What is the correct central ray entrance point for a lateral chest radiograph

Gilbert E Boswell, MD, Sione T Wolfgramm, MD, USAF, Raynard K Fong, USN (Ret), Daniel B Hawley, MD, USN Specialty Leader, Radiology, US Navy, Dual-Energy, Dual-Exposure PA and Lateral Chest Radiograph: Not Your Father’s Chest X-Ray, Military Medicine, Volume 188, Issue 1-2, January-February 2023, Pages 12–15, https://doi.org/10.1093/milmed/usac220

ABSTRACT

INTRODUCTION

In the mid 2000s, most military medical facilities began transitioning X-ray equipment to direct digital capture units. This allowed for digital archiving of images and networked distribution of images and reports, allowing rapid access to interpreting physicians and referring providers. Costly film processing, use of hazardous chemicals, and film archiving have been eliminated, reducing operational and storage costs. This has allowed for ready comparison with older examinations in an organized fashion, allowing for better evaluation of disease progression or stability. Digital datasets introduced the capability for mathematical processing, pixel by pixel, to optimize the images presented. It allowed a wider latitude in processing. By simply changing a level or window setting, visualization of lung parenchyma, mediastinum, and bones improved. Fewer radiographs needed to be repeated due to poor image quality. Digital detectors also introduced the usefulness of acquiring posterior to anterior (PA) views with two exposures of different energies allowing improved visualization of lungs and bones. This will be detailed further below.1

This article is presented to help any provider, whether in operational medicine or specialty practice, learn how the images are created and thereby appreciate how normal anatomy or disease processes are presented, depending on tissue density and elements in the body. It shows how advancements in computer processing lead to better use of physical principles separating tissue types. A unique aspect for military medicine is the application of these techniques in detecting unsuspected coronary calcium among our military members. As military providers our task is to be the best in preventive care and early detection in the care of our patients, so that they may perform the military mission. Chest radiographs are routine in our medical screening for many military operational roles. This article explains improvements and added capabilities of this test.

SIMPLE PHYSICS IN MODERN CHEST RADIOGRAPHY

To better understand the concept of dual-energy techniques, a short primer on X-ray physics is helpful. In projection radiography, as X-ray photons are transmitted through the patient, some of the photons are attenuated (absorbed) by the tissues before reaching the image detector. The probability of X-ray photon absorption is proportional to the atomic number (Z) of the target material and inversely proportional to the energy of the X-ray photon beam. Tissue composition with low atomic numbers (i.e., soft tissue), composed of hydrogen (Z = 1), oxygen (Z = 8), (water) and carbon (Z = 6) (protein, fat), will attenuate fewer photons than tissues with higher atomic numbers, such as calcium (Z = 20) (bone, calcified atherosclerotic plaque). Likewise, X-ray photons with a lower beam energy spectrum will have a higher attenuation differential in bone as compared to soft tissue. These principles allow for the creation of two image datasets obtained from different X-ray beam energy exposures, enabling the decomposition of tissue characteristics by mathematically computing and creating images that emphasize either soft tissue (low Z) or bone (high Z).

This technique also allows chemical element separation to “emphasize,” or “subtract” metal (nickel Z = 28, copper Z = 29, titanium Z = 22, iron Z = 26), implanted in, or overlying the chest, and characterize prostheses that may have silicon (Z = 14) or lithium (Z = 7) components. This allows for better identification of vascular access lines or a patient’s indwelling tubes or devices (coronary stents and left atrial appendage closure devices) and also demonstrates tissues otherwise hidden by them. This may allow the observer to “see” behind a pacemaker or jewelry on clothing or on the chest wall.

CURRENTLY, THERE ARE TWO CLINICAL IMAGING SYSTEMS AVAILABLE FOR DUAL-ENERGY RADIOGRAPHY

One system allows processing two images simultaneously from a single exposure by employing two imaging plates on the X-ray detector that are geometrically aligned and separated by a copper filter, which preferentially absorbs lower X-ray energies. A low-energy image and a high-energy image are acquired with a single X-ray beam. The advantage of this system eliminates any patient motion artifacts. The disadvantage is that the energy differential between the two images is small, resulting in a relatively less separation of the tissue characteristics. There is a low signal-to-noise ratio for the soft tissue and bone images at typical patient exposures.

