Abstract
Low back pain is common and relates to a variety of overlapping pathologies. Within the last few decades, almost every medical imaging modality has been applied in the evaluation of low back pain. Imaging of the spine has a high priority in the assessment of patients with low back pain, who seem to expect such procedures to be undertaken. However, the majority of conventional imaging techniques do not have adequate precision to identify the primary source of pain. Not only can this be frustrating to both clinicians and patients, but importantly, inadequate correlation between imaging findings and symptoms hampers the ability of clinicians to devise a specific treatment plan for the patient. Therefore, there is mounting interest in new imaging techniques of the lumbar spine that may increase the clinical correlation in low back pain. In this review, we will discuss the value and limitations of various lumbar spine imaging techniques with focus on new emerging technologies.
The imaging techniques of the lumbar spine and scope of the review
The overall prevalence of low back pain (LBP) increases with age until between 60 and 65 years of age , and because of the demographic changes in industrialized countries over the last few decades, LBP has received increased attention in global, regional, and national health policies due to the immense costs associated with this disorder . Most studies define LBP simply based on the reported symptoms; however, LBP may be subgrouped according to the underlying pathophysiology . This review will focus on the so-called mechanical back pain ( Table 1 ).
| Mechanical LBP (97%) | Nonmechanical spinal conditions (1%) | Visceral disease (2%) |
|---|---|---|
| Lumbar strain or sprain Degeneration of disc and facets Herniated disc Spinal stenosis Osteoporotic compression fracture Spondylolisthesis Traumatic fractures Congenital disease
Discogenic pain Segmental instability | Neoplasia
Paget’s disease | Pelvic organ involvement
Gastrointestinal involvement
|
The causes of mechanical LBP and pathologic degenerative changes in the lumbar spine are believed to be multifactorial, including fissures in the annulus and fatigue injury of the vertebral endplates , mechanical , inflammatory , genetic , and infectious . Back pain is believed to originate from a multitude of lumbar spinal structures including ligaments, facet joints, vertebral periosteum, paravertebral musculature and fascia, blood vessels, annulus fibrosus, and spinal nerve roots . Therefore, imaging of the lumbar spine has an established role in the evaluation of the LBP patient, and within the last few decades, almost every medical imaging modality has been applied in the evaluation of LBP. A documented problem prevailing in lumbar spine imaging is the poor correlation between imaging findings and the clinical presentation of the patient , and it is the well-known fact that degenerative findings are common in both individuals with and without LBP . Because of this overlap, it is usually difficult for clinicians to determine whether the described imaging abnormalities are clinically relevant. To complicate matters, patients today often seek information from a variety of secular and medical sources including the Internet and not infrequently seem to expect advanced imaging procedures to be undertaken understandably hoping to identify the cause of pain. This is a clinical paradox, as simply accommodating such wishes for advanced imaging is likely of little benefit to the patient and liberal use of imaging in LBP may even worsen long-term outcomes in some patients . Technical advances in the field of lumbar spine imaging are made at an increasingly high rate, and it seems likely that improved imaging procedures can indeed increase the understanding of LBP and aid the diagnostic precision and specificity. The aim of this review is to provide an extensive overview of both existing and emerging imaging modalities and to discuss strengths and weaknesses of each modality as well as applicability in the diagnostic workup of mechanical LBP.
Imaging specific degenerative changes of the lumbar spine
Radiography
Radiography can visualize bony structures and can be used in suspected cases of traumatic, osteoporotic or pathologic vertebral fractures, malalignment, congenital defects, and late stages of inflammatory and infectious diseases . Despite the inability to visualize soft tissue, indirect indications of late degenerative changes can be given (e.g., facet joint osteoarthritis, disc space narrowing, vertebral osteophytes, endplate sclerosis, and spondylolisthesis) . Radiography is an inexpensive and relatively fast technique and therefore still has its place in the initial screening of the spine. Myelography and functional radiography such as flexion/extension or lateral bending views are still widely used by the spine surgeons . Discography is also an invasive radiography technique in which iodine-based contrast agent is injected into the disc space under fluoroscopic radiographic guidance. Discography has high diagnostic accuracy for detecting painful disc levels in LBP with low false-positive rates . However, some studies have indicated that the procedure results in accelerated disc degeneration or herniation and the development of reactive endplate changes compared to matched controls . With improvements in advanced 3D tomographic imaging techniques such as computed tomography (CT) and especially magnetic resonance imaging (MRI), the need for invasive procedures in the diagnostic workup of patients with LBP has decreased.
