Photon-counting CT in Thoracic Imaging: Early Clinical Evidence and Incorporation Into Clinical Practice.

Photon-counting CT (PCCT) is an emerging advanced CT technology that differs from conventional CT in its ability to directly convert incident x-ray photon energies into electrical signals. The detector design also permits substantial improvements in spatial resolution and radiation dose efficiency and allows for concurrent high-pitch and high-temporal-resolution multienergy imaging. This review summarizes (a) key differences in PCCT image acquisition and image reconstruction compared with conventional CT; (b) early evidence for the clinical benefit of PCCT for high-spatial-resolution diagnostic tasks in thoracic imaging, such as assessment of airway and parenchymal diseases, as well as benefits of high-pitch and multienergy scanning; (c) anticipated radiation dose reduction, depending on the diagnostic task, and increased utility for routine low-dose thoracic CT imaging; (d) adaptations for thoracic imaging in children; (e) potential for further quantitation of thoracic diseases; and (f) limitations and trade-offs. Moreover, important points for conducting and interpreting clinical studies examining the benefit of PCCT relative to conventional CT and integration of PCCT systems into multivendor, multispecialty radiology practices are discussed.

Infrapopliteal Segments on Lower Extremity CTA: Prospective Intraindividual Comparison of Energy-Integrating Detector CT and Photon-Counting Detector CT.

BACKGROUND. The higher spatial resolution and image contrast for iodine-containing tissues of photon-counting detector (PCD) CT may address challenges in evaluating small calcified vessels when performing lower extremity CTA by energy-integrating detector (EID) CTA. OBJECTIVE. The purpose of the study was to compare the evaluation of infrapopliteal vasculature between lower extremity CTA performed using EID CT and PCD CT. METHODS. This prospective study included 32 patients (mean age, 69.7 ± 11.3 [SD] years; 27 men, five women) who underwent clinically indicated lower extremity EID CTA between April 2021 and March 2022; participants underwent investigational lower extremity PCD CTA later the same day as EID CTA using a reduced IV contrast media dose. Two radiologists independently reviewed examinations in two sessions, each containing a random combination of EID CTA and PCD CTA examinations; the readers assessed the number of visualized fibular perforators, characteristics of stenoses at 11 infrapopliteal segmental levels, and subjective arterial sharpness. RESULTS. Mean IV contrast media dose was 60.0 ± 11.0 (SD) mL for PCD CTA versus 139.6 ± 11.8 mL for EID CTA (p < .001). The number of identified fibular perforators per lower extremity was significantly higher for PCD CTA than for EID CTA for reader 1 (R1) (mean ± SD, 6.4 ± 3.2 vs 4.2 ± 2.4; p < .001) and reader 2 (R2) (8.8 ± 3.4 vs 7.6 ± 3.3; p = .04). Reader confidence for assessing stenosis was significantly higher for PCD CTA than for EID CTA for R1 (mean ± SD, 82.3 ± 20.3 vs 78.0 ± 20.2; p < .001) but not R2 (89.8 ± 16.7 vs 90.6 ± 7.1; p = .24). The number of segments per lower extremity with total occlusion was significantly lower for PCD CTA than for EID CTA for R2 (mean ± SD, 0.5 ± 1.3 vs 0.9 ± 1.7; p = .04) but not R1 (0.6 ± 1.3 vs 1.0 ± 1.5; p = .07). The number of segments per lower extremity with clinically significant nonocclusive stenosis was significantly higher for PCD CTA than for EID CTA for R1 (mean ± SD, 2.2 ± 2.2 vs 1.6 ± 1.7; p = .01) but not R2 (1.1 ± 2.0 vs 1.1 ± 1.4; p = .89). Arterial sharpness was significantly greater for PCD CTA than for EID CTA for R1 (mean ± SD, 3.2 ± 0.5 vs 1.8 ± 0.5; p < .001) and R2 (3.2 ± 0.4 vs 1.7 ± 0.8; p < .001). CONCLUSION. PCD CTA yielded multiple advantages relative to EID CTA for visualizing small infrapopliteal vessels and characterizing associated plaque. CLINICAL IMPACT. The use of PCD CTA may improve vascular evaluation in patients with peripheral arterial disease.