The other system uses a detector with fast read-out capability, allowing the use of two closely timed separate X-rays of different energies. The large differences in the effective energy of the X-ray beams result in a higher signal-to-noise ratio and better separation of tissue types. The first acquisition occurs with the high-energy beam (120 kVp), followed immediately by a low-energy beam (60 kVp). In the most current systems, the two exposures are 160 milliseconds apart. This system is more demanding on the technologist, to make sure the patient is not moving, and has suspended breathing before the chest radiography exposures.

The image data are captured on high-resolution direct photon to digital electronic signals and then processed by computer algorithms that allow the creation of images, which prioritize low-atomic-number tissues (water, carbohydrates, fat, protein, and air) and higher atomic number tissues and elements (calcium, silicon, and metals) (Fig. 1).

Dual-energy, dual-exposure lateral CXR, 60-year-old male with right upper chest pain. (A) Standard view obtained at 120 kVp. (B) Calculated “lung” subtraction image emphasizing lower atomic numbers demonstrate a 3 cm mass (arrow), obscured by ribs and spine on standard view. (C) Calculated “bone” subtraction image emphasizing higher atomic numbers. (D) CT image (inset) confirming lung mass posterior to the liver and obscured on the frontal projection. Biopsy confirmed adenocarcinoma.

The technology to generate dual-energy exams has been present since the early 2000s. However, the advent of faster digital detectors, faster computer processing, and advanced mathematical algorithms has recently allowed these applications to be applied in the lateral X-ray projection.2

Dual-Energy, Dual-Exposure PA and Lateral Radiographs in Current Practice

Dual-energy soft tissue–reconstructed images, often referred to as “lung” images, have been demonstrated to improve the detection of pulmonary nodules, consolidation, and lung disease.3 “Bone” images were initially designed to identify calcium in pulmonary nodules, thereby obviating the need for CT, as these could generally be classified as benign calcified pulmonary granulomas. This technique has been particularly beneficial in detecting noncalcified nodules that would otherwise be obscured by the clavicles or ribs. In the mid 2000s, Gilkeson published a series of reports on dual-energy imaging to evaluate coronary calcifications in the frontal projection.4,5 More recently, this technique has been modified to allow imaging in the lateral projection in addition to the frontal projection.

There are advantages of employing this technique on the lateral chest X-ray. The lateral projection presents a better view of the spine, central lungs and airways, and mediastinum. On the frontal projection, the overlapping shadows of the sternum, vertebrae, and calcified cartilage of the rib ends obscure the heart and coronary arteries and much of the aorta, central airways, and pulmonary arteries. The lateral projection has been a favorite point of instruction by academic chest radiologists since the publication of classic manuscripts on the lateral chest radiograph by Proto, a chest radiologist with the U.S. Air Force in the late 1970s.6,7 With some training, a reader can identify the anatomic structures of the mediastinum and central lungs well on the lateral projection. Dual-energy subtraction imaging with “lung” images remove the overlying ribs and spine, thereby allowing the reader to more readily detect regions of pulmonary consolidation or obscured pulmonary nodules. This allows better definition of the central airways and hila, aorta, and pulmonary arteries. Similarly, “bone” images can be created that better demonstrate the spine, calcified heart valves, and atherosclerotic calcifications of the coronary arteries, thoracic aorta, and great arteries of the aortic arch (Fig. 2).

Asymptomatic male mid 60s preparing for a marathon. (A) Standard view at 120 kVp does not demonstrate coronary calcium. (B) Calculated “lung” subtraction image emphasizing lower atomic numbers. Better visualization of lungs and hila. (C) Calculated “bone” subtraction image emphasizing higher atomic numbers. Heavy coronary calcium, unexpected finding. Much greater than expected for patient age. Right, left anterior descending, and left circumflex coronary arteries are demonstrated. (D) and (E) CT images (inset) from the subsequent calcium score inset to demonstrate anatomic correlation.