Computed tomography ( CT ) uses a 3D tomographic radiographic technique and is the standard of reference for imaging the vertebral bones, facet joints degeneration, and soft tissue calcification . Tissue contrast on CT is optimal in very dense structures such as bone. Dense soft tissues like tendons, ligaments, and annulus fibrosus can also, to some degree, be differentiated. This makes CT ideal for trauma imaging and fracture characterization in the spine. The method is also feasible for the diagnosis of disc herniations and spinal stenosis in patients who are not able to undergo MRI examination particularly for detecting extrusions . DUAL-energy CT ( DECT ) is a novel CT method based on the principle that various tissues have different and specific radiation absorptions and attenuation coefficients at two discrete energy levels (usually 140 and 80 kV). Thus, DECT has the potential to characterize the chemical composition of the different tissues . This technique may be used to identify crystal depositions (i.e., urate and calcium pyrophosphate) in the facet joints and soft tissue; however, the clinical implication of such findings in the spine is still unknown. DECT also has the ability to minimize metal implant artifacts, and CT is superior to MRI in delineating juxtaarticular osteophytes in osteoarthritis . Over the last decade, facet joint pathology has received increased attention ; for example, facet joint effusion (increased fluid) is correlated with spinal instability and LBP . Therefore, improvement of the CT technologies is ongoing; for example, multispectral CT scanners that have the potential to image bone marrow edema, ligaments, and tendons. Such advancements may enhance the value of CT in the diagnosis of the spine ( Fig. 1 ).
Conventional MRI of the lumbar spine
MRI plays an established role in the assessment of degenerative spinal disorders and is a rapidly growing technology with continuous increase both in the number of installed scanners and in the number of MRI examinations performed. Based on the data from the Organization for Economic Co-operation and Development (OECD), Japan has the highest number of MRI systems per capita (46.9 systems per million), followed by the United States with 35.9 per million population. In the United States, the (absolute) number of MRI examinations more than doubled between 2000 and 2013 . The conventional MRI units include a large cylindrical helium-cooled superconducting magnet, a radiofrequency coil, a receiver, and a computer system for image data postprocessing. For the lumbar spine, the patient is typically scanned supine with a supporting pillow under the knees . This results in slight flexion of the hip, which reduces the lumbar tension and the risk of movement artifacts. However, studies have indicated that the use of a leg supporting pillow may cause underestimation of spondylolisthesis and spinal stenosis, and it is proposed that patients should be imaged with straightened lower extremities . The relatively small bore in the magnet between 60 and 70 cm in diameter can induce claustrophobia, anxiety, or fear in some patients . This has led to the development of open MRI systems in which only the body part of interest is covered by the magnet .
