When back or leg pain makes a visit to the doctor necessary, the physician will review the patient’s history, perform a physical exam, and make a diagnosis based on those findings.
The precise etiology (or cause) of most back pain is difficult to determine, but typically will resolve with standard treatment. In most cases, diagnostic imaging studies are not required for effective treatment.
When imaging studies for back or leg pain are indicated, they are administered in the following situations:
The more specialized the doctor, the less imaging studies are generally needed, says Richard J. Herzog, MD, Chief of the Division of Teleradiology at Hospital for Special Surgery (HSS). “A general practitioner may order imaging studies sooner than a physiatrist or a spine surgeon,” he explains.
The information provided by imaging studies is utilized for both the diagnosis and treatment of back and leg pain. The quality of the imaging study and the accuracy of its interpretation will directly impact the care that a patient receives.
While patients present with a wide range of causes of back or leg pain, the fundamental question that needs to be resolved for all patients is, “where is the pain coming from?”
There are several potential causes of pain, Dr. Herzog notes, including abnormalities of the disc, nerve roots, or vertebrae.
Imaging studies such as x-ray, CT (computed tomography), and MRI (magnetic resonance imaging) are all employed to evaluate patients who present with back or leg pain. The choice of which examination to conduct depends upon the specific clinical question that needs to be answered.
Conventional radiographs, also known as plain films or x-rays, are particularly useful as the first imaging study in diagnosing the initial onset of back pain and before prescribing therapy. These x-rays should be obtained with the patient standing erect. Weight-bearing, standing x-rays are particularly important to determine the nature and extent of spinal deformities and treating problems of spinal alignment, such as scoliosis.
Gregory R. Saboeiro, MD, Chief of Interventional Radiology and CT at HSS, says that the ordering physician will prescribe a variety of different positions with the patient standing or bending, depending on their symptoms. “X-rays don’t provide the fine detail of CT or MRI,” he explains, “but they can be very important in evaluating the alignment and potential abnormal motion of the spine in the various positions obtained.”
“Certain spinal deformities that are demonstrated in x-rays taken when the patient is standing,” adds Dr. Herzog, “may not be detected when that same patient is lying down.”
Additional benefits of x-ray imaging include monitoring patients with spinal deformities or spinal imbalance and for follow-up during and after treatment or surgery.
Both doctors are quick to point out the drawbacks and limitations of x-ray imaging. X-rays provide little or no information when trying to diagnose a disc herniation or nerve root compression. And even though x-rays demonstrate vertebral bodies, they are less sensitive than other imaging studies in detecting bone destruction due to tumor or infection.
In most cases where additional information is needed to resolve a clinical question concerning back or leg pain, advanced imaging modalities such as MRI or CT will be employed.
Computed tomography, also known as a "cat" scan or CT scan, is an imaging study in which a series of digital images of the body are obtained as the patient moves through a doughnut-shaped machine that houses a thin x-ray beam. This method provides detailed cross-sectional multiplanar and 3D images not available on conventional x-ray and is excellent for evaluating the complex bony anatomy of the spine in particular, and also the surrounding soft tissues.
Its ability to detect bone conditions - especially fractures, spinal stenosis, and arthritic conditions - is unique because of the excellent resolution provided by current CT technology. This enables the scan to detect specific details in bone and adjacent soft tissue.
“CT scans are great for fine bone detail,” says Dr. Saboeiro. “In determining whether the vertebrae or the facet joints are abnormal, or if there are very subtle fractures, CT scans are the gold standard.” He goes on to explain that while another strong suit of CT scans is visualization of soft tissue structures, muscles, or nerves, MRI imaging is usually warranted to gain the most detail in these areas, if further information is necessary.
When a patient arrives for a lumbar or cervical spine CT, the radiologist will conduct a very brief patient history and describe the procedure. The patient will then lie down on the scanner table.
The CT machine looks similar to an MRI machine, but instead of the long tube that a patient moves through for an MRI, the CT has a short tube encircling the patient. This is typically a less claustrophobic environment for patients when compared to an MRI. The patient also spends less time in the CT scanner than with MR, as the exam typically lasts only approximately five minutes.
Magnetic Resonance Imaging, or MRI, involves a large apparatus containing a high strength magnet which surrounds the tube (or area in which a patient lies) and acquires images of the spine. MRI produces cross-sectional multiplanar images of the body and allows for the evaluation of all the components of the spine that may be the source of pain, including soft tissue and boney abnormalities.
