4
Imaging in Back Pain
Alberto Zerbi
■ Introduction
Aging, trauma, infections, tumors, and degenerative diseases can all cause low back pain (LBP). Hip arthritis, sacroiliac joint pathologies, and visceral pathologies of the pelvis or of the abdomen can mimic LBP symptomatology. As spine specialists, we have to consider all of the possible causes of LBP symptomatology. Thus, it is mandatory that an accurate clinical examination be performed first, and then, on the basis of this examination, the patient can be referred to a radiologist if necessary. We are, first, doctors in medicine and second, spine superspecialists, so don’t be a subspecialist!
In choosing the chapter subtitle “The Eye of the Tiger,” I vaguely recalled a romantic quotation from Rudyard Kipling or some other classic author, but instead I found a novel from John Edmund Delezen entitled Eye of the Tiger: Memoir of a United States Marine, Third Force Recon Company, Vietnam, a title and subtitle that well describe how radiologists must learn to survive in everyday medicine, being friendly with referring clinicians but using their claws when necessary and looking with the eye of a tiger at the images to comprehend every obvious or subtle aspect of the spine.
■ Referring a Patient to the Radiologist
When you refer a patient to the radiologist, submit a clinical summary that includes relevant anamnestic data and suspected pathology. Never request “L/S MRI, LBP” without first providing other important information, such as the precise site of pain, if left or right sided; the neurologic impairments, such as sciatica; previous surgery; previous trauma; major pathology, such as a breast cancer; and other relevant information.
If you do not provide the relevant information to the radiologist, he or she will not be able to determine the most appropriate examination protocol for your patient. Radiologists are imaging specialists, not spine specialists, and they need the most appropriate information from you in order to determine the correct diagnosis by using the least biologically invasive and the least costly diagnostic procedure possible.
■ Clinical Findings Determine the Need for Imaging
When the patient’s symptoms, clinical signs, and anamnestic data are not sufficiently specific to determine the pathology, ask first for the most widely available, the least costly, and the most readily obtainable imaging modality—the lumbar spine X-ray.
There are very few situations in which a lumbar spine X-ray should be bypassed as the first imaging modality. One such situation is when the clinical and anamnestic data clearly indicate that a nerve or the cauda is compressed by a disk herniation or by other causes. In this case, a magnetic resonance imaging (MRI) of the lumbar spine can be requested as the first imaging modality. In most other cases, a lumbar spine X-ray is the first imaging to be done.
■ Reasons for Requesting an X-Ray
The X-ray is a powerful, panoramic, inexpensive, and readily available imaging technique. Unfortunately, due to the difficulty in reading it and its less trendy appeal, it is frequently overlooked in favor of more expensive and more trendy techniques such as MRI or computed tomography (CT).
The difficulties in reading X-rays are mostly due to the fact that everything is shown in one bidimensional plane, so that more skill and experience is needed by the radiologist or the clinician to interpret the images than to interpret the imaging of other tomographic techniques such as CT or MRI. The X-ray, moreover, has several objective limitations compared with the other techniques, such as a lower sensitivity in recognizing lithic lesions of bone, and the inability to directly show soft tissue damages. The image quality of the X-ray is affected by the size and the compliance of the patient and the ability of the technician to obtain a correctly exposed and oriented film.
On the other hand, X-ray is the only technique that can assess the spine as a whole and demonstrate malalignments, deviations of the spine, malformations, and pelvic parameters. It is very helpful in assessing most of the pathologies affecting the spine, from fracture and dislocations, to infections and tumors, to degenerative modifications and metabolic pathologies. Moreover, being panoramic, it can show pathologies of other anatomic structures, for instance, near joints like the sacroiliac joints or hip joints that can frequently mimic symptoms related to the lumbar spine.
■ Determining the Type of X-Ray to Request
The standard radiological protocol is an anteroposterior (AP) and latero-lateral (LL) view of the lumbar spine obtained with the patient lying on the table. The AP field of view (FOV) is from T11 to the half sacrum and laterally barely includes the vertebral transverse processes. The LL view shows the lumbar spine from T11 to the midsacrum.
This standard X-ray protocol is no longer adequate to the modern clinical concepts about LBP and its origin, and is mostly useless in answering the needs of the spine specialists. This is the reason why the standard AP and LL views should be supplanted by the lumbopelvic views (De Seze view) with the patient in standing position (Fig. 4.1). The FOV of these views is wide enough to demonstrate, both in AP and LL, the spine from T11 to the head of the femurs. From these lumbopelvic views, we can obtain information about the lumbar spine, as with the standard protocol, as well as information about the hip and sacroiliac joints and the lumbopelvic parameters. Adapting the imaging protocol to the new clinical criteria, at the same cost, we acquire much more useful infor mation to better understand the causes of the patient’s back pain.
Fig. 4.1 Lumbopelvic X-ray: De Seze view in a 62-year-old patient with long-standing low back pain (LBP). Note hip and sacroiliac joints arthritis with subchondral sclerosis and osteophytes. (a) Anteroposterior (AP) view. (b) Latero-lateral (LL) view.
Flexion-extension views can be added to demonstrate reciprocal dynamic modifications of the vertebral bodies and of the disks. It is not yet established if this imaging should be done with the patient in the standing, sitting, or lying positions. It may be difficult for aging or ailing patients to perform maximum range of motion while sitting or standing without holding on to a device, impairing the results of the examination. Moreover, muscular function can impair the reciprocal movement of the vertebrae. For these reasons, it is more appropriate to obtain the dynamic, flexion-extension views with patients lying on their side.
There are few indications for obtaining oblique views because the information about the apophyseal joints and pars are more easily available with other techniques such as MRI or CT. However, oblique views are preferable to CT in young patients because of concerns about the radiation dose.
