Article

Shorter scan times for PET/MR of pediatric cancer patients

Cancer patients can be challenging to image. This could be because of the frequency of their imaging, the length of the scan, the comfort of the patient or the age of the patient. As a result, radiology departments must determine the best type of scan to image their patients and the amount of time the scan could potentially take.

Cancer patients often have to have multiple scans over a long period of time to monitor their tumors, metastases and treatment options. As a result, oncologists may be reluctant to image their patients using scans that include ionizing radiation, such as positron emission tomography (PET) or computed tomography (CT). This leaves them with the options of imaging with magnetic resonance imaging (MRI) or ultrasound (US). Often, the combined scan of PET/CT is used to image cancer patients, but PET/MR has recently begun to make an impact for pediatric cancer patients.

Imaging tests

  • PET: Positron emission tomography imaging can show how the cells in a patient's body process a radioactive contrast medium.1 This can provide details about the metabolic activity in the body. The patient may have to drink, inhale or have the contrast medium injected prior to their scan. Throughout the exam, the technologist obtains images of the dispersion process, which are later reviewed by a radiologist.
  • CT: Computed tomography uses X-rays to produce detailed images of the inside of the body.1 This study can aid doctors in pinpointing unusual activity in the body. CT scans can be done with or without contrast. As with other imaging exams, when contrast is used, the technologist is monitoring the body's reaction to the contrast to look for specific anomalies. CT scans provide detailed images of the body for the radiologist to read.
  • MRI: Magnetic resonance imaging utilizes the strong magnetic field created by the super-conducting magnet within the scanner.2 After the patient is positioned within the scanner, the technologist uses pulse sequences to change the magnetic field. These pulse sequences cause the protons within the patient's body to realign to the altered magnetic field and then relax to their original state. As a result, the body produces signals that can then be intercepted by the coils placed near the body, which then transmit the data to the computer. The resultant images depict the tissue structures and organs within the body in detail.
  • US: Ultrasound produces pictures of the inside of the body using sound waves.3 This is done using a probe and ultrasound gel that are placed on the skin. Sound waves are then transmitted into the body. When the sound waves bounce back, the computer can interpret them to create an image. Ultrasound can be used on various areas of the body and is sometimes used in cancer imaging.
  • PET/CT: Positron emission tomography and computed tomography is a common imaging modality for pediatric cancer patients.1,4 Like the name suggests, PET/CT scanners conduct both PET and CT imaging scans simultaneously to provide additional information about the patient and their body. PET/CT has been the preferred method for imaging pediatric cancer patients.4
  • PET/MR: Positron emission tomography and computed tomography can also be used to image pediatric cancer patients. In comparison to PET/CT, PET/MR showed equivalent detection rates in a study of 18 pediatric patients.5 PET/MR offers a reduction in radiation exposure compared to PET/CT.

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PET/MR takes over

Doctors Jing Qi and Nghia "Jack" Vo have experienced a shift in their pediatric oncologic imaging since the facility acquired a PET/MR scanner in 2017.4 Dr. Qi is an Assistant Professor at the Medical College of Wisconsin, while Dr. Vo is the Chief of Pediatric Radiology. The two initially noticed that the main disadvantage of PET/MR compared to PET/CT was the length of the scan.

To answer this challenge, the two and their team began to search for ways to reduce the time it took to scan their patients using PET/MR.4 They based their new MR protocols on the time it took to scan with PET. The team produced a 30-minute PET/MR scan that has already begun to help them scan their pediatric patients efficiently. This protocol has provided valuable information about patients with lymphoma and sarcoma so far.

With the reduced scan time protocol produced by Doctors Qi and Vo, their hospital has shifted many of their pediatric imaging scans from PET/CT to PET/MR, prompted in part by the reduced radiation. If their protocol begins to make its way to other radiology departments, these facilities may see similar results and may in turn want to shift their own imaging studies. PET/MR offers equivalent detection with reduced radiation from PET/CT. As many cancer patients have to have multiple imaging studies, PET/MR may provide an alternative to PET/CT that may be more appealing to physicians and patients alike.

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References

  1. PET-CT (positron emission tomography computed tomography). MassGeneral.orghttps://www.massgeneral.org/imaging/services/procedure.aspx?id=2250. Last accessed July 17, 2019.
  2. How MRI Works. HowStuffWorks.comhttps://science.howstuffworks.com/mri1.htm. Last accessed July 17, 2019.
  3. General ultrasound. RadiologyInfo.orghttps://www.radiologyinfo.org/en/info.cfm?pg=genus. Last accessed July 17, 2019.
  4. Thirty-minute PET/MR exam for pediatric cancer patients. SIGNA Pulse of MR. http://www.gesignapulse.com/signapulse/spring_2019/MobilePagedArticle.action?articleId=1488811&app=false#articleId1488811. Last accessed July 17, 2019.
  5. Simultaneous whole-body PET/MR  imaging in comparison to PET/CT in pediatric oncology: initial results. Radiology. https://www.ncbi.nlm.nih.gov/pubmed/24877983. Last accessed July 17, 2019.