 1.Radiology. 2004; 230: 329-337Crossref PubMed Scopus (337) Google Scholar). Improvements in image quality have, however, come at the cost of higher radiation dose, a significant concern given the widespread use of PA-CTA, and also make images more susceptible to motion artifact. Strategies to reduce radiation exposure and minimize artifact include improved reconstruction algorithms, high-pitch scanning, peak kilovoltage (keV) reduction, and using magnetic resonance pulmonary artery angiography, which may be performed without intravenous contrast and does not expose the patient to ionizing radiation (6Kalra M.K. Maher M.M. Toth T.L. et al.Strategies for CT radiation dose optimization.Radiology. 2004; 230: 619-628Crossref PubMed Scopus (828) Google Scholar, 7McCollough C.H. Bruesewitz M.R. Kofler Jr, J.M. CT dose reduction and dose management tools: overview of available options 1.Radiographics. 2006; 26: 503-512Crossref PubMed Scopus (642) Google Scholar). These techniques will be briefly reviewed here. The patient population most commonly in need of evaluation for PE (the elderly in respiratory distress) is also the most likely to have impaired respiratory function and is often unable to cooperate with breath-holding instructions. This will often produce artifactual filling defects in the pulmonary arteries that can be mistaken for emboli either directly due to motion or due to poor mixing of blood and contrast from transient interruption of the contrast column (8Wittram C. Maher M.M. Yoo A.J. et al.CT angiography of pulmonary embolism: diagnostic criteria and causes of misdiagnosis.Radiographics. 2004; 24: 1219-1238Crossref PubMed Scopus (269) Google Scholar). To a certain degree, this can be addressed by coaching the patient prior to the scan with careful breath-hold instructions. Reducing scan time, however, will significantly improve image quality in patients who cannot cooperate with breath-hold (9Remy-Jardin M. Tillie-Leblond I. Szapiro D. et al.CT angiography of pulmonary embolism in patients with underlying respiratory disease: impact of multislice CT on image quality and negative predictive value.Eur Radiol. 2002; 12: 1971-1978Crossref PubMed Scopus (131) Google Scholar). High-pitch scanning has been in use mainly in cardiac imaging and in pediatric patients due to faster scan times and a significant reduction of radiation exposure (10Koplay M. Kizilca O. Cimen D. et al.Prospective ECG-gated high-pitch dual-source cardiac CT angiography in the diagnosis of congenital cardiovascular abnormalities: radiation dose and diagnostic efficacy in a pediatric population.Diagn Interv Imaging. 2016; https://doi.org/10.1016/j.diii.2016.03.014Crossref Scopus (16) Google Scholar). With older, single-detector systems, the maximum pitch is limited to 1.5 to ensure gapless coverage along the scan axis. Helical pitch of higher than 3 has become possible using dual-detector systems. In recent studies, high-pitch scanning has shown promise in routine chest CT with a marked reduction of artifact from cardiac and diaphragmatic motion while maintaining diagnostic image quality even in patients who are free breathing (11Lim H.K. Ha H.I. Hwang H.J. et al.Feasibility of high-pitch dual-source low-dose chest CT: reduction of radiation and cardiac artifacts.Diagn Interv Imaging. 2016; 97: 443-449Crossref Scopus (10) Google Scholar, 12Liang T. McLaughlin P. Arepalli C.D. et al.Dual-source CT in blunt trauma patients: elimination of diaphragmatic motion using high-pitch spiral technique.Emerg Radiol. 2016; 23: 127-132Crossref Scopus (6) Google Scholar). Both dose reduction and reduction in scan times are maximized when dual-detector systems are used. Another concern with PA-CTA is the need for intravenous contrast, which in a significant subset of patients cannot be safely administered due to impaired renal function. Some patients with renal impairment may be scanned with a reduced contrast dose; this, however, often results in suboptimal scan quality due to decreased opacification of the pulmonary arteries. Contrast dose also may be significantly reduced with a reduced keV technique, which increases intravascular contrast resolution because iodine resorption is inversely proportional to tube potential. Consequently, administering a lower dose of contrast will not reduce diagnostic quality for PE. Radiation dose savings in the order of 30%–50% also result when keV is reduced, at the cost of modest increases in image noise (13Chung Y.E. You J.S. Lee H.