Imaging in Low Back Pain

Medical imaging plays an important role in the evaluation of low back pain (LBP). The choice of certain radiological method over other depends on many factors like patient’s presentation, presence of contraindication, availability, relative cost of the test, and the expected impact of the results on management. Radiological evaluation helps the physician reach the most likely cause of LBP, con-firm the provisional diagnosis, provide alternative one, or narrow the differential diagnosis. Plain X-ray radiograph is useful in initial general assessment. Magnetic resonance imaging (MRI) is the imaging modality of choice in the evaluation of LBP because of elegant demonstration of anatomical details and many pathologies. Computerized tomography (CT) can provide high-resolution images of the bony structures and is particularly invaluable in trauma. Other imaging modalities are rarely used usually as problem-solving or in selected conditions. For example, sonography may have a role in the evaluation of soft tissue lesions and the sacroiliac joints. Angiography is useful for vascular evaluation. Isotope imaging may be used in the elucidation of of hidden cause of pain (tumors or fracture). Conventional myelography and discography are virtually obsolete in current clinical practice because of the presence of much safer and accurate new modalities. Finally, interventional radiology has an increasing role in treating certain conditions.


Introduction
The main role of imaging in patients with low back pain (LBP) is to help physicians reach the most likely cause of the pain. The use of one or more of the different radiological investigations in a suitable manner will enhance the detection of the underlying cause of the LBP in a timely way. Moreover, radiology has a role not only in the diagnosis but also in the treatment of some conditions that lead to LBP. The rapidly advancing interventional radiology is increasingly utilized as an adjuvant or sole therapeutic option of a variety of conditions like vascular malformations and tumors.
There are many imaging modalities that can be used in the evaluation of LBP. Selection of the appropriate modality depends on different factors, like the patient's conditions, clinical state, availability, and cost of the test and the presence of certain contraindications.
We will discuss each imaging modality from different points of view starting with the most to the least commonly and widely used ones. body particularly the musculoskeletal system, brain, and spine. In patients with LBP, MRI is considered as the imaging modality of choice because of its excellent demonstration of both the anatomy of the lower back and most of regional pathologies [4]. 2. Imaging quality is affected by the patient's motion, so examination needs a cooperative and stable patient. Agitated and unstable patients result in bad images that are diagnostically not useful.

Advantages
3. Long examination time, usually 10-20 minutes (longer than X-ray and CT scan).
4. Not so accurate in certain conditions like cortical fractures and calcification (CT scan and lesser extent X-ray radiography are better).

5.
Clinical MRI mismatch. Many abnormal findings specially disc degeneration or mild herniation are seen incidentally on spinal MRI of asymptomatic persons and vice versa where MRI can be completely normal in the presence of significant LBP. This has raised the issue of importance of clinical and electrophysiological correlation and the controversial need for more specific diagnostic tests like discography [5].
2. Metallic object (shells, bullet, orthopedic fixation devices). However, if the fixation devices used were made of MRI-compatible metals (more expensive than regular ones), MRI examination can be safely done despite some artifacts that slightly reduce diagnostic imaging quality.
3. Early pregnancy. However, the chemical material sometimes may be injected intravenously (gadolinium) to enhance the images, and this should not be used throughout pregnancy.
4. Claustrophobia. Seen in about 5-10% of population, when the person is unwillingly afraid of being in closed space. This problem may be resolved by reassurance and careful description of the procedure to the patient. Sometimes the patient will need to be examined using a special type of MRI device called "open type" that has wider aperture and more space around the patient. Rarely sedative/antianxiety drugs may be prescribed.