Whole spine imaging is necessary when the alignment of the spine, the sagittal and frontal balance, and pelvic parameters need to be assessed. The protocol for this imaging must be carefully followed. The patient must be standing in a comfortable, unforced position on both feet. The technician must pay maximum attention in checking the position of the pelvis in AP to avoid rotation. The patient’s hands must be on the collarbones or on the forehead. If a holding device is necessary, or a step for the patient to stand on, this must be communicated by the technician and described by the radiologist. The FOV suggested is in AP from C3 to the femoral head, wide enough to show the iliac crests, and in LL from the acoustic foramina to the first 15 cm of the femurs.
Fig. 4.2 A standing X-ray of spine with the EOS® method in a 39-year-old patient who presents with LBP due to scoliosis. EOS uses two perpendicular X-ray beams collimated in two very thin, horizontal, fan-shaped beams along with two detectors. This enables obtaining (a) frontal and (b) lateral X-rays of a patient simultaneously.
To avoid excessive irradiation of the patient, the technician must pay maximum attention to precisely aiming the FOV and checking the position of the patient so that repetitions of the exposure are avoided. The EOS® system (EOS Imaging Inc., Cambridge, MA) is a revolutionary X-ray machine that enables the simultaneous exposure of the body in anterolateral (AP) and LL standing view (Fig. 4.2). For this reason, a three-dimensional (3D) reformation of the spine can be postprocessed to provide important information about the position of the spine and of the pelvis. Moreover, due to its revolutionary detectors, for which the inventor Georges Charpak won the Nobel Prize in Physics in 1992, the EOS machine gives one-tenth the radiation dose compared with standard modern X-ray machines. 1,2,3,4
■ When to Request an MRI
An MRI should be the second imaging modality requested unless the findings of the physical examination clearly indicate a root impingement pathology by disk herniation, thus precluding the need for X-ray and further imaging. But this rarely happens; when it does, it is based on the clinical skill of the clinician, who notes the typical pattern of the specific pathology of a disk herniation.
The MRI is the most powerful imaging modality for diagnosing inflammatory, degenerative, neoplastic, or traumatic lesions of the lumbar spine. Its ability to detect both bone and soft structures with anatomic and tissue quality precision is invaluable.
The quality of the imaging of the spine is affected by several factors with which the clinician may not be familiar but that are well studied by the radiologist doing the examination. The factor most affecting the MRI image quality is the magnet field strength, with 1.5 tesla (T) being the current gold standard, but with 3T rising in quality. With 1T or less powerful magnet units, the examination must be performed with time-consuming skill so that the image quality can come close to that of the most powerful units. Other factors affecting the quality of the image are the coil quality, the patient’s compliance (staying still during the examination), the choice of the sequences for the single magnet unit, and the knowledge of the suspected pathology, so that the protocol of examination can be aimed at the specific pathology. Other local problems can affect the image quality, such as the presence of foreign bodies like postsurgical devices (screws, hooks, bars, cages, etc.) and metal dust from surgical instruments. In these cases, artifacts cannot be completely avoided, but an imaging strategy to reduce these artifacts can be adopted. A two-point strategy is used in these cases: (1) the radiologist must be informed of the presence of such devices and be provided with an X-ray of the lumbar spine; (2) the radiologist uses a protocol for metal artifact reduction called the metal artifacts reduction sequence (MARS) that is adapted for each specific MRI unit.
■ Standard and Special Protocols and Sequences, and the Use of Gadolinium
The standard study protocol of the lumbar spine is based on at least three sequences. The choice of the sequences is the responsibility of the radiologist and it is based on the characteristics of the MRI unit. Two sagittal sequences (usually spin echo T1 and T2) and an axial T2 complete the examination.
The sagittal views must show the entire lumbar spine from the inferior half of T11 to S2 or S3, depending on the sacral inclination. The slices laterally must include not only the vertebral body but also the complete lateral foramina. That means that a 70-mm-wide volume must be studied with at least 15 slices of 4-mm thickness with a 0.6-mm gap in between.
The axial slice thickness is usually 3 or 4 mm with a 10% gap in between. The slices must be oriented parallel to the upper end plate of the lower vertebra of each disk space that must be completely studied from the pedicle of the upper vertebra to the pedicle of the lower one. The FOV must be wide enough to include the whole vertebra and the transverse and spinous processes. At the lumbosacral junction, a wider FOV must be used to demonstrate at least the upper part of the sacroiliac joint and the sacral wings.
To this standard study protocol other views and sequences can be added, depending on the appearance of the standard study or the clinical question to be answered.
Schematically, if a scoliosis is noted or a malformation or a far lateral herniation or a cord or roots lesion is seen or suspected, a coronal T2 or T1 sequence should be added, whereas if a deformation of a vertebral body with or without signal intensity alteration is seen, a fat-suppressed sequences (FAT SAT or STIR) must be added to facilitate the diagnosis of a fracture, a tumor, or an inflammatory process. Diffusion-weighted images can help distinguish between pathological and osteoporotic fractures (Fig. 4.3).
Endovenous (EV) injection of contrast medium (gadolinium) is generally used for a more complete diagnosis when a discitis or neoplastic lesion is suspected. In the postoperative spine, gadolinium is generally indicated as the gold standard for a differential diagnosis between scar tissue and disk herniation recurrence.5 Gadolinium is an expensive pharmaceutical with rare but well-known side effects; thus, it must be used by knowledgeable technicians. It can be suggested or proposed by the clinician but it cannot be ordered. In fact, its use is strictly the choice and responsibility of the radiologist, who, knowing the clinical question and checking the standard images, will decide whether or not to inject the drug in the patient, after considering the risks of administration (Fig. 4.4).