-J. et al.Possible contrast media reduction with low keV monoenergetic images in the detection of focal liver lesions: a dual-energy CT animal study.PLoS ONE. 2015; 10 (e0133170)Google Scholar, 14Delesalle M.A. Pontana F. Duhamel A. et al.Spectral optimization of chest CT angiography with reduced iodine load: experience in 80 patients evaluated with dual-source, dual-energy CT.Radiology. 2013; 267: 256-266Crossref PubMed Scopus (124) Google Scholar). Increased image noise is a concern with all types of low-dose CT techniques. The use of noise reduction filters in post processing has been shown to improve the image quality of scans acquired with reduced tube output (15Kalra M.K. Maher M.M. Sahani D.V. et al.Low-dose CT of the abdomen: evaluation of image improvement with use of noise reduction filters—pilot study.Radiology. 2003; 228: 251-256Crossref PubMed Scopus (134) Google Scholar). Image noise also can be reduced by using iterative reconstruction instead of filtered back projection, although this comes at the cost of increased reconstruction time or the need for more computing power (16Pontana F. Pagniez J. Flohr T. et al.Chest computed tomography using iterative reconstruction vs filtered back projection (part 1): evaluation of image noise reduction in 32 patients.Eur Radiol. 2011; 21: 627-635Crossref PubMed Scopus (164) Google Scholar, 17Silva A.C. Lawder H.J. Hara A. et al.Innovations in CT dose reduction strategy: application of the adaptive statistical iterative reconstruction algorithm.Am J Roentgenol. 2010; 194: 191-199Crossref PubMed Scopus (473) Google Scholar). Building on these concepts, in the current issue of Academic Radiology, the article by Martini et al. examines a free breathing high-pitch CT technique with reduced keV for PE-CTA. Image qualities were compared to standard-pitch scanning with breath-hold. The authors examined PA-CTA scans from 100 patients, 50 of whom were prospectively randomized into a high-pitch (pitch = 3) low keV (80 kVp) group scanned with free breathing and the remaining 50 were scanned with a low-pitch protocol (pitch = 1.2, 100 kVp). The patients were followed for any events related to PE for a 30-day period, during which no events were detected in either group. Dual-source scanners and iterative reconstruction were used in all patients. The authors found that in the high-pitch group, radiation dose was significantly lower, main PA attenuation was improved, and although image noise was higher signal-to-noise ratio was not reduced compared to the standard pitch group. In spite of free breathing, high-pitch scanning resulted in less motion artifact both from breathing and cardiac motion. Artifacts from transient interruption of the contrast column were also reduced in the high-pitch CT group. Image quality was deemed diagnostic in all patients in both groups, and although contrast-to-noise ratio was lower in the high-pitch group interpreter confidence was higher. Qualitative assessment of cardiac and bronchial structures was deemed more reliable in the high-pitch CT group. The authors conclude that high-pitch PA-CTA allows improved image quality and diagnostic confidence while reducing radiation dose. The role of magnetic resonance imaging (MRI) in PE scanning is not discussed in this CT-focused study. MRI has been shown to be accurate in detecting PE without exposing the patient to radiation (18Kluge A. Luboldt W. Bachmann G. Acute pulmonary embolism to the subsegmental level: diagnostic accuracy of three MRI techniques compared with 16-MDCT.Am J Roentgenol. 2006; 187: W7-W14Crossref PubMed Scopus (142) Google Scholar). A wide variety of MR techniques, some free breathing, have been used in PE evaluation, including magnetic resonance angiography, MR perfusion, and steady-state free precession protocols which allow PE scanning without the use of contrast (19van Beek E.J. Wild J.M. Fink C. et al.MRI for the diagnosis of pulmonary embolism.J Magn Reson Imaging. 2003; 18: 627-640Crossref PubMed Scopus (64) Google Scholar, 20Kluge A. Müller C. Hansel J. et al.Real-time MR with TrueFISP for the detection of acute pulmonary embolism: initial clinical experience.Eur Radiol. 2004; 14: 709-718Crossref PubMed Scopus (71) Google Scholar, 21Hochhegger B. Ley-Zaporozhan J. Marchiori E. et al.Magnetic resonance imaging findings in acute pulmonary embolism.Br J Radiol. 2014; 84: 282-287Crossref Scopus (33) Google Scholar, 22Fink C. Ley S. Schoenberg S.O. et al.Magnetic resonance imaging of acute pulmonary embolism.Eur Radiol. 2007; 17: 2546-2553Crossref PubMed Scopus (38) Google Scholar). Although MRI is a promising alternative to CT and may become more widespread in the future, its use is currently limited by a relative lack of interpreter familiarity, scanner availability, and higher cost compared to CT.