Training and Verification Requirements for Interpretation of Cardiac CT and MRI: AJR Expert Panel Narrative Review.

The use of cardiac CT and MRI is rapidly expanding based on strong evidence from large international trials. The number of physicians competent to interpret cardiac CT and MRI may be unable to keep pace with the increasing demand. Societies and organizations have prescribed training requirements for interpreting cardiac CT and MRI, with recent updates focusing on the increased breadth of competency that is now required due to ongoing imaging advances. In this AJR Expert Panel Narrative Review, we discuss several aspects of cardiac CT and MRI training, focusing on topics that are uncertain or not addressed in existing society statements and guidelines, including determination of competency in different practice types in real-world settings and the impact of artificial intelligence on training and education. The article is intended to guide updates in professional society training requirements and also inform institutional verification processes.

Multimodality Imaging in Metabolic Syndrome: State-of-the-Art Review.

Metabolic syndrome comprises a set of risk factors that include abdominal obesity, impaired glucose tolerance, hypertriglyceridemia, low high-density lipoprotein levels, and high blood pressure, at least three of which must be fulfilled for diagnosis. Metabolic syndrome has been linked to an increased risk of cardiovascular disease and type 2 diabetes mellitus. Multimodality imaging plays an important role in metabolic syndrome, including diagnosis, risk stratification, and assessment of complications. CT and MRI are the primary tools for quantification of excess fat, including subcutaneous and visceral adipose tissue, as well as fat around organs, which are associated with increased cardiovascular risk. PET has been shown to detect signs of insulin resistance and may detect ectopic sites of brown fat. Cardiovascular disease is an important complication of metabolic syndrome, resulting in subclinical or symptomatic coronary artery disease, alterations in cardiac structure and function with potential progression to heart failure, and systemic vascular disease. CT angiography provides comprehensive evaluation of the coronary and systemic arteries, while cardiac MRI assesses cardiac structure, function, myocardial ischemia, and infarction. Liver damage results from a spectrum of nonalcoholic fatty liver disease ranging from steatosis to fibrosis and possible cirrhosis. US, CT, and MRI are useful in assessing steatosis and can be performed to detect and grade hepatic fibrosis, particularly using elastography techniques. Metabolic syndrome also has deleterious effects on the pancreas, kidney, gastrointestinal tract, and ovaries, including increased risk for several malignancies. Metabolic syndrome is associated with cerebral infarcts, best evaluated with MRI, and has been linked with cognitive decline. ©RSNA, 2024 Test Your Knowledge questions for this article are available in the supplemental material. See the invited commentary by Pickhardt in this issue.

Mitral Annular Disease at Cardiac MRI: What to Know and Look For.

Accurate evaluation of the mitral valve (MV) apparatus is essential for understanding the mechanisms of MV disease across various clinical scenarios. The mitral annulus (MA) is a complex and crucial structure that supports MV function; however, conventional imaging techniques have limitations in fully capturing the entirety of the MA. Moreover, recognizing annular changes might aid in identifying patients who may benefit from advanced cardiac imaging and interventions. Multimodality cardiovascular imaging plays a major role in the diagnosis, prognosis, and management of MV disease. Transthoracic echocardiography is the first-line modality for evaluation of the MA, but it has limitations. Cardiac MRI (CMR) has emerged as a robust imaging modality for assessing annular changes, with distinct advantages over other imaging techniques, including accurate flow and volumetric quantification and assessment of variations in the measurements and shape of the MA during the cardiac cycle. Mitral annular disjunction (MAD) is defined as atrial displacement of the hinge point of the MV annulus away from the ventricular myocardium, a condition that is now more frequently diagnosed and studied owing to recent technical advances in cardiac imaging. However, several unresolved issues regarding MAD, such as the functional significance of pathologic disjunction and how this disjunction advances in the clinical course, require further investigation. The authors review the role of CMR in the assessment of MA disease, with a focus on MAD and its functional implications in MV prolapse and mitral regurgitation. ©RSNA, 2024 Supplemental material is available for this article. See the invited commentary by Stojanovska and Fujikura in this issue.

Milestones in CT: Past, Present, and Future.