The conventional whole-body MRI system magnetic field strength typically ranges from 1.5 to 3.0 Tesla (T), often defined as “high-field MRI” , whereas open MR systems usually operate with lower field strength between 0.2 and 1.0 T. High-field scanners are usually preferred because of better image quality primarily from a higher signal-to-noise ratio and the ability to perform spectral fat saturation. Although low-field systems in general provide lower image quality, yet this does not necessarily yield lower diagnostic potential in LBP evaluation , but imaging times tend to be longer to achieve appropriate signal-to-noise ratio. The usual MRI protocols of the lumbar spine include sagittal T1-weighted (T1w) turbo/fast spin echo (TSE/FSE), sagittal and axial T2-weighted (T2w), TSE/FSE images. Adding a sequence in the coronal plane at a later stage has been recommended to allow a better visualization of transitional vertebras, extraforminal disc herniations, extraforaminal nerve roots compression, degenerative vertebral translation, and paravertebral osteophytes . The availability of robust 3D sequences may obviate having to acquire multiplanar acquisitions. In T1w images without spectral fat saturation, areas of high signal intensity indicate high fat concentration, whereas in T2w images without fat saturation, high signal intensity is seen in both fluid containing tissue, fluid collections, and fatty tissue. Thus, T2w images are well suited to assess disc degeneration in which loss of water and degenerative changes in the disc matrix involving collagen structures occur . The presence of fatty tissue in the bone marrow may obscure underlying pathologies on the T2w images without fat saturation . To overcome this issue, spectral fat suppression T2w or fluid sensitive such as the Short Tau Inversion Recovery (STIR) sequences have been used . In STIR sequences, fluid is bright because the contrast is a combination of T1w and T2w images. STIR is the most robust fat-suppression technique in the lumbar spine sensitive for detection of edema and may serve as a screening sequence for many neoplastic, infectious, and traumatic pathologies. However, it is less useful for degenerative conditions because of a high degree of noise and low resolution . So-called Dixon imaging has been developed to provide adequate fat suppression in areas of the spine otherwise susceptible to inadequate spectral fat suppression on T2w images. The technique has been further refined with Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation ( IDEAL ) sequences to provide more robust fat suppression sequences, for example, in the presence of metallic implants . The applicability of IDEAL in mechanical LBP remains largely unexplored.
Finally, it should be noted that despite increased image quality and improved signal-to-noise ratio, conventional high-field MRI still has a high rate of both false-positive and false-negative results , which limits its accuracy in identifying the clinical phenotype of patients with LBP.
Ultrasonography
As an inexpensive and readily available imaging tool, some investigators have assessed ultrasonography (US) as a possible alternative for the investigation of patients with LBP. The measurement of spinal canal diameter using US was first reported in 1976 . Different researchers have assessed such measurements, using both a posterior method and an anterior/abdominal approach . Using a technique called Doppler imaging of vibration , some authors have applied US in the evaluation of patients with sacroiliac joint dysfunction and in patients with pelvic pain during pregnancy . The sacroiliac joint can be the primary source of pain in some patients with LBP, and intra-articular injection with steroids and analgesics is the gold standard for diagnosis. US is known to be very operator-dependent. Perhaps for this reason, US has not received much scientific attention in relation to LBP imaging. Further below, additional US-based measurements in LBP are described.
Axial-loaded MRI/CT
Axial-loaded MRI attempts to simulate gravity and loadbearing during supine imaging using a compression device, which applies axial load through controlled traction force on a harness worn by the patient while the feet are positioned against a fixed footplate and knees remain in the extended position ( Fig. 2 A). A small pillow under the lumbar spine can be applied to retain the lumbar lordosis . When loading the body by approximately 50% of the patient’s body weight, axial loaded MRI is found to simulate the morphological changes observed in the upright position . Several studies have compared conventional imaging of the lumbar spine with axial-loaded images, and have reported that the use of axial-loaded MRI led to additional useful information and better correlation with the patient reported outcomes for the diagnosis of degenerative lumbar spondylolisthesis and spinal stenosis . The advantage of this approach is that it can be used in conventional MRI and CT scanners . Adverse events have been reported during the examination with axial-loading, and studies have reported interruption or noncompletion rates of 5–10% due to new-onset or worsening pain and neuropathy during scanning . One study by Willén et al. found that information gained from axial-loaded MRI influenced neurosurgical treatment decisions, changing from conservative to operative treatment in 10 out of 20 investigated patients suspected for spinal stenosis . Axial-loaded MRI provides a higher diagnostic sensitivity for degenerative lumbar spondylolisthesis and spinal stenosis; however, the existing evidence for axial-loading MRI/CT for other pathologies remains at an early development stage .
