Since the discovery of x-rays by Wilhelm Röntgen in 1895, imaging has been employed in a vast number of diagnostic applications. For hundreds of years, the only way to peer inside a patient was to make an incision; however, with the advent of diagnostic imaging, this could suddenly be done non-invasively. It is therefore not surprising that an editorial in the New England Journal of Medicine listed medical imaging as one of the top 11 most important developments in the past 1000 years of medicine [1].

Recently, there has been increasing public awareness and concern regarding radiation exposure to patients undergoing diagnostic imaging. From 1980 to 2006, radiation exposure related to medical imaging increased from 0.54 millisieverts (mSv) per person to 3.0, nearly a sixfold rise [2]. While radiologists and radiation physicists are charged with having true expertise in the field of radiation biology, non-radiologists should be prepared to answer questions from patients about potential health risks when ordering imaging tests.

Ionizing radiation , as opposed to non-ionizing radiation, can cause biologic damage to cells and has been linked to an increased risk of developing malignancies in a cumulative, dose-dependent fashion. There are three imaging modalities commonly used in the evaluation of pain, which employ ionizing radiation: radiography, computed tomography (CT), and fluoroscopy. In contrast, there is no ionizing radiation exposure from MRI or US, and therefore no increased risk for malignancy.

The potential diagnostic benefits from medical imaging must always be weighed with the risk of cancer induction. To gain perspective on the relative risks, the average American is exposed to 3.1 mSv per year of “background” radiation, which is a combination of terrestrial and cosmic sources [2]. By comparison, the doses of antero-posterior (AP) and lateral views of the lumbar spine are approximately 0.7 and 0.3 mSv, respectively [3]. CT uses a higher dose of radiation than conventional radiography. A CT of the lumbar spine is approximately 6 mSv. The risk of developing fatal cancer has been estimated to increase by 5.5% for each 1 Sv (1000 mSv) received; thus, a CT of the lumbar spine would theoretically increase one’s risk of cancer by approximately 0.03%. This is a gross estimate and it is important to understand that children are more at risk of cancer induction from radiation exposure, whereas the elderly are less at risk. Of note, the baseline risk of developing fatal cancer in the U.S. population is approximately 20% [4].

In the current climate of healthcare economics, cost-effectiveness is extremely important to keep in mind when ordering expensive diagnostic studies. While imaging has the potential to easily demonstrate pathologic findings, it would be prohibitively expensive to indiscriminately order imaging studies without stratifying patients based on risk and need.

Since 1993, the American College of Radiology (ACR) has published evidence-based guidelines called “Appropriateness Criteria ” for a variety of clinical presentations and conditions, which are free to the public online. Imaging modalities are rated from 1 to 9, which is based on their appropriateness in the evaluation of each clinical situation. Additionally, each modality is designated as one of six relative radiation levels, which give non-radiologists insight into the risk conferred to patients.

Essentially, radiography and CT are both performed by placing a patient between a radiation source and a detector, sending a concentrated beam of ionizing radiation into the patient and creating an image based on the distribution and intensity of radiation which passes through the patient and hits the detector. Radiographs are two-dimensional images, while CTs are three-dimensional volume sets.

While radiography is able to evaluate the bones, it does not have the ability to evaluate the soft tissues or to provide cross-sectional imaging. For example, chronic discitis/osteomyelitis could cause destruction of the vertebral endplates, which would be visible on radiograph; however, acute infections are radiographically occult. Similarly, degenerative disk disease may cause narrowing of the intervertebral distance and endplate changes; however, radiographs cannot reveal the extent of spinal cord or nerve root impingement. CT is able to evaluate soft tissues, but is often inadequate for spinal pathology.

Magnetic resonance imaging (MRI) is performed by placing the patient in an extremely strong magnetic field, sending multiple radiofrequency pulses into the patient, detecting subtle differences in the time it takes for hydrogen atoms in the body to realign themselves, and plotting this information as shades of gray in space in order to make pictures that can be interpreted for diagnosis. Contrast is not needed for diagnosing many causes of pain, such as spondylosis or fractures. However, contrast is indicated in cases of suspected infection or malignancy. As MRI utilizes an extremely strong magnet, patients with ferromagnetic implantable devices, which include cardiac pacemakers, cannot undergo this study.

Nuclear medicine encompasses a group of modalities, which are less commonly used in the field of pain management than the tests listed above; however, nuclear medicine is extremely useful in certain situations.

The following sections discuss the imaging workup of common entities, which are managed by pain specialists.

Headaches are extremely common, affecting up to 60% of the population [5]. In the majority of cases, clinical history and physical examination can be used to accurately make the diagnosis. Only patients with the following “red flag” clinical features should be further evaluated with medical imaging, which can be easily remembered with the mnemonic “SNOOP” [6]:

In addition, imaging should be considered if the patient does not respond to conventional therapy. Even in patients with symptoms that are concerning enough to warrant imaging, a significant finding is made in only 0.4% of examinations [7]. CT and MRI are the two imaging modalities used to evaluate headaches. MRI is more sensitive than CT for all disease processes, but is also more expensive.