In 1971, the first patient CT examination by Ambrose and Hounsfield paved the way for not only volumetric imaging of the brain but of the entire body. From the initial 5-minute scan for a 180° rotation to today's 0.24-second scan for a 360° rotation, CT technology continues to reinvent itself. This article describes key historical milestones in CT technology from the earliest days of CT to the present, with a look toward the future of this essential imaging modality. After a review of the beginnings of CT and its early adoption, the technical steps taken to decrease scan times-both per image and per examination-are reviewed. Novel geometries such as electron-beam CT and dual-source CT have also been developed in the quest for ever-faster scans and better in-plane temporal resolution. The focus of the past 2 decades on radiation dose optimization and management led to changes in how exposure parameters such as tube current and tube potential are prescribed such that today, examinations are more customized to the specific patient and diagnostic task than ever before. In the mid-2000s, CT expanded its reach from gray-scale to color with the clinical introduction of dual-energy CT. Today's most recent technical innovation-photon-counting CT-offers greater capabilities in multienergy CT as well as spatial resolution as good as 125 µm. Finally, artificial intelligence is poised to impact both the creation and processing of CT images, as well as automating many tasks to provide greater accuracy and reproducibility in quantitative applications.

Cardiac MRI: State of the Art.

Cardiac MRI plays an important role in the evaluation of cardiovascular diseases (CVDs), including ischemic heart disease, cardiomyopathy, valvular disease, congenital disease, pericardial disease, and masses. Large multicenter trials have shown the positive impact of MRI-based management on outcomes in several CVDs. These results have made MRI an indispensable technique in the evaluation of these diseases, and cardiac MRI has an important role in multisociety guidelines. MRI is the reference standard for quantification of ventricular volumes and function. Flow imaging enables accurate quantification of flow and velocities through valves, shunts, and surgical conduits or baffles. Late gadolinium enhancement and parametric mapping techniques enable tissue characterization and yield prognostic information. In the past decade, cardiac MRI technology has seen rapid advances in both hardware and sequences. Multiple novel sequences, such as parametric mapping and four-dimensional flow, are increasingly being incorporated into routine clinical practice. Acceleration strategies have matured, allowing faster acquisition of cardiac MRI sequences in patients with arrhythmia and poor breath holding. Challenges of cardiac MRI at high-field-strength magnets and in patients with indwelling cardiac devices or severe renal dysfunction have been mitigated. Artificial intelligence techniques are decreasing the complexity of MRI acquisition and postprocessing. This article reviews the current state of the art and emerging techniques in cardiac MRI.

Clinical applications of photon counting detector CT.

The X-ray detector is a fundamental component of a CT system that determines the image quality and dose efficiency. Until the approval of the first clinical photon-counting-detector (PCD) system in 2021, all clinical CT scanners used scintillating detectors, which do not capture information about individual photons in the two-step detection process. In contrast, PCDs use a one-step process whereby X-ray energy is converted directly into an electrical signal. This preserves information about individual photons such that the numbers of X-ray in different energy ranges can be counted. Primary advantages of PCDs include the absence of electronic noise, improved radiation dose efficiency, increased iodine signal and the ability to use lower doses of iodinated contrast material, and better spatial resolution. PCDs with more than one energy threshold can sort the detected photons into two or more energy bins, making energy-resolved information available for all acquisitions. This allows for material classification or quantitation tasks to be performed in conjunction with high spatial resolution, and in the case of dual-source CT, high pitch, or high temporal resolution acquisitions. Some of the most promising applications of PCD-CT involve imaging of anatomy where exquisite spatial resolution adds clinical value. These include imaging of the inner ear, bones, small blood vessels, heart, and lung. This review describes the clinical benefits observed to date and future directions for this technical advance in CT imaging. KEY POINTS: • Beneficial characteristics of photon-counting detectors include the absence of electronic noise, increased iodine signal-to-noise ratio, improved spatial resolution, and full-time multi-energy imaging. • Promising applications of PCD-CT involve imaging of anatomy where exquisite spatial resolution adds clinical value and applications requiring multi-energy data simultaneous with high spatial and/or temporal resolution. • Future applications of PCD-CT technology may include extremely high spatial resolution tasks, such as the detection of breast micro-calcifications, and quantitative imaging of native tissue types and novel contrast agents.

