Routine Dose Reduction with Biograph mCT and Time of Flight

Imaging techniques to reduce the radiation burden for PET patients

Dale L Bailey, PhD, Elizabeth A Bailey, PhD, Kathy P Willowson, Geoffrey P Schembri and Paul J Roach, MD

Case study data provided by Department of Nuclear Medicine, Royal North Shore Hospital Sydney, and University of Sydney, Sydney, Australia

 |  2012-12-14

Introduction
Radiation dose from medical imaging is currently receiving widespread attention. Both manufacturers and end-users are implementing methods to reduce the radiation exposure received by the subject. This is becoming increasingly recognized in PET/CT scanning where, with modern therapies for cancer, many subjects have repeated follow-up scans to monitor their response over time. This concern is exacerbated in the pediatric cancer patient. Also, governments and health providers are increasingly concerned about the costs associated with the modern technologies available to diagnose, treat, stage and monitor disease response and progression. Siemens Biograph™ mCT with extended axial field of view (TrueV), time of flight capability and point spread function (PSF) resolution (ultraHD•PET) recovery reconstruction processing has consistently enabled the lowering of the standard injected dose of fludeoxglucose F18* (18F FDG). The TrueV extended field of view option is uniquely suited for such a strategy of lowering administered dose and the consequent decrease in patient radiation burden. Substantial radiation dose reduction can also be achieved on CT using iterative reconstruction** techniques. Following optimization investigations, the 18F FDG dose, which was defined as standard for Biograph mCT was 250 MBq for patients <90 kg in weight and 300 MBq if body weight was >90 kg. There was no adjustment of scanning time per bed position for different patient weights. The standard acquisition parameter was 2 to 2.5 minutes per bed position. Recent software upgrades have enabled variation of time per bed for individual bed positions (e.g., shortened acquisition times when scanning lower limbs, extended time per bed during scanning of selected areas of interest), which provides greater flexibility. The default CT settings for whole-body PET•CT are 120kVp, 80 mA with automatic exposure control (AEC). A selection of typical clinical examples using low-dose protocols is shown in the following 3 cases.

 

*Siemens' PETNET Solutions is a manufacturer of fludeoxyglucose F 18 injection (18F FDG). Indication and important safety information as approved by the US Food and Drug Administration can be found at the bottom of the page for 18F FDG, adult dose 5-10 mCi, administered by intravenous injection.

Case 1

History
A 61-year-old male presented with a past history of plasmacytoma. His body weight was 89 kg/196 lbs and 289 MBq 18F FDG (3.2 MBq/kg) was injected with PET•CT acquisition performed after a post injection delay of 82 minutes. Lowdose CT was followed by PET acquisition of 2.5 minutes per bed position.

 Diagnosis
The PET•CT study (Figure 1) shows multiple 18F FDG avid focal areas in the left inguinal/femoral region as well as in the lower lumbar and mid thoracic vertebrae, ribs and mediastinum. Clinical impression was that of multiple metastases. Radiation dose estimate from injected 18F FDG: 4.3 mSv.1 Note the sharp delineation and high contrast of small focal lesions in spite of the low injected dose.
 

Case 2
History

A 77-year-old male presented with a past history of anaplastic thyroid cancer. He weighed 95 kg/209 lbs and was injected with 242 MBq 18F FDG (2.5 MBq/kg). Scanning commenced approximately 62 minutes after injection with low dose CT followed by a PET acquisition of 2 minutes per bed position.

 Diagnosis
The PET•CT study (Figure 2) shows significant uptake of 18F FDG in the thyroid bed, bilaterally in lymph nodes in the neck and in a nodule in the left lung. This is clinically suggestive of residual thyroid tissue with cervical nodal and lung metastases. Radiation dose estimate from injected 18F FDG: 3.4 mSv.
 

