Approaches to Dose Reduction in Molecular Imaging
Dose Reduction Technology for PET·CT
As concern over radiation dose to patients increases, hybrid modalities are coming under intense scrutiny due to their dual imaging nature. The two components of the PET•CT scan both impose a certain radiation burden. Consequently, both modalities must maximize the diagnostic information obtained while reducing the injected radiopharmaceutical dose and the applied radiation from the CT scan. Siemens innovations in both areas have resulted in reducing the radiation dose by 50–60%.
The following sections describe the major innovations contained in the Siemens Biograph™ family of PET•CT scanners. Many of the innovations are unique to the Biograph, offering a wide range of flexibility in patient dose reduction and scan acquisition speed.
Biograph mCT offers the ability to reduce the patient dose while simultaneously increasing scan speed. No longer must the choice be made between the two.
FlowMotion – Eliminating Over-Scanning
Because the CT is used for attenuation correction, the scan length must exactly match the length of the PET range. This creates a problem with conventional stop-and-go imaging, as the sequential bed dimensions often force the PET and CT acquisitions to expose an area beyond what is needed. Commonly known as over-scanning, this issue is an inherent drawback of stopand-go PET/CT systems. FlowMotion takes Siemens CARE commitment to the next level byallowing physicians to precisely and continuously plan and scan only the desired areas. As a result, only the targeted tissue is irradiated, completely eliminating over-scanning and the associated CT dose.
Furthermore, the new Biograph mCT Flow, coupled with TrueV, is the only solution that enables continuous scanning at twice t he speed of conventional PET/CT. Thanks to the 34.6% wider axial FOV of 22.09 cm– the largest in the PET/CT industry,1– a 70% increase in count rate performance is realized. When combined with ultraHD•PET, five-minute scanning becomes a reality for routine clinical performance. The arrival of high-speed, 10 mm/second FlowMotion PET scanning creates an opportunity that extends beyond providing the fastest examination. It is poised to advance future areas of research like breath-hold PET lung imaging for scans virtually free of motion.
Biograph mCT Flow meets the dual mandate for high patient safety while achieving maximum efficiency. Biograph mCT Flow is capable of achieving a 5-minute diagnostic PET•CT, with flexible CT-like range planning and zero CT overscan. With FlowMotion, physicians can truly offer As Low as Reasonably Achievable (ALARA) dose to every patient2 and referring physician.
1 Based on competetive literature available at time of publication. Data on file.
2 Patients up to 227 kg (500 lbs).
LSO Crystal Technology
In the 1990s Siemens pursued the development of a new scintillator crystal for clinical PET scanning called lutetium oxyorthosilicate (LSO). This new crystal has the properties of emitting more light than previously used materials when struck with a 511 keV photon from a PET radiopharmaceutical. LSO also emits the light more rapidly and has a shorter “afterglow.” Both of these qualities enabled the PET detector system to become more efficient, without losing any necessary information or reducing image quality. This, in turn, allows the injected dose of radiopharmaceutical to be reduced while maintaining image quality and speed of the examination.
Along with dose reduction, the improved characteristics of LSO allowed a second innovation, called HI-REZ, to take place within the detector configuration. The additional light output enabled changes to be made to the crystal configuration while simultaneously increasing the resolution. The detector went from 6 mm square pixels to 4 mm square pixels, enabling an increase in resolution of approximately 250%.
The Biograph family of PET•CT systems possesses a unique feature in the market, called TrueV, that is directly aimed at dose reduction. TrueV increases the amount of detector material and extends the field of view (FOV) of the PET detectors in the z-direction of the patient in order to capture more information in each PET bed position. Biograph mCT employs 3D acquisition technology. By applying this method, any extension of the field of view leads to tremendous benefits in data acquisition. Increasing the field of view by 34.6% actually yields an improvement in scan productivity of about 70%. Conventional PET scanning uses bed positions or stop-and-go acquisition technique in order to cover the patient’s whole body. With TrueV,
it takes less bed stops to cover the same patient volume than with other systems. In addition, each bed stop is more sensitive. By combining both of these benefits, the injected dose to the patient can be reduced while keeping all the other scan parameters the same, and image quality is maintained.
ultraHD•PET further reduces the image noise by using the HD•PET algorithm, which improves image resolution and uniformity across the field of view (FOV), and adds time-of-flight (TOF) information in the PET data. By measuring the arrival times of both incoming photons onto the detectors, the sophisticated electronics are able to determine where the measured event took place along the line of response. Because photons travel at the speed of light, the timing resolution of the PET system must be able to measure a difference in arrival times of the photons in the picosecond range. By being able to localize the event in the body along the line of response with this time-of-flight information, the reconstruction process is able to reduce image noise further to a point where the injected dose to the patient can be reduced by half and the image quality will match a full dose scan.
Biograph mCT and Biograph mCT Flow are the only PET•CT systems that combine the extended FOV of TrueV with ultraHD•PET to offer a system that can reduce the injected dose to the patient by 50% and of fer improvement in scan speed at t he same time, eliminating the need to choose between a fast scan or a low-dose scan.
Low-Dose CT CARE for Biograph
With CARE (Combined Applications to Reduce Exposure), Siemens has been highly successful in integrating many innovations that significantly reduce CT radiation dose. STRATON™ X-ray tube features a unique Adaptive Dose Shield lowering CT dose by up to 25%. Moreover, SAFIRE (Sinogram Affirmed Iterative Reconstruction) allows up to a 60% dose reduction* by utilizing raw-data-based iterative reconstruction. Finally CARE kV, the only automated CT voltage setting technology, optimizes the contrast-to-noise-ratio to reduce dose by up to 60%.
* In clinical practice, the use of SAFIRE 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.