Imaging of Cardiac Fibrosis: An Update, From the AJR Special Series on Imaging of Fibrosis.

Myocardial fibrosis (MF) is defined as excessive production and deposition of extracellular matrix (ECM) proteins, resulting in pathologic myocardial remodeling. Three types of MF have been identified: replacement fibrosis from tissue necrosis, reactive fibrosis from myocardial stress, and infiltrative interstitial fibrosis from progressive deposition of non-degradable material such as amyloid. While echocardiography, nuclear medicine, and CT play important roles in the assessment of MF, MRI is pivotal in the evaluation of MF, using the late gadolinium enhancement (LGE) technique as a primary endpoint. The LGE technique focuses on the pattern and distribution of gadolinium accumulation in the myocardium and assists the diagnosis and establishment of the etiology of both ischemic and non-ischemic cardiomyopathy. LGE MRI aids prognostication and risk stratification. In addition, LGE MRI is used to guide management of patients being considered for ablation for arrhythmias. Parametric mapping techniques, including T1 mapping and extracellular volume measurement, allow detection and quantification of diffuse fibrosis, which may not be detected by LGE MRI. These techniques also allow monitoring of disease progression and therapy response. This review provides an update on imaging of MF, including prognostication and risk stratification tools, electrophysiologic considerations, and disease monitoring.

Palliative Procedures for Congenital Heart Disease: Imaging Findings and Complications.

Palliative procedures are performed for congenital heart diseases that are not amenable for definitive surgical procedures or as a component of hybrid procedures along with transcatheter interventions. Multimodality imaging plays an important role in the follow-up of these palliative procedures, mainly for the timely detection of complications and for planning any subsequent palliative or definitive procedure. Echocardiography is the first-line imaging modality, with CT and MRI used as complementary techniques in indeterminate cases. MRI provides anatomic, functional, flow, and tissue characterization information. CT is performed for the evaluation of vascular anatomy and when MRI cannot be performed due to contraindications, challenges, or artifacts. The modified Blalock-Taussig shunt procedure is the most common systemic-pulmonary artery (PA) shunt procedure, with thrombus being the most serious complication. Other complications of systemic-PA shunts include shunt stenosis, infection, pulmonary overcirculation, and cardiac failure. The Glenn shunt procedure is the second stage of palliation in single ventricle physiology, with thrombus, stenosis, superior vena cava syndrome, and infection being the common complications. The Fontan shunt procedure is the third stage of palliation in single ventricle physiology. Complications can be cardiovascular (heart failure, valve regurgitation, thromboembolism, shunt stenosis, arteriovenous malformation), venolymphatic (collaterals, protein-losing enteropathy, plastic bronchitis), or hepatic (congestion, cirrhosis, portal hypertension). PA banding is used to decrease pulmonary flow or to train the systemic ventricle. Complications include stenosis, thrombus, erosion, pseudoaneurysm, and subaortic obstruction. Atrial septostomy and atrial switch procedures are performed for increasing intracardiac mixing. Complications of atrial septostomy can be mechanical, traumatic, embolic, or electrical. Complications of the atrial switch procedure include baffle stenosis, baffle leak, and systemic ventricle failure. The authors review the role of multimodality imaging in the evaluation of these palliative procedures. © RSNA, 2023 See the invited commentary by Bardo and Popescu in this issue. Quiz questions for this article are available through the Online Learning Center.

RadioGraphics Update: Pictorial Guide to CAD-RADS 2.0.

Editor's Note.-RadioGraphics Update articles supplement or update information found in full-length articles previously published in RadioGraphics. These updates, written by at least one author of the previous article, provide a brief synopsis that emphasizes important new information such as technological advances, revised imaging protocols, new clinical guidelines involving imaging, or updated classification schemes.

Utility of CT and MRI in Tricuspid Valve Interventions.