Case 3
History

A 29-year-old female (67 kg/148 lbs) presented with a history of metastatic pancreatic neuroendocrine tumor. She had a distal pancreatectomy and splenectomy one year prior to the scan. Due to a low yield from the radiopharmaceutical synthesis she was injected with only 54 MBq of 68Ga DOTATATE*** (0.8 MBq/kg), while the standard dose for such a study was 150-200 MBq. Scanning commenced approximately 48 minutes after injection with low dose CT followed by a PET scan of 5 minutes per bed position to partially compensate for the low amount of radioactivity injected.

Diagnosis
68Ga DOTATATE*** PET images (Figure 3) show multiple foci of increased uptake in the liver suggestive of hepatic metastases. Radiation dose estimate from 68Ga DOTATATE***: 1.3 mSv. Note the clear delineation of small liver metastases with adequate lesion contrast in spite of the low injected dose.

 

Comments
Low-dose protocols on Biograph mCT consistently produce high-quality images with accurate and reproducible quantification while maintaining acceptably short scanning durations. The extended field of view (TrueV) configuration has a key contribution to this ability along with further contributions from time of flight and HD•PET PSF resolution recovery as part of the iterative reconstruction process. In view of the low radioactivity dose injected, the bulk of the radiation burden to the patient is delivered by the CT.
A retrospective series of 500 consecutive patients scanned with 18F FDG PET•CT using the above mentioned protocol was reviewed. The average radiation doses calculated were 4.9 mSv for PET (for~250 MBq 18F FDG) and 8.2 mSv for the CT component (120 kVp, 80 mA with AEC – CARE Dose®) giving an average of just over 13 mSv1 for the total examination. The radiation burden for PET patients could be reduced** from generally seen levels today of over 20-30 mSv (when administering 550 MBq of 18F FDG and from CT using normal reconstruction techniques) to around 10 mSv in total using this methodology of lower 18F FDG dose requirement and iterative CT reconstruction with lower beam current usage without compromise of the patient throughput and image quality.

 

References:

  1. Willowson KP, Bailey EA, Bailey DL. A retrospective evaluation of radiation dose associated with low dose FDG protocols in whole-body PET•CT. Australas Phys Eng Sci Med. 2012 Dec 10; 35(1):49-53


*Fludeoxyglucose F 18 Injection

INDICATIONS AND USAGE
Fludeoxyglucose F 18 injection (18F FDG) is indicated for positron emission tomography (PET) imaging in the following setting:
Oncology: For assessment of abnormal glucose metabolism to assist in the evaluation of malignancy in patients with known or suspected abnormalities found by other testing modalities, or in patients with an existing diagnosis of cancer.

 

IMPORTANT SAFETY INFORMATION
Radiation Risks
Radiation-emitting products, including fludeoxyglucose F 18 injection, may increase the risk for cancer, especially in pediatric patients. Use the smallest dose necessary for imaging and ensure safe handling to protect the patient and health care worker.

Blood Glucose Abnormalities

In the oncology and neurology setting, suboptimal imaging may occur in patients with inadequately regulated blood glucose levels. In these patients, consider medical therapy and laboratory testing to assure at least two days of normoglycemia prior to fludeoxyglucose F18 injection administration.

Adverse Reactions
Hypersensitivity reactions with pruritus, edema and rash have been reported; have emergency resuscitation equipment and personnel immediately available.

 

Full Prescribing Information for Fludeoxyglucose F 18 Injection

Fludeoxyglucose F 18 injection is manufactured by Siemens' PETNET Solutions, 810 Innovation Drive, Knoxville, TN 39732

** In clinical practice, the use of Iterative Reconstruction PET•CT may reduce CT patient dose depending on the clinical task, patient size, anatomical location, and clinical practice. A consultation with a radiologist and a physicist should be made to determine the appropriate dose to obtain diagnostic image quality for the particular clinical task.

*** 68Ga DOTATATE is not currently recognized by the U.S. Food and Drug Administration (FDA) or other regulatory agencies as being safe and effective, and Siemens does not make any claims regarding its use.

The statements by Siemens customers described herein are based on results that were achieved in the customer's unique setting. Since there is no "typical" hospital and many variables exist (e.g., hospital size, case mix, level of IT adoption) there can be no guarantee that other customers will achieve the same results.