Diagnostic Imaging and Pain Management

Onassis A. Caneris

I have a little shadow that goes in and out with me,

And what can be the use of him is more than I can see.

He is very, very like me from the heels up to the head;

And I see him jump before me when I jump into my bed.

—Robert Louis Stevenson, 1850–1894

In recent years, there have been advances in understanding the pathophysiology and mechanisms of pain; concomitantly, there have been advances in diagnostic imaging. Diagnostic imaging is an essential tool for the pain physician, who uses it to understand, diagnose, and treat pain. Although plain x-rays remain the mainstay of diagnostic imaging, advanced modalities including computerized tomography (CT), magnetic resonance imaging (MRI), and nuclear medicine studies have proved to be valuable diagnostic tools for patients with pain. There has been an increasing interest in understanding the dynamic nature of pain processes; functional imaging modalities, including positron emission tomography (PET) scanning and functional magnetic
resonance imaging (fMRI), have added to our knowledge in this area. Over the past decade, the use of new technologies has resulted in a 50% increase in health care costs. It becomes increasingly important for the pain physician to have a clear understanding of imaging studies and to optimize the use of diagnostic imaging. A radiologist or imaging specialist can help pain physicians select the most cost-effective test and identify causative pathology.

I. IMAGING TECHNIQUES AND STUDIES

1. Plain Films

Plain x-rays (static x-rays) generate two-dimensional (2D) images that primarily display skeletal tissue, and, in addition, soft tissue anatomy is either seen or inferred. Contemporary x-ray technology generally produces high-quality images with minimal radiation exposure. X-rays produced as electrons from a cathode are accelerated by electric current toward an anode target. The x-ray beam is differentially absorbed as it passes through a section of the patient and then goes on to expose the film. Radiopaque contrast materials given orally, locally, intravenously, and intrathecally may be used to aid the study. Most contrast materials used with plain x-rays are iodine based. Plain x-rays remain the first-line examination for many conditions.

2. Fluoroscopy

The principles of fluoroscopy are the same as those of plain x-rays. The primary difference is that the transmitted radiation is viewed on a fluorescent screen rather than on a static film and that the patient can be imaged in real time in fluoroscopy. The image is generally amplified by an image intensifier. Fluoroscopy can be used both in diagnostic studies and in assisting therapeutic treatment.

3. Computerized Tomography

The prototype CT scanner was developed in the 1960s. First-generation scanners took days to collect data and then hours to reconstruct the images. In the early 1970s, CT scanning for imaging the brain became available. Today’s fourth-generation scanners have considerably improved on quality, and the imaging time is considerably shortened. In CT imaging, the x-ray tube produces a beam of energy that passes through a single section of the patient. This beam is then detected by a circular array of detectors on the opposite side. Both the detector and the x-ray source rotate around an axis of the patient and produce exposures at small intervals of rotation. Subsequently, computer reconstruction results in a display of the targeted area. The resolution can be as low as 0.5 mm. Intravenous contrast can be used to enhance the imaging of vascular structures and normal tissues.

CT scanning offers the advantage of three-dimensional (3D) images, but they are generally in standard cross-sectional or axial planes. Quantitative CT scanning is particularly useful in measuring bone density for the assessment of osteoporosis. 3D CT scan also allows postreconstruction images to be rotated at various angles. CT scan displays soft tissues fairly well and is
used for soft tissue imaging if MRI (which provides superior soft tissue contrast) is not available or if the patient cannot tolerate MRI because of claustrophobia or because it is a more lengthy process.

4. Magnetic Resonance Imaging

As early as the 1940s and 1950s, nuclear magnetic resonance (NMR) was used to image chemical compounds by exposing them to strong magnetic fields. By the mid-1980s, clinical NMR had become common, and the name was changed to MRI because of public anxiety engendered by the word nuclear.

A major difference between MRI scanning and CT scan as well as x-rays is that MRI uses no ionizing radiation. In MRI, signals are obtained by subjecting the tissues to strong magnetic fields, which influence hydrogen ions in the tissues to align in a certain direction. Tiny radio frequency signals are emitted as the hydrogen ions “relax” when the magnetic field is removed. The image represents the intensities of the electromagnetic signals emitted from the hydrogen nuclei in the patient. A tissue such as fat, which is rich in hydrogen ions, gives a bright signal, whereas bone gives a void or essentially no signal. Abnormal tissue generally has more free water and displays different magnetic resonance (MR) characteristics.