Transcatheter tricuspid valve interventions (TTVIs) comprise a variety of catheter-based interventional techniques for treatment of tricuspid regurgitation (TR) in patients at high surgical risk and those with failed previous surgeries. Several TTVI devices with different mechanisms of action are either currently used or in preclinical evaluation. Echocardiography is the first-line modality for evaluation of tricuspid valve disease that provides information on tricuspid valve morphology, mechanism of TR, and hemodynamics. Cardiac CT and MRI have several advantages for a comprehensive preprocedure evaluation. CT and MRI provide complementary information to that of echocardiography on the mechanism and cause of TR. MRI can quantify the severity of TR using indirect or direct techniques that involve two-dimensional or four-dimensional flow sequences. MRI and CT can also accurately quantify right ventricular volumes and function, which is crucial for timing of intervention. CT provides comprehensive three-dimensional information on the morphology of the valve, annulus, subvalvular apparatus, and adjacent structures. CT is the procedure of choice for evaluation of several device-specific measurements, including tricuspid annulus dimensions, annulus-to-right coronary artery distance, leaflet morphology, coaptation gaps, caval dimensions, and cavoatrial-to-hepatic vein distance. CT allows evaluation of the vascular access as well as optimal procedure fluoroscopic angles and catheter trajectory. Postprocedure CT and MRI are useful in detection of complications such as paravalvular leak, pseudoaneurysm, thrombus, pannus, infective endocarditis, and device migration. © RSNA, 2023 Quiz questions for this article are available in the supplemental material.

Diastology with Cardiac MRI: A Practical Guide.

Diastolic filling of the ventricle is a complex interplay of volume and pressure, contingent on active energy-dependent myocardial relaxation and myocardial stiffness. Abnormal diastolic function is the hallmark of the clinical entity of heart failure with preserved ejection fraction (HFpEF), which is now the dominant type of heart failure and is associated with significant morbidity and mortality. Although echocardiography is the current first-line imaging modality used in evaluation of diastolic function, cardiac MRI (CMR) is emerging as an important technique. The principal role of CMR is to categorize the cause of diastolic dysfunction (DD) and distinguish other entities that manifest similarly to HFpEF, particularly infiltrative and pericardial disorders. CMR also provides prognostic information and risk stratification based on late gadolinium enhancement and parametric mapping techniques. Advances in hardware, sequences, and postprocessing software now enable CMR to diagnose and grade DD accurately, a role traditionally assigned to echocardiography. Two-dimensional or four-dimensional velocity-encoded phase-contrast sequences can measure flow and velocities at the mitral inflow, mitral annulus, and pulmonary veins to provide diastolic functional metrics analogous to those at echocardiography. The commonly used cine steady-state free-precession sequence can provide clues to DD including left ventricular mass, left ventricular filling curves, and left atrial size and function. MR strain imaging provides information on myocardial mechanics that further aids in diagnosis and prognosis of diastolic function. Research sequences such as MR elastography and MR spectroscopy can help evaluate myocardial stiffness and metabolism, respectively, providing additional insights on diastolic function. The authors review the physiology of diastolic function, mechanics of diastolic heart failure, and CMR techniques in the evaluation of diastolic function. ©RSNA, 2023 Quiz questions for this article are available in the supplemental material.

Seeing More with Less: Clinical Benefits of Photon-counting Detector CT.

Photon-counting detector (PCD) CT is an emerging technology that has led to continued innovation and progress in diagnostic imaging after it was approved by the U.S. Food and Drug Administration for clinical use in September 2021. Conventional energy-integrating detector (EID) CT measures the total energy of x-rays by converting photons to visible light and subsequently using photodiodes to convert visible light to digital signals. In comparison, PCD CT directly records x-ray photons as electric signals, without intermediate conversion to visible light. The benefits of PCD CT systems include improved spatial resolution due to smaller detector pixels, higher iodine image contrast, increased geometric dose efficiency to allow high-resolution imaging, reduced radiation dose for all body parts, multienergy imaging capabilities, and reduced artifacts. To recognize these benefits, diagnostic applications of PCD CT in musculoskeletal, thoracic, neuroradiologic, cardiovascular, and abdominal imaging must be optimized and adapted for specific diagnostic tasks. The diagnostic benefits and clinical applications resulting from PCD CT in early studies have allowed improved visualization of key anatomic structures and radiologist confidence for some diagnostic tasks, which will continue as PCD CT evolves and clinical use and applications grow. ©RSNA, 2023 Quiz questions for this article are available in the supplemental material. See the invited commentary by Ananthakrishnan in this issue.

