I need a scan for my back, doc!

You may be surprised but you probably do not need an X-ray, CT scan, or MRI for acute back pain. There are some good reasons for this.

Radiation from x-rays and CT scans can cause harm

The image obtained is the end results of how various tissues absorb radiation to prevent passage of the radiation to a detector. An x-ray provides a two dimensional image from a flat film. Although, now a digital detector is used rather than wet chemistry, just like photography. The radiographer positions you and the machine up to takes the image, whilst a radiologist is a doctor who interprets the image.

A CT or computed tomography scan, makes use of computer-processed combinations of many x-ray measurements taken from different angles to produce cross-sectional (tomographic) images. Hence, you lie down in a machine that looks like a giant donut. The virtual slice of your body are in reproduced in various shades of grey depending again on how much the tissue absorbed the radiation. Some illness or injury will affect the degree of absorption of the radiation affecting the final image. CT scans, through more computerised processing can be reconstructed in a 3-D image of parts of the human body. Here is a link to Radiopedia’s page on what a normal CT and x-ray of the lumbar spine looks like.

There are no published studies that prove a direct link between medical imaging and increased cancer risk. The assumed increased risk of cancer from CT scans and x-rays is based on individuals exposed to high doses  due to atomic bombs and nuclear accidents.

Things have moved on from the early days of x-rays such as shown in the illustration where both doctor and patient are receiving a massive dose of radiation.  This article talks about some of the early injuries from x-rays.

Here is a table that shows the relative amount of radiation required for various x-ray and CT scans. Radiation exposure is measured in millisieverts or mSv. The average radiation dose to a person in Australia from natural sources is about 1-2mSv per year.

There are calculators used to determine the risk from a radiological study. So for a lumbar spine x-ray series I would receive a dose of 1.5 mSv of radiation, which is equivalent to back ground radiation I am exposed to over a whole year.

What about an MRI?

MRI or magnetic resonance imagining uses strong electromagnets and radiowaves to make water molecules move in a specific way. The information collected at a detector then enables computers to build up an image on internal structures in thin slices. An MRI allows a better image of soft tissues, hence it is more useful than x-rays and CT-scans to view nerves, ligaments and intraverbral discs. There may be no radiation risk from an MRI, but it is not without physical hazards; it can be difficult to tolerate the tunnel if you are claustrophobic, it would be wise not to be in the way of metals objects being attracted to the magnets, and movement or heating of metallic implants can cause tissue damage, and pigments in tattoos can cause burns.

Here is a link to Radiopedia that shows a normal MRI of the lumbar spine.

Imaging tests can be expensive

Here is the Medicare rebate for three studies of the lumbar spine. Of course the cost to you may include a gap set by the radiology practice. It may pay to shop around. Remember, you pay not only for the quality of the scan, but also the radiologist’s interpretation of what may be wrong. l have to confess that I have NOT done five year of training to be a fellow of the Royal Australian and New Zealand College of Radiologists, and so am not be the best person to interpret an MRI done by “MRI R US”.

  • X-ray  $47
  • CT scan $177
  • MRI  $358.40, (but be aware that a GP may not be able to order an medicare rebatable MRI)

Imaging may lead to the unnecessary

As mentioned above, a radiologist will interpret the x-ray or scan to answer a clinical question posed to them. Hopefully it will more than “what is the cause of this chaps back pain?” An incidentaloma is a shadow that is seen that represents a lesion of dubious or uncertain clinical significance. This may leads to increased anxiety on the part of the doctor or the patient and may well lead to more scanning, extra tests, surgery or a biopsy to find out if the spot is of clinical significance. In some cases, it may be a “lucky” find such as a resectable pancreatic cancer, but more often is just a distraction. 

Sometimes, incidental findings can be quite enlightening. In this case from Radiopedia, the patient did not know about the pregnancy.  Two months later she gave birth to a healthy child.

And now to the elephant in the room

As suggested by this review paper, “these imaging findings must be interpreted in the context of the patient’s clinical condition”.

