The selection of thoracic imaging studies ideally should be based on carefully designed studies which determine the sensitivity, specificity, and positive and negative predictive values of the test. Many times, such specific data is not available and clinicians must base their choice on local practice or non evidence-based recommendations from books or other publications. One of the goals of this article is to establish recommendations available in the literature and to place them in the context of the evidence from which they were derived. This article also hopes to identify areas in need of further study.

Thoracic imaging studies range from those used routinely and frequently, such as the chest radiograph, to those that the practicing emergency physician will probably consider relatively infrequently and perhaps only in highly acute situations, such as imaging for aortic dissection. The chest radiograph is so ubiquitous that it is often ordered routinely without much consideration for the indications. For example, the portable anteroposterior chest radiograph is, along with a lateral cervical spine film and an anteroposterior pelvis, part of the basic screening radiology evaluation of the trauma patient, as recommended by the American College of Surgeons Advanced Trauma Life Support Course (ATLS).¹ While this recommendation is appropriate for the major trauma patient, its application to many patients with far less than major, multi-system trauma as part of the "cookbook" approach to the trauma patient, has unquestionably led to countless unnecessary studies.

The ATLS recommendations are voluminous and detailed. However, this particular set of expert consensus guidelines does not specify the methodology used to create the recommendations and does not specify the strength of the evidence upon which they are based. Therefore, the user cannot easily differentiate between strong recommendations based on prospective studies using a gold standard, and weak recommendations based on case studies and anecdotal experience. While each chapter of the ATLS text contains a compendium of references, recommendations are not specifically referenced to the source(s). ¹

The American College of Radiology has developed a novel set of guidelines for the use of imaging studies and has published these as "ACR Appropriateness Criteria."^2-8 Each guideline was developed by a panel of experts and begins with a summary and critique of the literature. This review is then used to assess the appropriateness of individual imaging studies for various clinical situations and the appropriateness is rated on a scale of one to nine, with one being the least appropriate and nine being the most appropriate. For example, for adults less than 65 years of age with possible rib fracture, rib films are given an appropriateness rating of two, while the chest radiograph is given an appropriateness rating of eight.²

The indications for chest radiography in patients of various ages with respiratory symptoms have been outlined in guidelines from the American College of Radiology, the American Thoracic Society, and the American College of Emergency Physicians.^3,-6,8-10 In addition, there are a number of prospective studies covering this topic.^11-15Similarly, the American College of Radiology and a number of prospective studies have verified the lack of utility of the chest radiograph in acute asthma exacerbations.^4,16-19However, this conclusion cannot be expanded to patients with chronic obstructive pulmonary disease, where chest radiograph has been shown to have a much higher level of appropriateness.^4,20

The use of ultrasonography in the evaluation for thoracic pathology is a rapidly developing field. Prospective studies to determine the sensitivity of ultrasound for various clinical problems have been published, but, in most cases, results have to be considered preliminary due to the limited number of confirmatory studies. This remains an area in need of study and an opportunity for emergency medicine researchers to perform well-designed prospective studies and meaningfully add to the emergency medicine literature.

Similarly, with the advent of multi-row detectors, the use of computed tomography (CT) scanning of the chest has greatly expanded. As a result of the technological advances, the sensitivity and specificity of the test for various indications have increased. Strong, validated evidence shows that CT is now the modality of choice for the diagnosis of mediastinal hemorrhage, aortic trauma, aortic dissection, and aortic aneurysm. ^21,22

The evaluation of the patient with dyspnea and potential pulmonary embolism has also undergone marked change. This has been an area of intense study with many robust studies of the use of CT scanning. ^23-33There are large, well-designed, prospective studies in this area, as well as meta-analyses and thoughtful editorials to assist in assimilating this data.^34-39

The use of imaging in patients with possible acute coronary syndromes is also undergoing rapid change. The American College of Cardiology (ACC), the American Heart Association (AHA), and the American Society of Echocardiography have issued joint guidelines for the clinical use of echocardiography.⁴⁰  With the American Society for Nuclear Cardiology, the ACC and AHA have issued joint guidelines for the use of nuclear medicine scanning in the diagnosis of acute coronary syndromes.⁴¹Recently, multi-detector computed tomography (MDCT) for non-invasive coronary angiography has received attention in regard to diagnosing acute coronary syndromes.^42-48 However, a recently published report of a large multicenter trial found a high rate of unevaluable segments, leading to questions regarding the clinical role of MDCT in evaluating coronary artery stenosis.⁴⁹