Weight-bearing MRI
Weight-bearing MRI is performed in open MRI scanners allowing imaging in the standing or seated position—known as positional MRI (pMRI) . The open configuration also allows images to be obtained during dynamic maneuvers (e.g., flexion-to-extension, left-to-right rotation or bending)—known as kinetic MRI (kMRI) ( Fig. 2 B–D). Some scanners even allow capturing real-time series of lumbar spine movement—called dynamic MRI (dMRI) ( Table 2 ).
| Supine/recumbent MRI | rMRI | Recumbent refers to the unloaded position with the patient lying down; however, most studies uses the term supine MRI as the patient is typically scanned in this position with a pillow supporting the lower extremities |
| Axial-loading MRI | aMRI | A compression device applies an axial force during supine MRI to simulate the gravity in the upright position |
| Positional MRI | pMRI | Imaging in varying weight-bearing positions (e.g., standing, seated or in the position which worsens symptoms) |
| Kinetic MRI | kMRI | Imaging during static kinetic maneuvers (e.g., flexion, extension, rotation, lateral bending) |
| Dynamic MRI | dMRI | MRI while the spine is moving. Serial images show the dynamic movement of morphology |
Flexion-extension kMRI simulates lumbar myelography in quantitative assessment of the sagittal dural sac diameter and allowing more valid estimation of the foraminal size as the contrast medium in myelography tends to increase the apparent dimensions of the foramina . Several kMRI studies have found that the upright extension of the lumbar spine decreases the dural sac dimensions and foraminal size compared to the upright flexed or the supine position. This may be partly explained by an increased thickness of the ligamenta flava also found in the extended position . kMRI seems to be a feasible alternative to myelography, which also suffers from risk of infection, adverse contrast agent reactions, and spinal headache. In theory, standing pMRI should most closely approximate the in vivo situation as the lumbar spinal will be affected by the normal tone in the core muscles of the torso . During standing pMRI, the size of posterior disc protrusions or herniations has been shown to enlarge compared to conventional supine MRI of the same patient . However, lumbar extension seems to produce similar effects . Standing pMRI seems to be more sensitive to detect spinal stenosis and spondylolisthesis than the conventional supine MRI ; however, similar findings have been reported in patients scanned in the supine position with straightened lower extremities ( Fig. 3 ) . Therefore, the influence of gravity on the spinal discs and the spinal morphology is not fully understood.
Weight-bearing MRI is more sensitive than conventional MRI to detect a variety of degenerative findings in LBP patients, including hidden high intensity zones (HIZ) , translational intervertebral movement , juxtaarticular-facet joint cysts , and instability . Adverse events have been reported in both pMRI and kMR with interruption or noncompletion of the upright scan due to worsening of pain or neuropathy . Standing pMRI is associated with a small risk of orthostatic syncope, which may be reduced by a crural pumping device . Furthermore, the image quality can be impaired by movement artifacts .
At present, there are no international evidence-based recommendations for the use of weight-bearing MRI, and existing knowledge about how positional changes in the lumbar spine can be interpreted into a clinical context is very limited. Clinicians and radiologists have also expressed concerns about the image quality of devices with low magnetic field strength, including weight-bearing scanners . A recent study has demonstrated that low-field (0.25 T) spine imaging has comparable diagnostic potential to high-field (3 T) systems for LBP evaluation . Weight-bearing MRI must still be regarded as an add-on examination to the conventional MRI evaluation. From a clinical perspective, it seems logical to scan patients with LBP in the position worsening their symptoms—typically the upright position. Therefore, randomized controlled and longitudinal follow-up studies reporting outcomes beyond anatomical changes are needed before weight-bearing MRI can be regarded as providing a higher diagnostic specificity or additional benefit to LBP patients.
