Applied Radiology

Dr. Anzilotti received his MD from Jefferson Medical College at Thomas Jefferson University in Philadelphia, PA. He is currently a fourth-year resident at Yale University, New Haven, CT, where he will be pursuing a fellowship in neuroradiology.

Low back pain is the most common cause of disability in the United States and an expensive healthcare problem, with an estimated total cost in 1990 exceeding $24 billion. ^1,2 In recent years, magnetic resonance imaging (MRI) has become the imaging modality of choice for the evaluation of the spine. ³ Not only has this imaging modality nearly replaced computed tomography (CT) and myelography, but several studies have shown a continued increase in the utilization of spinal MRI. ^2,4 As a result, although imaging quality and protocols have been shown to be highly variable, spinal MRI has become a large part of most MRI practices. ⁵

Recently, Rao and colleagues ⁴ showed a relative disparity between the growth rates of spinal MRI and spinal surgeries in the Medicare population. From 1996 through 1998, the growth rate of spinal MRI was 25%, compared to an 11% growth rate in the number of spinal surgeries performed. One conclusion that can be drawn from this data is that MRI of the spine is becoming more sophisticated in its ability to detect and define spinal pathology and therefore better select patients that may benefit from surgery.

As spinal MRI has developed and become more sophisticated a myriad of imaging techniques and sequences have been developed. ^6,7 Nevertheless, the evolution of this imaging technique is far from over. Several new techniques are currently being developed, some of which will no doubt revolutionize MRI's ability to diagnose spinal pathology. Several of these techniques are reviewed below with emphasis on the unique diagnostic qualities each one may offer over standard spinal imaging.

Intrathecal gadolinium­ enhanced MR myelography

MRI provides excellent depiction of the anatomic structures and most pathologic abnormalities within the spine. However, several pathologic states require specific evaluation of the cerebrospinal fluid (CSF) and its function. For example, evaluation of possible CSF obstruction or leak, and cystic masses in the intradural and paradural spaces require evaluation of surrounding CSF hemodynamics. Traditionally these questions have been addressed with myelography using iodinated contrast agent and either standard radiography or CT.

In general gadopentatate dimeglumine (GD) has been shown to be a safe contrast agent for MRI. ^8,9 Although adverse reactions to intravenous GD administration have been described, the majority of these are transient and minor; typically nausea, vomiting, headache, or dizziness, with an extremely low reported incidence (0.2% to 0.42%). ^8,9

While intrathecal injection of GD is not FDA approved, several animal studies have been reported ^10-12 and recently Zeng et al ¹³ published a pilot study describing this technique in humans. In addition, Krumina et al ¹⁴ report a series in which 52 patients underwent intrathecal-GD-enhanced MR myelography without significant adverse reaction and excellent depiction of spinal pathology. Using a standard lumbar puncture technique, 0.2 to 1 cc of GD was infused into the subarachnoid space after being mixed with 3 to 5 cc of CSF or normal saline. MRI was then performed, including T1-weighted sequences, resulting in excellent depiction of several pathologic abnormalities such as spinal stenosis, herniated disks, vascular malformations, CSF leaks, and paraspinal masses (figure 1). There were no serious adverse reactions in this series and the incidence of nausea, vomiting, and headache was similar to standard myelography.

The advantage of this procedure is its ability to marry the excellent anatomic resolution of MRI with the functional evaluation of the CSF provided by myelography and therefore combine two commonly utilized imaging procedures into one. While long-term safety data has not been collected, preliminary data suggests that this new imaging technique is safe in humans, with an adverse reaction rate similar to standard myelography. ¹⁴ Since a lumbar puncture is a relatively simple and commonly performed procedure, the imaging advantages of this technique are gained with little or no need of added procedure expertise, making this a technique that could be easily integrated into any radiology practice.

Kinematic spinal MRI

As described by Haughton, "the spine which appears immobile in our static images, is far from a rigid structure." ¹⁵ Imaging the spine in the supine position completely discounts the dynamic nature of this structure. Effects of axial loading, flexion, extension, and rotation cannot be assessed with supine MRI but have been shown to affect spinal stenosis from disk disease and spinal symptoms. ^16-18 By imaging patients in the standing, flexing, and extending position, degenerative disk abnormalities can be depicted more accurately.

Jinkins et al ¹⁹ have developed a .6T magnet that not only allows supine and upright positioning of the patient but also has an open configuration that allows flexion, extension, and rotation during MRI. ¹⁹ These techniques and positions allow better delineation of degenerative disk disease and its effect on the spinal cord and nerve roots in different positions. Specifically, disk protrusions and herniations are often more pronounced in the upright and extended positions (figure 2).

Muhle et al ²⁰ have already described a classification system of cervical spondylitic myelopathy based on severity of disease during flexion and extension. Further work in this field may shed more light degenerative disk disease and possible reveal a better correlation between finding and symptoms.

