Radiology of low back pain

After taking a thorough history, and performing a detailed physical examination, radiological confirmation is important.  After all, this is a picture which tells the physician whether or not there is something mechanically present which may explain the symptoms.

An AP (anterior to posterior ) film of the lumbar spine is looking through the body, from front to back.   This type of view can spot fractures, and is often useful for early detection of tumors involving the bone.      

A lateral x ray of the lumbar spine is excellent for determining alignment of the spine.  Compression fractures can detected easily.  In addition, collapse of the disk spaces may indicate degenerative disk disease. 

A CT (computed axial tomography) scan shows "slices" through the lumbar spine.  This type of image uses x rays, delivered in a circular manner circumferentially around the body.  A sophisticated computer then adds the various rays, calculates densities of bones and soft tissue within the center of the rays, and produces a picture.  The first CT scanner was made by EMI (which also produced Beatles records), and was extremely slow, and the pictures were relatively poor, by today's standards.  Present day CT scans are fast, and quite sharp.  CT is best for looking at bone, while soft tissue within the lumbar spine (such as nerves and herniated disks) are less optimally seen.

A myelogram is a study in which a spinal tap is performed within the lumbar spine, and radioopaque (shows up on x-ray) dye is placed within the spinal fluid.  X-rays are then taken, and CT scans are taken as well.  The dye provides an outline of the nerves, and any abnormality, such as a herniated disk, can be seen as an indentation or defect in the normal filling of dye.  In the myelogram on the left, the normal column of white dye is significantly narrowed by compression upon the nerves.

MRI (magnetic resonance imaging) provides exquisite views of the nerves and other soft tissues around the lumber spine.  While they don't provide as much detail about the bone as a CT scan does, they do offer the advantage of showing images in many different planes (axial, coronal, sagittal).  Sometimes herniated disks which are subtle and difficult to see in one plane, are easily seen in another.  These scans rely on magnetization of hydrogen atoms, and so far have not been found to have any long lasting adverse risks.  Some patients have difficulty because the "bore" into which they must lie is relatively narrow, and "open" MRI scanners suit these patients better.  Unfortunately there is some compromise of image quality in the open MRI scanners, and the closed machines of comparable technology still provide the best pictures.

A discogram is a study in which radioopaque dye is injected into the disk space.  This is both an anatomical study as well as a functional study.  It looks at the anatomy of the disk space, and can show when dye leaks through rents in the annulus fibrosis.  But just as or even more important, it is a functional test.  When the physician injects dye into the disk space, the patient reports, on a scale from 1 to 10, how much pain has been produced.  If the pressure injection of dye into the disk space reproduces the patients usual low back pain, then surgical fusion of the disk space may have some benefit in improving the pain.  As far as the low back team knows, it is the only test in which we hope a patient has significant pain, because that means the disk is likely a cause of the pain, and surgical fusion may help.  This is usually a method of last resort, after all else has failed.