Imaging specific degenerative changes of the lumbar spine
Radiography
Radiography can visualize bony structures and can be used in suspected cases of traumatic, osteoporotic or pathologic vertebral fractures, malalignment, congenital defects, and late stages of inflammatory and infectious diseases . Despite the inability to visualize soft tissue, indirect indications of late degenerative changes can be given (e.g., facet joint osteoarthritis, disc space narrowing, vertebral osteophytes, endplate sclerosis, and spondylolisthesis) . Radiography is an inexpensive and relatively fast technique and therefore still has its place in the initial screening of the spine. Myelography and functional radiography such as flexion/extension or lateral bending views are still widely used by the spine surgeons . Discography is also an invasive radiography technique in which iodine-based contrast agent is injected into the disc space under fluoroscopic radiographic guidance. Discography has high diagnostic accuracy for detecting painful disc levels in LBP with low false-positive rates . However, some studies have indicated that the procedure results in accelerated disc degeneration or herniation and the development of reactive endplate changes compared to matched controls . With improvements in advanced 3D tomographic imaging techniques such as computed tomography (CT) and especially magnetic resonance imaging (MRI), the need for invasive procedures in the diagnostic workup of patients with LBP has decreased.
Computed tomography ( CT ) uses a 3D tomographic radiographic technique and is the standard of reference for imaging the vertebral bones, facet joints degeneration, and soft tissue calcification . Tissue contrast on CT is optimal in very dense structures such as bone. Dense soft tissues like tendons, ligaments, and annulus fibrosus can also, to some degree, be differentiated. This makes CT ideal for trauma imaging and fracture characterization in the spine. The method is also feasible for the diagnosis of disc herniations and spinal stenosis in patients who are not able to undergo MRI examination particularly for detecting extrusions . DUAL-energy CT ( DECT ) is a novel CT method based on the principle that various tissues have different and specific radiation absorptions and attenuation coefficients at two discrete energy levels (usually 140 and 80 kV). Thus, DECT has the potential to characterize the chemical composition of the different tissues . This technique may be used to identify crystal depositions (i.e., urate and calcium pyrophosphate) in the facet joints and soft tissue; however, the clinical implication of such findings in the spine is still unknown. DECT also has the ability to minimize metal implant artifacts, and CT is superior to MRI in delineating juxtaarticular osteophytes in osteoarthritis . Over the last decade, facet joint pathology has received increased attention ; for example, facet joint effusion (increased fluid) is correlated with spinal instability and LBP . Therefore, improvement of the CT technologies is ongoing; for example, multispectral CT scanners that have the potential to image bone marrow edema, ligaments, and tendons. Such advancements may enhance the value of CT in the diagnosis of the spine ( Fig. 1 ).
Conventional MRI of the lumbar spine
MRI plays an established role in the assessment of degenerative spinal disorders and is a rapidly growing technology with continuous increase both in the number of installed scanners and in the number of MRI examinations performed. Based on the data from the Organization for Economic Co-operation and Development (OECD), Japan has the highest number of MRI systems per capita (46.9 systems per million), followed by the United States with 35.9 per million population. In the United States, the (absolute) number of MRI examinations more than doubled between 2000 and 2013 . The conventional MRI units include a large cylindrical helium-cooled superconducting magnet, a radiofrequency coil, a receiver, and a computer system for image data postprocessing. For the lumbar spine, the patient is typically scanned supine with a supporting pillow under the knees . This results in slight flexion of the hip, which reduces the lumbar tension and the risk of movement artifacts. However, studies have indicated that the use of a leg supporting pillow may cause underestimation of spondylolisthesis and spinal stenosis, and it is proposed that patients should be imaged with straightened lower extremities . The relatively small bore in the magnet between 60 and 70 cm in diameter can induce claustrophobia, anxiety, or fear in some patients . This has led to the development of open MRI systems in which only the body part of interest is covered by the magnet .