Techniques
The patient is asked to remove any removable metallic objects like rings and watches. The examination usually takes 10-20 min to complete according to the clinical indication and requested sequences. MRI examination is usually composed of two or more of "sequences." Body tissues and pathologies appear differently on each sequence, and the most commonly used are T1-weighted and T2-weighted sequences. Other sequences used are Short tau inversion recovery (STIR) (suppresses fat signal in the bone marrow and fatty areas), diffusion-weighted image (DWI) (to assess certain features of some lesions according to water molecules diffusion) and fluid attenuation inversion recovery (FLAIR) (suppresses signal of the fluid). Sometimes, when vascular assessment is important, magnetic resonance angiography (MRA) is also conducted. Spinal pathologies can cause both structural and morphological changes. Degeneration of the intervertebral disc, for instance, can be diagnosed when the normal high signal (white) of the disc is lost (disc appears black). Disc prolapse is diagnosed when a part of the disc is seen outside the normal contour of the disc bulging into the spinal canal or compressing the nerve roots within the intervertebral canal.

Diagnostic value
From the diagnostic point of view, MRI can help in: 1. Comprehensive evaluation of the lower back (lumbosacral region) in which the most common causes of LBP arise. Variable pathologies from degenerative, neoplastic, infective, and congenital abnormalities can be elegantly demonstrated. Important and common causes of LBP like intervertebral disc herniation, nerve root compression, lumbar canal stenosis, and degeneration of the small joints at the back of the spine (facet or apophyseal joints) and intraspinal ligaments (ligamentum flavum) can be seen (Figure 2).

Computerized tomography (CT) scan
The main physical principle underlying CT scan is X-radiation. The use of CT scan in the assessment of the patients with LBP is limited to certain conditions that either need further evaluation after MRI, patients who cannot be examined by MRI, or when the clinical scenario necessitates CT from the start (like trauma).

Excellent depiction of the bony anatomy and bony changes at the lower back
including lumbosacral spine and pelvis [7]. Bony changes can be readily seen on CT even before appearance on X-ray radiographs.
2. Can compete with MRI in the visualization of lumbar disc prolapse and bony spinal canal stenosis.
3. Relatively more widely available and accessible than MRI.
5. Less affected by the patient's motion during examination than MRI.
6. Short time of examination (usually less than minute).
7. Can be safely done for patients with metallic implants, shells, and magnetic fixation devices (but some image compromise).

Limitations
1. High risk of radiation exposure [7]. This is indeed the most important disadvantage of CT scan. CT scan delivers a huge amount of radiation dose to the patient (almost 100 times that of chest X-ray).
2. Poor demonstration of intraspinal neural content like cord and nerve root and early bone marrow infiltration [8].
3. Artifacts from metallic fixation devices or shells may badly affect the quality of image and can obscure some anatomical and pathological findings.

Techniques
The patient is routinely examined in supine position (back on the table), and the exposure done while the patient is asked not to move. The examination is relatively rapid (less than minute).

Diagnostic value
Considering a case of LBP, CT scan can show: 1. Abnormal bone densities of the lower spine and pelvis including osteolytic (blacker) and osteoblastic (whiter) lesions, with a lot of possible causes, from incidental nonsignificant (bone islands), benign (hemangiomas, osteomas) to malignant (metastases, multiple myeloma) lesions.
2. Abnormal configuration and deformity, whether of congenital origin or as sequel of old trauma or surgery.
3. Traumatic findings like linear fracture, partial or complete vertebral compression, and burst vertebra (Figure 3). CT scan can detect even tiny fractures of the bony cortex particularly of the posterior spinal elements (laminae and pedicles) which are usually difficult to be seen and may be missed on X-ray radiographs [3].
4. Size, shape, and exact location of the metallic foreign body or shell or bullet. 6. Many (but not all) features of spinal degeneration like intervertebral disc space narrowing, marginal osteophytes, and facet joints sclerosis.

Bone erosion secondary to inflammation, infection, or tumor.
7. Calcification. CT scan is the best imaging modality in defecting calcification in paraspinal soft tissues, ligaments, muscles, or within a mass. The presence of calcification has important diagnostic impact as it helps narrow the differential diagnostic list or may reach final diagnosis like hydatid cyst or para-articular calcification in some chronic arthropathies.

