Approaches to Dose Reduction in Molecular Imaging
In molecular imaging the effective dose is proportional to the administered activity of the radiopharmaceutical, which indicates the amount of the isotope. Activity is measured in Becquerel (Bq), where 1 Bq equals 1 decay per second, and Curies (Ci), where 1 Ci equals 3.7 x 1010 decays per second.
Additional physical parameters are physical half-life, and type and properties of the emitted radiation. Isotopes with a physical half-life in the range of the total examination duration should be applied. Thus, unnecessary radiation is prevented that would result from using isotopes with longer half-lives. Pure gamma emitters such as 99mTc or pure positron emitters such as 18F are preferred. Additional radiation components do not contribute to the diagnostic image, but contribute to the overall effective dose.
Dose & Radiation Risk
Biological parameters are the pharmacokinetic properties of the radiopharmaceutical, which influence the biological half-life and distribution pattern, and change dynamically after the application. Effective half-life combines the effects of physical decay (physical half-life) and excretion (biological half-life):
1/T1/2eff = 1/T1/2phys + 1/T1/2biol
The calculation of effective dose in an individual varies due to differences in excretion time, as well as anatomical differences. Dose calculation reflects the absorbed radiation in the body both from the radiopharmaceutical residing in that region and the radiation dose from distant organs/regions. Thus, some simplifications – such as standardized anatomy – are made for the assessment of dose for diagnostic purposes, as in Organ Level INternal Dose Assessment
Package inserts describe the effective doses per administered MBq as well as the organ doses per MBq for, at minimum, critical organs for a standard patient with respect to size, weight, distribution pattern and excretion.
The impact of the molecular imaging system on the dose is only influenced by the system’s minimum activity requirements for a given image quality and acquisition duration.
There is a wide range of typical effective doses for different nuclear medicine procedures. Some are far below 1 mSv (e.g. the Schilling test), while others may exceed 10 mSv (e.g. gallium scintigraphy). The effective dose for a typical 18F-FDG injection used for a PET•CT is 5–7 mSv. Most procedures, however, result in doses between 1 and 10 mSv.
In hybrid devices, the burden of radiation stems not only from the use of radiopharmaceuticals but also from the CT component as well. Thus, advances in scanner technology allow the reduction of injected activity, and hence, the effective dose.
In addition to activity reduction, other considerations include:
1. The usage of ionizing radiation always requires a justified indication. Can the same clinical result reasonably be achieved using any other examination?
2. Can the overall diagnostic dose be reduced by judiciously combining examination types? Good clinical planning may reduce the number of CT examinations if diagnostic CT and CT for hybrid imaging are combined in a single scan.
3.Isotopes with a shorter half-life and favorable radiation type can reduce radiation exposure dramatically. For example, the use of 123I-MIBG instead of 131I-MIBG in the diagnosis of neuroendocrine tumors improves image quality and reduces radiation exposure, due to a much shorter half-life (13 hours instead of eight days) and radiation type (gamma emitter with a main peak of 151 keV versus combined beta and gamma emitter). Another example is the use of 18F-FDG-PET instead of 67Ga scintigraphy in lymphoma and inflammation cases. In addition to a much lower radiation exposure, image quality and precision are much better and the overall time from injection to end of scan is substantially reduced (1.5–2 hours versus 3–4 days).
4. The choice of the radiopharmaceutical can also reduce the radiation burden. 99mTc-MAG3 shows a much higher renal uptake and clearance than 99mTc-DTPA. This allows injected activity to be reduced by a factor of 2 while maintaining the
same image quality and precision with respect to urodynamics.
5. Not every clinical question requires striving to achieve the best possible image quality through the combination of high injected activity and advanced acquisition and reconstruction tools. In some cases, a somewhat reduced image quality, which permits lower injected activity, still fulfills the clinical need.
6. Several countries have issued guidelines for maximum standard activities according to examination type. Pediatric activities can be calculated adherent to special rules. In the past, effective doses in children were rather high due to insufficient adjustments to the guidelines, which have since changed.
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.
Dose Reduction Technology for SPECT
SPECT•CT imaging procedures are categorized into four main clinical areas – cardiology, oncology, general purpose imaging, and neurology. Minimizing dose is equally important in all clinical segments, but – unlik e CT or MRI – h ybrid imaging in nuclear medicine (SPECT•CT and PET•CT), permits dose to be optimized in two ways: by reducing the dose of the injected radiopharmaceutical and by minimizing the CT dose emitted from the hybrid scanner. While pressure is increasing to minimize the CT dose, it is also critically important to reduce the injected dose in SPECT imaging to help the physician improve patient safety.