While this imaging technique requires a dedicated magnet for optimal utilization, further research in this field may make the use of these systems more prevalent.

Perfusion imaging in the spine

Perfusion weighted MRI (PWI) is an imaging technique that measures the relative amount of blood volume flowing to a particular anatomic location. While perfusion measurements can be obtained with MRI using either an injected contrast agent (GD) or based on endogenous contrast techniques, the former seems to provide improved physiologic and tissue contrast. ²¹

First proposed by Villringer et al, ²² contrast-enhanced PWI has been shown to have several applications in the brain, including evaluation of "tissue at risk" in stroke patients, differentiating recurrence of hypervascular tumor from nonneoplastic tissue, staging gliomas based on vascularity, and evaluation of patients with Alzheimer's disease. ^23-26

Recently, Hinman and colleagues ²⁷ showed how PWI could be used to diagnose infectious and neoplastic processes within the spine. Using a 2D-fast spoiled gradient echo (FSPGR) PWI, they plotted the slope of enhancement following GD injection and showed a statistically significant difference between the slope of increasing signal intensity between normal vertebral bodies and those with neoplastic or infectious processes (figure 3). This strong correlation of histological diagnosis and PW imaging characteristics may provide a significant advantage in the diagnosis of neoplastic or infectious processes in the spine, which can sometimes be difficult to determine with standard MRI techniques. Similar findings have been reported by Stabler et al ²⁸ for the evaluation of myeloma in the spine.

Diffusion imaging in the spine

Diffusion-weighted MRI (DWI) has drastically changed the way in which ischemia of the brain is evaluated. ²⁹ Based on the current theory that it can detect differences in the free or relatively restricted motion of water molecules, this technique has been shown to be very sensitive to pathologic states in the brain. ³⁰ Ischemia, demyelination, and even neoplasm all effect cell membranes and change the normal diffusion coefficient of neuronal tissues making them conspicuous on DWI.

In the past, application of DWI to the spine has been limited because of motion artifact associated with the spine and the difficulty in obtaining accurate motion corrected images. However, recently DWI has been used to differentiate malignant versus bland compression fractures in the spine. ³¹ In addition, DWI evaluation of the spine for ischemia should become more applicable as newer coils and sequences are developed.

Perhaps most exciting is DWIs ability to evaluate the spine for axonal injury and cord healing. ^32,33 Because of the innate anisotropic diffusion of the spine in the craniocaudal direction, cord injury, which disrupts nerve fibers, is readily detected with DWI. This technique will likely continue to play an increasingly important role in the evaluation of the traumatized spine, particularly as treatment for these disorders evolve.

Ultrafast MRI of the lumbar spine

As described above, low back pain is a major cause of disability and health care expenditure in the United States, with MRI being the gold standard for evaluation of back disorders. ^1-3 However, in the current healthcare environment, decreasing reimbursement, coupled with increasing utilization of MRI, have created a great need to increase efficiency of imaging.

As with MRI in other parts of the body, there is a constant trade-off between imaging time and quality. In the spine, attempts to decrease imaging time are particularly difficult, given the added unique problems of CSF flow and motion artifacts. Sze and colleagues ³⁴ addressed the need to shorten imaging time by showing that fast spin-echo (FSE) imaging of the spine decreased imaging time while maintaining accurate depiction of spinal pathology.

Recently, Mastromatteo and colleagues described an "Ultrafast MRI" technique that drastically decreased scan time. ³⁵ Using a 1.5T Vision System (Siemens Medical Systems, Iselin, NJ) to evaluate 79 patients, the researchers reduced the total scan time for the lumbar spine to 1.38 minutes, compared with 16.53 minutes for a standard spine MRI series. The entire Ultrafast MRI study consists of the following three sequences; sagittal HASTE (256 matrix, 1.17 * 1.09 pixel size, 5-mm slice thickness), sagittal 3D T1 gradient echo isotropic 256 sequence (1.95 * 1.95 1.95 voxel size), and axial T2 HASTE (1.04 * .98 pixel size with 5-mm slice thickness).

Statistical comparison of the Ultrafast imaging sequence to standard sequences showed an almost perfect diagnostic agreement while significantly decreasing scan time and reducing costs by a factor of 11.97 (figure 4). ³⁵

Although additional studies are needed before this new imaging technique can be incorporated into mainstream imaging, reducing scan time and costs, to the degree these authors have reported, could potentially increase access to MRI for patients with back disorders, decrease patient anxiety and discomfort, and greatly increase the efficiency of MRI centers.

Conclusion

Several new imaging techniques specifically tailored to evaluate the spine have been described. These imaging techniques have the potential to revolutionize the way we evaluate the spine, by better depicting known pathologic disorders as well as shedding new light on disorders we currently do not fully understand.