The conventional whole-body MRI system magnetic field strength typically ranges from 1.5 to 3.0 Tesla (T), often defined as “high-field MRI” , whereas open MR systems usually operate with lower field strength between 0.2 and 1.0 T. High-field scanners are usually preferred because of better image quality primarily from a higher signal-to-noise ratio and the ability to perform spectral fat saturation. Although low-field systems in general provide lower image quality, yet this does not necessarily yield lower diagnostic potential in LBP evaluation , but imaging times tend to be longer to achieve appropriate signal-to-noise ratio. The usual MRI protocols of the lumbar spine include sagittal T1-weighted (T1w) turbo/fast spin echo (TSE/FSE), sagittal and axial T2-weighted (T2w), TSE/FSE images. Adding a sequence in the coronal plane at a later stage has been recommended to allow a better visualization of transitional vertebras, extraforminal disc herniations, extraforaminal nerve roots compression, degenerative vertebral translation, and paravertebral osteophytes . The availability of robust 3D sequences may obviate having to acquire multiplanar acquisitions. In T1w images without spectral fat saturation, areas of high signal intensity indicate high fat concentration, whereas in T2w images without fat saturation, high signal intensity is seen in both fluid containing tissue, fluid collections, and fatty tissue. Thus, T2w images are well suited to assess disc degeneration in which loss of water and degenerative changes in the disc matrix involving collagen structures occur . The presence of fatty tissue in the bone marrow may obscure underlying pathologies on the T2w images without fat saturation . To overcome this issue, spectral fat suppression T2w or fluid sensitive such as the Short Tau Inversion Recovery (STIR) sequences have been used . In STIR sequences, fluid is bright because the contrast is a combination of T1w and T2w images. STIR is the most robust fat-suppression technique in the lumbar spine sensitive for detection of edema and may serve as a screening sequence for many neoplastic, infectious, and traumatic pathologies. However, it is less useful for degenerative conditions because of a high degree of noise and low resolution . So-called Dixon imaging has been developed to provide adequate fat suppression in areas of the spine otherwise susceptible to inadequate spectral fat suppression on T2w images. The technique has been further refined with Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation ( IDEAL ) sequences to provide more robust fat suppression sequences, for example, in the presence of metallic implants . The applicability of IDEAL in mechanical LBP remains largely unexplored.
Finally, it should be noted that despite increased image quality and improved signal-to-noise ratio, conventional high-field MRI still has a high rate of both false-positive and false-negative results , which limits its accuracy in identifying the clinical phenotype of patients with LBP.
Ultrasonography
As an inexpensive and readily available imaging tool, some investigators have assessed ultrasonography (US) as a possible alternative for the investigation of patients with LBP. The measurement of spinal canal diameter using US was first reported in 1976 . Different researchers have assessed such measurements, using both a posterior method and an anterior/abdominal approach . Using a technique called Doppler imaging of vibration , some authors have applied US in the evaluation of patients with sacroiliac joint dysfunction and in patients with pelvic pain during pregnancy . The sacroiliac joint can be the primary source of pain in some patients with LBP, and intra-articular injection with steroids and analgesics is the gold standard for diagnosis. US is known to be very operator-dependent. Perhaps for this reason, US has not received much scientific attention in relation to LBP imaging. Further below, additional US-based measurements in LBP are described.
Axial-loaded MRI/CT
Axial-loaded MRI attempts to simulate gravity and loadbearing during supine imaging using a compression device, which applies axial load through controlled traction force on a harness worn by the patient while the feet are positioned against a fixed footplate and knees remain in the extended position ( Fig. 2 A). A small pillow under the lumbar spine can be applied to retain the lumbar lordosis . When loading the body by approximately 50% of the patient’s body weight, axial loaded MRI is found to simulate the morphological changes observed in the upright position . Several studies have compared conventional imaging of the lumbar spine with axial-loaded images, and have reported that the use of axial-loaded MRI led to additional useful information and better correlation with the patient reported outcomes for the diagnosis of degenerative lumbar spondylolisthesis and spinal stenosis . The advantage of this approach is that it can be used in conventional MRI and CT scanners . Adverse events have been reported during the examination with axial-loading, and studies have reported interruption or noncompletion rates of 5–10% due to new-onset or worsening pain and neuropathy during scanning . One study by Willén et al. found that information gained from axial-loaded MRI influenced neurosurgical treatment decisions, changing from conservative to operative treatment in 10 out of 20 investigated patients suspected for spinal stenosis . Axial-loaded MRI provides a higher diagnostic sensitivity for degenerative lumbar spondylolisthesis and spinal stenosis; however, the existing evidence for axial-loading MRI/CT for other pathologies remains at an early development stage .