Military Relevance and Current Use

Military treatment facilities provide care to service members—individuals who are selected to be fit and healthy for worldwide deployment. They are often deployed away from specialty medical care in stressful and often hazardous environments and often for prolonged period of time. Given these considerations, military medicine has a low threshold for imaging, whether this is a screening exam for an accession physical, a recruit with suspected pneumonia, a routine 5-year aviation or dive physical, or an occupational health physical. We also care for many retired members who are eligible beneficiaries and who may continue to work as government employees or contractors at military facilities.

Additional importance for sensitive chest radiography in the military population is due to the legacy of high smoking rates in the military, which directly leads to higher rates of atherosclerosis, heart disease, osteoporosis, and lung cancer among other well-known diseases.8,9 Importantly, chest radiography is not an established or recognized means of screening for coronary disease or lung cancer. However, when chest X-rays are obtained for other indications, dual-energy techniques allow for the opportunistic detection of coronary atherosclerotic calcifications or suspicious pulmonary nodules that otherwise may not be detected.10–12

THE SIGNIFICANCE OF CORONARY CALCIUM BY AGE

It is not inevitable that everyone will develop coronary atherosclerotic calcification with age.13 In fact, 10%-15% of patients in their eighth and ninth decade will not have any coronary calcium. As a general rule, the median age for males to develop their first scorable coronary calcium by CT is 50. For women, it is 60. Therefore, if we detect coronary calcium by dual-energy CXR in individuals at younger ages, these patients are identified as outliers and at increased risk for coronary event. In our experience, we have detected unsuspected coronary atherosclerotic calcifications in active duty members in their 30s and 40s. Furthermore, the greater the calcium burden identified by age, the greater the risk of a cardiac event. Chiles et al. demonstrated that a trained reader can semiquantify the burden of calcium by CT and assign risk categories.14

When coronary calcium is detected, it is important for the referring provider to recognize the significance of this, put it in context with the patient’s age and other risk factors, and use this information to add guidance in lipid management if the patient does not have symptoms referable to coronary disease. If the patient does have symptoms, then appropriate referral for further evaluation should be initiated. Similarly, it should be emphasized that CXR is not a screening tool for coronary atherosclerosis, but can be useful when detected on an examination ordered for other indications. Not detecting coronary calcium on a CXR does not exclude coronary disease.

PITFALLS TO RECOGNIZE WHEN VIEWING THESE EXAMINATIONS

Dual-energy, dual-exposure chest radiographs are obtained by performing two examinations in rapid succession. These data are then used to create “subtraction” images, which then best demonstrate lung and soft tissues and calcified or bony tissues. Because this requires two exposures, there will be some cardiac motion artifact. If the patient is actively breathing during the exposures, imaging artifacts will also occur. Heart motion will result in artifacts adjacent to the heart and mediastinal borders. On the subtraction images, this should not be mistaken for pneumomediastinum. Similarly, if there is chest wall or diaphragm motion, there could be confusing artifacts, which one may mistake as pneumothorax. If these entities are suspected, one should refer to the standard view images (120 kVp) to avoid this pitfall.

SUMMARY

With this introduction of the dual-energy radiography technique and brief discussion of the advantages for the detection of important pathology in our military population, it is our hope that the reader will better understand how to interpret these images and understand the value added as these digital subtraction images become commonplace in military treatment facilities. Reference 2 will present the reader with more examples, illustrating the applicability of the techniques and more details about physical principles behind dual-energy chest radiography.

FUNDING

None declared.

CONFLICT OF INTEREST STATEMENT

None declared.

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The views expressed herein are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, DoD, or the U.S. Government.

We are military service members or employees of the U.S. Government. This work was prepared as part of my official duties. Title 17, U.S.C. 105 provides that copyright protection under this title is not available for any work of the U.S. Government. Title 17 U.S.C.101 defines a U.S. Government work as a work prepared by a military service member or employee of the U.S. Government as part of that person’s official duties.

Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2022. This work is written by (a) US Government employee(s) and is in the public domain in the US.

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