Additionally, but no less importantly, there are only five nuclear reactors worldwide that produce 100% of the most commonly used SPECT tracer – technetium (99mTc). Some of these reactors have been shut down for temporary maintenance, and the availability of 99mTc, consequently, has been reduced worldwide. This limited availability has brought a need for more efficient use of technetium. Therefore, injecting less dose per patient is very important so that hospitals can continue to scan their full patient load.
Siemens SymbiaTM SPECT·CT product line is pioneering dose savings in both SPECT and CT. Not only does Siemens have the most integrated diagnostic SPECT·CT systems, but also provides features that can save up to 75% of the dose while maintaining image quality. The following sections provide a short overview of the dose-saving features of Symbia.
Features pioneered by Siemens to reduce dose in Symbia SPECT·CT include (Figure 1).
Conventional collimators have uneven septa wall thickness, which decreases sensitivity and increases the dose or scan time needed to acquire a valuable image.
Siemens is the only equipment manufacturer that designs and produces its collimators inhouse. AUTOFORM collimators use a proprietary design that provides uniform septa wall thickness. The proprietary collimator design offers the industry’s highest sensitivity,1 with up to 26% more counts1, while maintaining image resolution.
Siemens AUTOFORM collimators offer the highest SPECT LEHR sensitivity in the market today that enables:
- Up-to 26% lower injected dose
- Up-to 26% faster imaging
1 Based on competitive literature available at time of publication. Data on file.
Flash 3D Iterative Reconstruction
Inaccurate image reconstruction impacts image quality and can result in both false positives and false negatives. Most reconstructed images are created using algorithms that do not account for the physical characteristics of the image acquisition system.
Symbia SPECT•CT scanners with Flash reconstruction use a measured 3D collimator beam model in the iteration process. Correct modeling of the collimator distributes the activity over the slices for more accurate reconstruction. With Flash, the spatial resolution of the collimator is modeled to maintain the precise shape of the lesion. As a result, images are reconstructed with more counts in the correct volume, increasing image contrast.
Flash enables up-to 45% higher reconstructed resolution according to NEMA specifications1 that ultimately enables:
- Up-to 50% less injected dose
- Up-to 50% faster imaging
1 Based on competitive literature available at time of publication. Data on file.
Automatic Quality Control
Scanners require regular quality control (QC) tests to validate calibration of the detectors. For conventional scanners, quality control is performed manually, and typically requires 20 minutes to an hour for daily QC, and about six hours for monthly QC. Because the tests require preparation and handling of open radioactive sources, the process must be rotated among the staff to avoid overexposure. Dose spillage adds further risk to the process.
With Siemens AQC, the process is performed automatically overnight, with a report provided for the technologist to review the next morning. Siemens AQC facilitates performance trending, eliminates the risk of open-source spillage and reduces staff radiation exposure.
The benefits of Automatic Quality Control (AQC) include:
- Reliable, consistently reproducible QC
- Eliminated risk of spillage with open sources
- Operator dose reduction
- Can run automatically overnight saving time for the operator
Routine cardiac studies can take up to 20 minutes, limiting the number of patient scans. To achieve the desired image quality, patients are often injected with high doses that increase radiation exposure.
Siemens IQ•SPECT is the only technology that performs ultra-fast cardiac imaging with a general purpose camera. Its unique collimator design, cardio-centric image acquisition and advanced reconstruction technology acquires four-times more counts than conventional methods for the highest quality images independent of age, body shape or size.
With IQ•SPECT, physicians can perform a myocardial perfusion imaging study in on of the following protocols options:
- Up-to 75% less injected dose in 16 minutes
- Up-to 50% less injected dose in 8 minutes
- Up-to 25% less injected dose in 4 minutes
Low-Dose CT AC
In hybrid imaging one of the most commonly used CT protocols is attenuation correction. Highend SPECT/CT scanners that incorporate CT systems with a higher number of slices may require more energy, potentially exposing the patient to higher levels of radiation. Additionally, conventional SPECT/CT scanners do not allow selection of lower X-ray tube voltages for attenuation correction scans.
Siemens high-end 16-slice CT system delivers up to 74% less radiation1 to the patient during attenuation correction and the user can adjust X-ray tube voltages to further reduce dose exposure. For the three most common nuclear medicine attenuation correction procedures, Siemens CT technology delivers the lowest dose compared to conventional scanners.
1 Based on competitive literature available at time of publication. Data on file. Dose values compared are based upon factory default protocols from GE NM670.