Weight-bearing MRI
Weight-bearing MRI is performed in open MRI scanners allowing imaging in the standing or seated position—known as positional MRI (pMRI) . The open configuration also allows images to be obtained during dynamic maneuvers (e.g., flexion-to-extension, left-to-right rotation or bending)—known as kinetic MRI (kMRI) ( Fig. 2 B–D). Some scanners even allow capturing real-time series of lumbar spine movement—called dynamic MRI (dMRI) ( Table 2 ).
| Supine/recumbent MRI | rMRI | Recumbent refers to the unloaded position with the patient lying down; however, most studies uses the term supine MRI as the patient is typically scanned in this position with a pillow supporting the lower extremities |
| Axial-loading MRI | aMRI | A compression device applies an axial force during supine MRI to simulate the gravity in the upright position |
| Positional MRI | pMRI | Imaging in varying weight-bearing positions (e.g., standing, seated or in the position which worsens symptoms) |
| Kinetic MRI | kMRI | Imaging during static kinetic maneuvers (e.g., flexion, extension, rotation, lateral bending) |
| Dynamic MRI | dMRI | MRI while the spine is moving. Serial images show the dynamic movement of morphology |
Flexion-extension kMRI simulates lumbar myelography in quantitative assessment of the sagittal dural sac diameter and allowing more valid estimation of the foraminal size as the contrast medium in myelography tends to increase the apparent dimensions of the foramina . Several kMRI studies have found that the upright extension of the lumbar spine decreases the dural sac dimensions and foraminal size compared to the upright flexed or the supine position. This may be partly explained by an increased thickness of the ligamenta flava also found in the extended position . kMRI seems to be a feasible alternative to myelography, which also suffers from risk of infection, adverse contrast agent reactions, and spinal headache. In theory, standing pMRI should most closely approximate the in vivo situation as the lumbar spinal will be affected by the normal tone in the core muscles of the torso . During standing pMRI, the size of posterior disc protrusions or herniations has been shown to enlarge compared to conventional supine MRI of the same patient . However, lumbar extension seems to produce similar effects . Standing pMRI seems to be more sensitive to detect spinal stenosis and spondylolisthesis than the conventional supine MRI ; however, similar findings have been reported in patients scanned in the supine position with straightened lower extremities ( Fig. 3 ) . Therefore, the influence of gravity on the spinal discs and the spinal morphology is not fully understood.
Weight-bearing MRI is more sensitive than conventional MRI to detect a variety of degenerative findings in LBP patients, including hidden high intensity zones (HIZ) , translational intervertebral movement , juxtaarticular-facet joint cysts , and instability . Adverse events have been reported in both pMRI and kMR with interruption or noncompletion of the upright scan due to worsening of pain or neuropathy . Standing pMRI is associated with a small risk of orthostatic syncope, which may be reduced by a crural pumping device . Furthermore, the image quality can be impaired by movement artifacts .
