Feature article

Advances in pain imaging using PET/MR

Millions of people across the world experience pain on a regular basis. Whether the pain is acute or chronic, short or long-lived, it all comes back to the nerves in our bodies. In most cases, nerves send pain signals in response to touches or sensations that are painful and can be harmful. However, there are times when the pain we experience does not have to do with something that is normally considered a painful experience.

What is pain?

Pain can be anything from a prick to a tingling to a burning sensation and can be sharp or dull.1 Pain is your body's way of telling you that something may be wrong. It is triggered by signals that your nervous system sends out and can be helpful in diagnosing a problem. Pain frequently keeps people from doing things that may seriously harm them. Without pain, it would be hard to know what has been or will be injured. Different treatments may reduce a patient's pain, even when it is not curable.1 If someone begins experiencing pain that may cause them to look further into the cause of their pain, especially if the cause is unknown.

What causes pain without an injury?

Unfortunately, chronic pain, which lasts for an extended length of time, may result from a number of different things, including acute pain and certain medical conditions.2 For example, after amputation, some patients experience phantom limb pain. This feels like it's coming from the body part that is no longer there. It is similar to phantom limb sensation, but it is pain instead of the feeling that they still have the body part that was removed. In some cases, phantom pain can show up on magnetic resonance imaging (MRI) or positron emission tomography (PET) in the form of activity in the portion of the brain neurologically connected to the nerves.2 This type of pain may fade over time and is not the result of the amputation itself.

Some chronic medical conditions can also cause pain in patients, including migraine. Migraines are often recurring headaches that can cause varying levels of pain. Patients who get migraine may be more likely to experience pain that accompanies something that is not normally painful.3 This pain is referred to as allodynia, which is a central pain sensitization, or a pain processing dysfunction in the nervous system, that follows normally non-painful, repetitive stimulation. It is important to note that this condition is not an increased sensitivity to pain or the result of physical damage. Allodynia can be static, dynamic or thermal and results from different sensations. Static allodynia is pain that results from a light touch on the skin, while dynamic allodynia results from movement across the skin. The third type, thermal, results from mild changes in temperature. Allodynia can occur with a variety of other painful conditions as well.

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What does PET/MRI tell us about pain?

Some recent studies, including those completed by Dr. Sandip Biswal, who is an Associate Professor of Radiology (Musculoskeletal Imaging) at Stanford University Medical Center, have begun measuring changes that occur with a painful sensation. Dr. Biswal studies the uptake of the PET contrast fludeoxyglucose, or 18FDG, to monitor these changes on PET/MRI in an attempt to understand the origin of a patient's pain.4,5 His attempts to understand pain in patients with sciatica, which is back pain associated with the sciatic nerve, have led to a better understanding of their pain.

Dr. Biswal used 18FDG in 14 patients, nine of which had chronic sciatica and the other five were healthy.5 Throughout the PET/MRI scan, the amount of contrast that was observed. The research team noticed an increase in the uptake of 18FDG in patients with chronic sciatica. This was observed in the lesions in all of the patients and was correlated with pain symptoms. The lesions were found in areas not associated with spine pain as well. Because of this, Dr. Biswal's team believes that this could lead to a possible method to determine pain generators in chronic sciatica.5 Though this method is still being research, it could potentially help physicians in the future to understand their patient's pain and what treatments may work the best.

Patients may see their doctors because of pain that they are experiencing, especially considering that the body treats pain as a warning sign to stop doing something. The injuries that pain can prevent are numerous, but patients with chronic, inexplicable pain may struggle to recognize what pain is normal for them and what isn't. Dr. Biswal and other researchers are attempting to understand the source of chronic pain and hopefully help these patients. If the method they proposed works, it could lead to better treatment for those patients who experience pain on a regular basis.

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References

1. "Pain." MedlinePlus.gov. Web. 4 June 2019. <https://medlineplus.gov/pain.html>.

2. Mayo Clinic Staff. "Phantom pain." MayoClinic.org. 30 October 2018. Web. 4 June 2019. <https://www.mayoclinic.org/diseases-conditions/phantom-pain/symptoms-causes/syc-20376272>.

3. "What to Know About Allodynia." AmericanMigraineFoundation.org. 7 February 2019. Web. 4 June 2019. <https://americanmigrainefoundation.org/resource-library/what-to-know-about-allodynia/>.

4. Mary Beth Mussat. "Localizing pain with PET/MR." SIGNA Pulse. Spring 2019. Web. 4 June 2019. <http://www.gesignapulse.com/signapulse/spring_2019/MobilePagedReplica.action?pm=1&folio=8#pg8>.

5. Peter W. Cipriano, et al. "18F-FDG PET/MRI in Chronic Sciatica: Early Results Revealing Spinal and Nonspinal Abnormalities." JNM. 2 November 2017. Web. 4 June 2019. <http://jnm.snmjournals.org/content/59/6/967.short>.