The MR signal is a complex function of the concentration of deflected normal hydrogen ions, buildup and relaxation times of the magnetic field (T1 and T2, respectively), flow or motion within the sample, and the MR sequence protocol. Three types of MR sequences are used: spin echo, gradient echo, and inversion recovery. MRI is easily able to provide multiplanar images. Its advantage over CT scan is its superior contrast of soft tissues, especially neural tissues. The addition of gadolinium as a contrast material aids in defining tumors and inflammatory processes.

5. Myelography

Injection of radiocontrast material into the intrathecal space, followed by imaging using conventional x-ray techniques or CT scanning, provides diagnostic information about potential structural abnormalities affecting the spinal nerves. When noninvasive imaging with either MRI or CT scanning does not provide adequate information, myelography, which was once the gold standard for assessing the spine, remains an option for diagnosing structural spine disease. It is also useful for imaging patients who have had spinal instrumentation, which tends to produce extensive artifact on CT imaging.

Postmyelogram CT imaging is sometimes useful for detecting subtle spinal nerve impingement caused by far-posterior lateral intervertebral disc herniation that has been missed by MRI. Its disadvantages are that it is invasive and, unlike MRI, it uses ionizing radiation.

6. Bone Scans and Nuclear Medicine

The field of nuclear medicine followed the discovery of radioactivity in 1896. There are three types of radioactive emissions: positive particles (α particles), negative particles (β particles), and high-penetration radiation (γ radiation). The scintillation events
are detected by a scintillation camera and are mapped in 2D space. Nuclear medicine uses the tracer principle, which essentially tags certain physiologic substances in the body and measures its distribution and flow or its presence in a target system. A radiopharmaceutical agent is injected into the patient and the radioactive decay is detected by a detection device (e.g., a γ counter).

Bone scans are commonly used to evaluate complaints of skeletal pain. Radiopharmaceuticals labeled with technetium 99 m localize areas of increased bone turnover and blood flow that represent increased rates of osteoblastic activity. Bone scans are more sensitive than x-rays in detecting skeletal pathology. One third of patients with pain and known malignant disease with normal x-rays have metastatic lesions on bone scans. The specificity of bone scans is not high, which can sometimes be a problem.

7. Discography

Discography involves injecting the nucleus pulposus of an intervertebral disc with contrast material under fluoroscopic guidance. This process can provide objective structural and anatomic information about the intervertebral disc. In addition, it can provide subjective information on whether a specific disc is the source of a patient’s axial lumbar pain. This information is helpful in recommending specific treatment modalities.

8. Positron Emission Tomography

In PET, positron emissions are detected with a circular array of detectors. The number of decays is displayed to produce an image of specific metabolic processes. PET is an excellent tool for quantification of various metabolic and physiologic changes and processes, making it a functional imaging device. PET scanning is being increasingly used to unravel pain processes, and the literature on PET scanning and functional neuroimaging of pain is growing.

9. Functional Magnetic Resonance Imaging

In the early 1990s, a number of centers reported that MRI could be used for functional imaging of the human brain. Functional imaging, including fMRI, has helped identify mechanisms that are critical targets for more effective and specific treatments for neuropathic pain. The technique utilizes the principle that functional activation of brain regions are reflected by increases in the blood oxygen level dependent (BOLD) signal in fMRI. This modality has been used to examine the contribution of thalamic and cortical areas to the human pain experience. The cortical areas identified include the primary and secondary somatosensory cortex (S1 and S2), the anterior insula, and the anterior cingulate cortex. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with the amplification of the thalamic, insular, and SII responses.

II. HEADACHE

Headache is a frequent presentation in both the primary care physician’s office and the pain clinic. The pain physician must be familiar with the indications for imaging in the assessment of
patients with headache. Most patients who complain of headache and whose neurologic examinations reveal normal findings show normal findings from a CT imaging study. Careful history and neurologic examination are crucial before deciding whether to order a diagnostic test.

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