Sonography
Sonography (or ultrasound scan, echography) utilizes sound waves of very high frequency that are normally not audible by human beings. Its application in the evaluation of patients with LBP is limited because the ultrasound waves are badly affected by tissues like the bone and air. 2. Highly device-dependent. The quality of the ultrasound machine has high impact on the quality and subsequent diagnostic outcome of the examination, particularly of the musculoskeletal ones. Many advanced technologies (both software and hardware) are currently emerging that help improve diagnosis which are typically supplied to the mid or even only high-level (expensive) machines.
3. Demonstrates only selected anatomical details of lower back.

Techniques
No special preparation is usually required. Routinely, a gray-scale (B-mode) scan is applied. Color Doppler scan is used when the assessment of blood flow and vascular imaging is required. Advanced techniques like 3D, 4D, and elastography are increasingly utilized both in research and specialized centers in musculoskeletal imaging.

Techniques
Depending on the purpose of the examination, the isotope material (usually incorporated with another carrier substance) is administrated to the patient. The patient is then imaged by the device.

Diagnostic value
1. Generally, metabolically active lesions appear of higher signal than adjacent tissues, so-called "hot spots," and vice versa, inactive lesions have lower signal, so-called cold spots.
2. Can provide invaluable decision about the nature of a vague or suspicious lesion, for example, in the pelvis or lower spine, and whether it is significant and metabolically active or not by assessing its activity.
3. Helps detect many lesions that are metabolically active (like cancer, active inflammation, healing fractures, postoperative) and that are not well or difficult to be seen by the common modalities (CT scan and MRI) [9,11].
4. Monitoring the response of cancer after radiotherapy and/or chemotherapy and assessing the presence of a residual/recurrent tumor mass after surgical resection.

Angiography
Many methods are utilized in angiography (visualization of the vascular system), commonly by injecting a contrast material into the vessels and then taking images. In the assessment of patient with LBP, angiography generally has very limited role like in further evaluation or treatment of a vascular lesion [12]. Opportunity of the therapeutic option as some lesions can be treated directly by the angiography by, for example, injecting a special material to occlude and "embolize" the feeding vessel of the mass or vascular anomaly. This is useful for patients who cannot undergo the surgery and with difficult or risky surgical access.
4. High radiation exposure both to patient and staff.
5. Risk of contrast material particularly the iodine-based material (in conventional angiography and CT angiography (CTA)) like allergy and renal damage.
needle inserted at the lower back, and the patient is imaged by either conventional X-ray (conventional myelography) or by CT scan (CT myelography) [10]. With the advent of MRI, it became possible to do myelography noninvasively (without need for any intraspinal injection). This so-called MRM is a great advance in spinal imaging. Nowadays, conventional myelography is mostly superseded by CT myelography which is in turn mainly reserved to those who cannot undergo MRI.

Diagnostic value
The interpretation of myelography is usually done after the complete evaluation of other imaging modalities like X-ray, CT, and MRI. The main diagnostic values are: 1. Filling defects in the myelography can be caused by many intraspinal lesions.
Therefore, accurate localization of these lesions whether they are extradural, intradural, extramedullary (outside the cord), or intramedullary (within the cord) has great diagnostic value in reaching the most probable cause.
2. MRM can improve the inter-observer and intra-observer agreement in the assessment of lumbar spinal canal stenosis [13], helping more consistent diagnosis when surgical intervention is considered.

Discography
Discography means the visualization of the content of intervertebral disc after injection of a radiopaque iodine-based contrast material directly into the disc. It is considered as the most accurate method in deciding which disc level is responsible for the LBP and used as reference test in medical researches. However, because it is a very invasive and painful test with many complications and contraindications (like infection, bleeding and severe spinal compromise), its clinical use now is highly restricted for carefully selected patients (e.g., continuous pain with normal noninvasive imaging or sometimes before surgery) and only under experienced interventionalists [14].