“MRI provides exquisite detail of soft tissue structures, including nerve roots, disc material, and tumors,” says Dr. Herzog. “CT provides excellent visualization of bone or tissue containing calcification.”
In preparation for the exam, the patient will be screened for possible causes of MRI incompatibility, such as cardiac pacemakers and some metal implants. The patient is asked to remove all metal, including jewelry that may causes problems when the patient is placed into the MRI equipment.
During the exam the patient is typically imaged on his/her back after the application of a specific magnetic coil to the body part being imaged. Every MRI subject at HSS receives MRI-compatible headphones and is offered a wide range of music to help them relax and protect their ears during the exam. The patient is made as comfortable as possible when they are placed in the magnet. A few patients – approximately 3-5%, by Dr. Herzog’s estimation – may experience claustrophobia and may require some type of sedation to get them through the exam.
Depending on the patient’s symptoms and initial diagnosis, the MRI exam can be tailored to any specific part of the spine, whether cervical (neck), thoracic (chest), or lumbar (lower back). The part of the spine that is to be evaluated, e.g. cervical, thoracic or lumbar, is positioned in the center of the magnetic field.
The average exam takes approximately 20-30 minutes to complete. The patient should be awake during the exam, since any movement will degrade the quality of the scan. Before each sequence (there are usually five or six per exam), the patient will be requested to breathe quietly and remain as immobile as possible in order to achieve optimal images. With the advances in MRI technology, these exams are much shorter in duration and easier for a patient to complete.
With some patients who are experiencing pain, medications to relieve the pain are administered prior to the MRI exam. It also may be necessary to shorten the MRI exam depending upon the patient’s condition. In certain postoperative spine cases, a contrast agent, gadolinium, will be injected intravenously (into the vein), in order to better enhance the detection of certain spinal abnormalities.
At HSS, all MRI studies are monitored by a musculoskeletal radiologist in order to maximize the amount of useful diagnostic information generated by each exam.
“Many patients who come to HSS for a serious problem or for an operation end up having both CT and MRI examinations done,” Dr. Saboeiro explains. “The two exams are very complimentary in terms of what they demonstrate.”
“CT is often used as an adjunct to MRI, particularly in patients with spinal stenosis,” adds Dr. Herzog. In patients with stenosis, MRI is used to evaluate the soft tissue structures and CT is employed to evaluate bony proliferation that may narrow the spinal canal.
“Depending on abnormalities suspected or seen on imaging studies,” says Dr. Saboeiro, “the referring physician may also ask for a myelogram or discogram, which are more advanced diagnostic tests requiring an interventional procedure.”
Myelograms are performed in order to gain even more detailed information about the spinal cord and nerve roots. In this procedure, a needle is placed in the lumbar spinal canal with injection of a contrast agent into the area. A series of x-rays, along with a CT scan, are then obtained and this contrast provides the radiologist with improved visualization of the soft tissue anatomy and nerve roots, as well as the bones.
This procedure is often performed in addition to MRI scanning, and also in patients with contraindications or claustrophobia.
Discograms are performed in order to evaluate the intervertebral disc (a sort of cushion between spinal vertebrae) and to distinguish whether the patient’s pain is being caused by a disc abnormality, and if so, where the abnormality is located. The nature of this injection requires the patient to relay their level of pain to the radiologist as he/she injects contrast agent into each disc area under suspicion. This helps the radiologist reproduce the source of the pain and localize the specific area of abnormality while detecting nerve root irritation and compression.
Both x-rays and a CT scan are administered in conjunction with discograms to provide detailed information about the condition of the disc.
Nerve root injections, spinal canal injections, and facet joint injections are generally used to minimize pain caused by a variety of spine conditions. These therapies are not intended to fix the underlying problem, but in those cases where the body heals itself, it helps in the alleviation of pain in the process. For those with high levels of pain that will require surgery, these injections can ease the pain until the day of surgery.
All musculoskeletal imaging is not equal. The resolution and information provided by imaging systems vary from hospital to hospital and center to center. At HSS, the imaging equipment is state of the art and the imaging protocols have been optimized to maximize their diagnostic information.
This higher quality of imaging and advanced imaging equipment results in a faster imaging process and minimized doses of radiation, and provides the option of image-guided therapeutic interventions.
Most importantly, all imaging exams are interpreted at HSS by the nation’s top musculoskeletal radiologists, working closely with the nation’s top spine physicians.
Interview and summary by Mike Elvin
Associate Attending Radiologist, Hospital for Special Surgery
Associate Professor of Clinical Radiology, Weill Cornell Medical College