At present, there are no international evidence-based recommendations for the use of weight-bearing MRI, and existing knowledge about how positional changes in the lumbar spine can be interpreted into a clinical context is very limited. Clinicians and radiologists have also expressed concerns about the image quality of devices with low magnetic field strength, including weight-bearing scanners . A recent study has demonstrated that low-field (0.25 T) spine imaging has comparable diagnostic potential to high-field (3 T) systems for LBP evaluation . Weight-bearing MRI must still be regarded as an add-on examination to the conventional MRI evaluation. From a clinical perspective, it seems logical to scan patients with LBP in the position worsening their symptoms—typically the upright position. Therefore, randomized controlled and longitudinal follow-up studies reporting outcomes beyond anatomical changes are needed before weight-bearing MRI can be regarded as providing a higher diagnostic specificity or additional benefit to LBP patients.
Disc degeneration – collagen and water content measurements
The lumbar intervertebral disc undergoes degenerative morphological and cellular changes with age, generally without any symptoms or pain . However, higher disc degeneration grades are found in individuals with LBP compared to individuals without LBP of the same age . Therefore, many clinicians make a distinction between ‘‘LBP-related’’ and ‘‘age-related’’ disc degeneration. Degeneration of the disc seems to be related to an initial structural defect involving the endplate or annulus . The degeneration causes loss of pressure in the nucleus and high stress–strain concentrations arise in the annulus, especially in the posterior part of the disc. These changes leads to impaired nucleus cell matrix synthesis and altered biochemistry of the disc . Further, inflammatory mechanisms are involved in degrading the matrix and further stimulate a catabolic response with loss of hydrophilic glycosaminoglycans (GAGs) within the nucleus pulposus . These changes result in reduced disc water content and subsequently bulging of the annulus and disc height reduction . The decreased water content accounts among other for reduced T2 signal intensity on MRI, and the discs appear as “black discs” or “dehydrated discs” on T2w images . The Pfirrmann disc degeneration evaluation system ranks the signal intensity and morphology changes in the disc into five grades ( Fig. 4 ). However, conventional MRI merely detects water loss rather than actual structural changes in collagen and glycosaminoglycan composition in the annulus and endplates . This has led to increasing interest in new MR-imaging methods for direct and more objective quantification of such changes.
T2-mapping MRI is a parametric technique that uses T2 relaxation times to provide a highly reproducible quantitative measurement of the intervertebral disc water content, and thereby represents a tool to study the structural integrity of the extracellular matrix indirectly. Changes in T2 relaxation times are found to decline with age and more rapidly with degeneration ( Fig. 5 ). Albeit, the method seems very sensitive to changes in the intervertebral disc water content, and it could be argued that it does not provide much information over categorical MRI scores (e.g., Pfirrmann grading). However, continuous measurements may enable clinicians to follow the degenerative process during treatment for degenerative disc disease and have the potential to distinguish early painful degenerative changes from age-related changes . T1rho MRI is an alternative MRI mapping method, which is sensitive to changes in the interactions between macromolecules and water in cartilage . Since the macromolecules (e.g., GAGs) maintain the disc’s osmotic pressure, this technique seems to be relevant in the quantification of lumbar disc degeneration . Further, T1rho MRI tends to have a high sensitivity and specificity for predicting pain using provocative discography as gold standard , but the technology is still in its early developmental phase. Delayed gadolinium-enhanced MRI of cartilage ( dGEMRIC ) uses a gadolinium-based contrast agent injected intravenously with a delayed MRI scan after 90–120 min to quantify the change in the T1w relaxation time, which is assumed to be inversely proportional to the concentration of GAGs. The technique was originally intended for joint cartilage assessment but seems promising in evaluation of macromolecules in the disc . Spectroscopy/nuclear magnetic resonace ( NMR ) allows the study of molecular groups carrying hydrogen nuclei, and has been used to study the chemical composition of the discs. Using a 3T scanner, metabolic changes associated with degeneration of intervertebral discs have been found with NMR . Sodium MRI quantifies the Na + concentration, which is strongly correlated with the GAG concentration. This makes it a promising diagnostic tool for measuring the degeneration of the macromolecules in the early phases of disc degeneration . However, the MRI procedure is technically challenging and requires dedicated and expensive hardware. The method is also very sensitive to movement artifacts.
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