Pre- and Postprocedure Imaging of Transcatheter Pulmonary Valve Implantation.

Transcatheter pulmonary valve replacement (TPVR) is a minimally invasive procedure for treatment of right ventricular outflow tract (RVOT) dysfunction in surgically repaired congenital heart diseases. TPVR is performed in these patients to avoid the high risk and complexity of repeat surgeries. Several TPVR devices are now available to be placed in the right ventricle (RV) to pulmonary artery (PA) conduit, native RVOT, or surgical bioprosthetic valves. Imaging is used before TPVR to determine patient eligibility and optimal timing, which is critical to avoid irreversible RV dilatation and failure. Imaging is also required for evaluation of contraindications, particularly proximity of the RVOT to the left main coronary artery and its branches. Cross-sectional imaging provides details of the complex anatomy in which the TPVR device will be positioned and measurements of the RVOT, RV-PA conduit, or PA. Echocardiography is the first-line imaging modality for evaluation of the RVOT or conduit to determine the need for intervention, although its utility is limited by the complex RVOT morphology and altered anatomy after surgery. CT and MRI provide complementary information for TPVR, including patient eligibility, assessment of contraindications, and key measurements of the RVOT and PA, which are necessary for procedure planning. TPVR, performed using a cardiac catheterization procedure, includes a sizing step in which a balloon is expanded in the RVOT, which also allows assessment of the risk for extrinsic coronary artery compression. Follow-up imaging with CT and MRI is used for evaluation of postprocedure remodeling and valve function and to monitor complications. ©RSNA, 2022 Online supplemental material is available for this article.

Myocardial Strain Evaluation with Cardiovascular MRI: Physics, Principles, and Clinical Applications.

Myocardial strain is a measure of myocardial deformation, which is a more sensitive imaging biomarker of myocardial disease than the commonly used ventricular ejection fraction. Although myocardial strain is commonly evaluated by using speckle-tracking echocardiography, cardiovascular MRI (CMR) is increasingly performed for this purpose. The most common CMR technique is feature tracking (FT), which involves postprocessing of routinely acquired cine MR images. Other CMR strain techniques require dedicated sequences, including myocardial tagging, strain-encoded imaging, displacement encoding with stimulated echoes, and tissue phase mapping. The complex systolic motion of the heart can be resolved into longitudinal strain, circumferential strain, radial strain, and torsion. Myocardial strain metrics include strain, strain rate, displacement, velocity, torsion, and torsion rate. Wide variability exists in the reference ranges for strain dependent on the imaging technique, analysis software, operator, patient demographics, and hemodynamic factors. In anticancer therapy cardiotoxicity, CMR myocardial strain can help identify left ventricular dysfunction before the decline of ejection fraction. CMR myocardial strain is also valuable for identifying patients with left ventricle dyssynchrony who will benefit from cardiac resynchronization therapy. CMR myocardial strain is also useful in ischemic heart disease, cardiomyopathies, pulmonary hypertension, and congenital heart disease. The authors review the physics, principles, and clinical applications of CMR strain techniques. Online supplemental material is available for this article. ©RSNA, 2022.

The current and future role of imaging of thoracic aortic disease: a North American society for cardiovascular imaging commentary on the 2022 AHA/ACC guidelines for the diagnosis and management of aortic disease.

The 2022 AHA/ACC Guidelines for the Diagnosis and Management of Aortic Disease introduced important updates for managing thoracic aorta aortic disease (TAD). In particular, the Guidelines underscore multimodality imaging's role in diagnosis, risk assessment, and monitoring of patients with TAD. This commentary aims to distill key imaging aspects from the Guidelines to provide a concise reference for the cardiovascular imaging community. Primary areas of focus include: (1) The importance of imagers in the multidisciplinary TAD care team, (2) Appropriate imaging techniques along with their strengths and weaknesses, (3) Aortic measurement methods and how aortic size and growth should contribute to TAD risk assessment, (4) Imaging evaluation of acute aortic syndrome. We have also highlighted several areas of ongoing uncertainty and confusion, specifically related to aortic measurement techniques and descriptive terminology. Finally, a perspective on the future of TAD imaging is discussed with a focus on advanced imaging tools and techniques as well as the potential role of artificial intelligence.