Low back pain is common in Australia, affecting over two-thirds of adults at some point in their lifetime. Findings such as disc degeneration, facet hypertrophy, and disc protrusion are often interpreted as causes of back pain, triggering medical and surgical interventions in the hope of alleviating suffering. Sometimes, this does works, but for some it doesn’t, leaving a person worse off after surgery, addicted to opiate pain relievers, believing that they will never get better.

Several studies have demonstrated that imaging findings of spinal degeneration are present in a large proportion of asymptomatic individuals. I have taken this table from a 2016 paper in the American Journal of Neuroradiology that studied the age-specific prevalence of the following imaging findings in asymptomatic individuals: disk degeneration, disk signal loss, disk height loss, disk bulge, disk protrusion, annular fissures, facet degeneration, and spondylolisthesis. You can see that as you age, a scan is more likely to pick up abnormal findings. But even in a 20 year old, a third of people scanned had a disc bulge which is often blamed for that annoying sciatica.

Age-specific prevalence estimates of degenerative spine imaging findings in asymptomatic patients

So are there times when I may consider imaging?

Indeed there are things which may increase the the chance of finding an significant abnormality which may indicate a scan. These are often clumped together as red flags and may indicate the four serious causes that is cancer, infection, a fracture or spinal cord damage (cauda equina). This then leads your doctor to consider a serious of questions that at first may seem odd and not connected before going on to examine your back.

  • Duration of pain
  • Nighttime pain affecting sleep
  • Progressiveness worsening pain
  • Trauma
  • Osteoporosis
  • Sudden back pain with spinal tenderness
  • Recent surgery
  • A history of cancer, immunocompromise, corticosteroid use (eg prednisolone) or intravenous drugs use
  • Unexplained weight loss
  • Age under 22 or over 55
  • Fever, chills or rigors, sweats
  • Loss of bowel control, inability to pass urine or loss of sensation in the saddle or perineal area
  • Altered sensation or strength the legs
  • Diminished or absent deep tendon reflexes, like a patella jerk
  • Blood tests that suggest infection , eg elevated ESR or CRP.

A review in the European Spinal Journal in 2016, of these and other red flags, however did find their usefulness is also limitted. This was backed up by a Cochrane review of red flags for malignancy due to lower back pain. This article recently published in the The Journal of Bone and Joint Surgery, suggests that maybe the red flags should be recoloured. For instance, for patients with no recent history of infection and no fever, chills, or sweating, the presence of night pain was a false-positive finding for infection in over 96% of the time. The authors recommendations were

  1. Recent trauma should raise the vertebral fracture, in patients who are over 50 years old and especially in those over 70 years old. The presence of both recent trauma and an age of over >50 years carries a 13.1% probability of a vertebral fracture  and 20.5% probability of vertebral fracture in an over 70 years.
  2. The presence of a history of cancer in the setting of low back pain carries a 10.6% probability of having a spinal malignancy. A history of unintentional weight loss, alone, carries a 3% probability of having a spinal infection; however, when both red flag were present, the probability of having a spinal malignancy was 14.3%.
  3. A recent history of infection in the setting low back pain carries a 10.2% probability of having a spinal infection. A history of fever, chills, or sweating, alone, carries a 2% probability of having a spinal infection; however, when both red flag questions were present, the probability of having an infection in the setting of low back pain was 13.8%.
  4. A loss of bladder control and loss of bowel control has a the probability of having cauda equina syndrome of 1.2%.
  5. Classically, night pain has been associated with malignancy and infection; however, this symptom was a false positive >85% of the time for malignancy, and >96% of the time for infection when no other associated red flag symptoms were present.
  6. Other than an age of >50 years and a diagnosis of vertebral fracture, the absence of any individual red flag or combination of red flags does not help to rule out a particular red flag diagnosis, as noted by the low change in probabilities, sensitivities, and negative likelihood ratio of red flag questions.

So back to the original request for an x-ray, CT or MRI, it is therefore not unreasonable to suggest that imaging the back is of such low yield that you are better off being reassured that your back pain is likely to be self-limited and benign. 

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