In patients who present with an atraumatic thunderclap headache , or the “worst headache ever”, it is necessary to evaluate the patient for a subarachnoid hemorrhage, which may be due to underlying cerebral aneurysm or arteriovenous malformation. CT is an extremely useful modality in the detection of subarachnoid blood. A study of 3132 patients who visited the emergency department with this presentation found the sensitivity and specificity of CT for the detection of subarachnoid hemorrhage was 92.9 and 100%, respectively, which improved to 100 and 100%, respectively, within 6 hours of headache onset [8]. Acute subarachnoid blood appears denser (brighter) than surrounding CSF on CT and decreases in density with time.

Once subarachnoid blood is detected, either by CT or lumbar puncture, there is some debate as to the next step in management. CT angiography (CTA) and MR angiography (MRA) are both respected as useful diagnostic tools, with high sensitivities for cerebral aneurysms [9, 10]. Although these sensitivities may be good enough in other realms of diagnostic imaging, the prognosis of ruptured cerebral aneurysms is so poor and the consequences of missing the diagnosis are so severe that their negative predictive values are often not felt to be sufficiently high enough to send the patient home without further workup. For this reason, catheter-based angiography is usually performed for further evaluation, regardless of whether or not the CTA or MRA is positive. This has led some to argue for skipping the CTA or MRA, which could potentially save both time and money.

If there is clinical concern for a brain tumor, MRI of the brain, with and without a gadolinium-based contrast agent, is the examination of choice. CT can often demonstrate large masses and associated vasogenic edema; however, small masses are often completely invisible by CT.

The diagnosis of meningitis is made clinically and by CSF analysis. Although leptomeningeal enhancement may be seen on MRI, the role of imaging is primarily to evaluate for complications such as subdural empyema, ventriculitis, and hydrocephalus.

Many causes of elbow pain can be diagnosed clinically and managed conservatively, with diagnostic imaging available for refractory cases. For example, tennis elbow (aka lateral epicondylitis) can be diagnosed clinically by eliciting reproducible tenderness to palpation of the lateral epicondyle with pain upon wrist extension; however, imaging may be employed if there is clinical concern for a radial collateral ligament injury.

Radiographs are the best initial test for the evaluation of elbow pain. In patients with acute elbow pain, radiographs can make the diagnosis of fracture, dislocation, joint effusion, lipo-hemarthrosis, and soft tissue swelling. In patients with chronic elbow pain, radiographs can make the diagnosis in cases of osteochondral unit injuries, intra-articular loose bodies, osteoarthritis, or calcium pyrophosphate deposition disease. Anteroposterior (AP), lateral, and oblique views are considered standard [11]. A variety of stress positions can also be utilized to elicit dynamic instability caused by underlying ligamentous injuries. If radiographs are negative, the next best step is MRI, which can characterize abnormalities of the cartilage, ligaments, muscles, and subcutaneous tissues.

If there is specific clinical concern for intra-articular loose bodies or synovial abnormality, such as clicking, locking, or limited range of motion, and if the radiographs are negative, CT or CT arthrography may be performed; however, MR and MR arthrography are widely preferred. MR arthrography involves the intra-articular injection of a diluted gadolinium-based contrast agent or normal saline.

Ultrasound examination of the elbow and other joints is a less expensive alternative to MRI for evaluation of ligaments and tendons. However, musculoskeletal ultrasound is highly operator-dependent, and a statement put forth by the Musculoskeletal Ultrasound Task Force of the American Medical Society of Sports Medicine has indicated that an operator should perform at least 50 ultrasound examinations of a joint or other anatomic structure, before achieving proficiency [12]. Also, US cannot evaluate the bone marrow.

Nuclear bone scan is usually performed to evaluate for malignancy, but can also be used for the detection of stress fractures, healing fractures, and infection.

Evaluation of hand and/or wrist pain should begin with a history and physical examination, with imaging available if necessary. As with other areas of the appendicular skeleton, the first imaging test should be radiography. The standard views are anteroposterior (AP), lateral, and an oblique. The lateral view is useful for evaluating for soft tissue swelling. Specific views for the parts of the anatomy, such as the scaphoid and Norgaard views, can be performed to address specific clinical questions. Dynamic views such as power grip and radial or ulnar deviation can be useful to evaluate for instability, which may be indicative of ligamentous injury [13]. In patients with arthritis, plain films are extremely useful in evaluating the extent and distribution of disease.

MRI is important in the workup of wrist pain given its ability to evaluate the soft tissues. Its utility is evidenced by the finding in one study that the diagnosis and treatment was changed in 50% of cases after the MRI was performed and interpreted [14]. MR arthrography can be performed to enhance the diagnostic yield when evaluating internal structures (ligaments, cartilage, and the TFCC).