PET Scanning Meets High Definition
PET images can be somewhat limited in terms of image quality. FDG-PET scans are highly sensitive for detection of certain types of cancerous lesions; however, specificity can be low, which can thus limit the clinical utility of conventional FDG-PET images. PET/CT helps increase the specificity by adding anatomical data to PET images; nevertheless, room for improvement of the PET images remains. A new technology debuting this month, high definition PET (HD•PET) from Siemens Medical Solutions promises to bring PET/CT scanning to the next level.

Analogous to HDTV, HD•PET provides uniform resolution across the entire field of view to offer significant improvements in the PET/CT imaging process. HD technology enables visualization of structures with two millimeter resolution and 2X improvement in signal to noise across the entire field of view (FOV); it offers the potential to deliver superior detection of small lesions, improved staging and enhanced treatment planning.

PET Scanning Meets High DefinitionConventional PET image shows hypermetabolic cervical lymph node metastases with FDG avid brown fat at left, in contrast to the HD•PET image at right that provides sharper delineation of nodes. Standard reconstruction shows increased uptake in the region without sharp delineation of individual nodes. The study was acquired on Siemens Biograph 64 at the University of Erlangen, Erlangen, Germany.

The technology inside
“The first PET/CT scanners introduced in 2001 were limited to 6 to 8 millimeters in terms of spatial resolution,” recalls David Townsend, PhD, director of the molecular imaging and translational research program at the University of Tennessee Graduate School of Medicine in Knoxville. Siemens introduced high-resolution PET/CT in 2003, which offered improved spatial resolution. Even so, resolution is optimal at the center of the field of view, decreasing towards the edges. In practice, even with a high-resolution scanner, spatial resolution varies from about 4.5 millimeters in the center of the field to about 6 millimeters at the edges of the field. The reality of the human anatomy is that the region of interest may lie outside the center of the FOV.

Siemens TrueV option, introduced in 2006, addresses some of the challenges inherent in PET/CT scanning by extending the axial field of view from 16.5 centimeters to 21.5 centimeters, providing physicians with the flexibility to decrease scan time by a factor of two for comparable image quality. 

Next-generation reconstruction technology
HD•PET reconstruction technology overcomes the variability in spatial resolution across the entire field of view. “An important objective in PET/CT imaging is early detection of disease. Uniform resolution and detectability are important because lesions can, of course, be located anywhere throughout the field of view,” explains Townsend. 

HD•PET technology combines superior PET resolution with uniform resolution across the entire transverse field of view, resulting in higher lesion contrast. It improves spatial resolution to an unparalleled 2 millimeters throughout the entire field of view. The advances in spatial resolution and uniformity potentially translate into a wide array of clinical benefits — including improved visualization of metastases, superior edge detection to help clinicians develop highly accurate radiation therapy plans and improved visualization of smaller lesions in all patients, regardless of size.

The clinical difference
HD•PET provides significant improvements in clinical image quality and is expected to enhance the clinical utility of PET/CT. Until recently, FDG-PET/CT often confirmed findings already known from CT. However, the real promise of the marriage between functional and anatomical imaging is the detection of malignancies not yet visible on CT. Consider, for example, lymph nodes malignancies. HD•PET may make it possible to detect FDG uptake in lymph nodes that are malignant but not yet enlarged (negative on CT), thus leading to earlier detection and treatment.

PET Scanning Meets High DefinitionConventional PET (top) and HD•PET (bottom) images of a 66-year-old female show bony and pelvic lymph node metastases. Two separate nodes are identified by HD•PET, while standard reconstruction shows one large extended node. Images courtesy of the University of Erlangen, Erlangen, Germany.

The technology also may benefit overweight and obese patients by providing uniformity across the entire field of view, thus helping physicians detect lesions at the periphery as well as the center of the field of view.

HD•PET is expected to benefit patients in multiple clinical scenarios. For example, HD•PET:
  • Improves image quality and visualization of small lesions, especially in cases with obese patients
  • Provides superior edge detection of lesions for improved region of interest delineation and improved accuracy of radiotherapy planning in lung cancer, pancreatic cancer and retroperitoneal metastases.
  • Improves delineation of doubtful small abdominal lesions
  • Improves visualization of small retroperitoneal lymph node metastases
  • Improves delineation of early axillary lymph node metastases in primary breast cancer and chest wall recurrences in post-surgery and post-radiation therapy patients in treated breast cancer patients referred for restaging
  • Clearly delineates doubtful hilar and mediastinal nodal metastases in lung cancer, especially in post-radiation therapy and post-surgery conditions associated with fibrosis, effusion and pneumonitis
  • Better identifies small early recurrences in head and neck malignancies, especially in post-operative and post-radiation patients with distorted anatomy
  • Improves identification of small primary or metastatic cerebral tumors and delineation of hypodense small epileptic foci
  • Improves visualization of small lesions in studies with non-FDG PET radiopharmaceuticals

The future of PET/CT imaging
HD•PET is available on all Siemens Biograph TruePoint PET/CT solutions and as an upgrade for current Biograph TruePoint systems. It optimizes image uniformity and spatial resolution with improvements in signal-to-noise ratio  to produce high-definition PET/CT images, which, in turn, may help physicians better identify lesions in a wide array of cancers. Earlier and improved detection also could translate into enhanced accuracy of radiation therapy planning.

HD Clarity = HD Uniformity + HD Resolution + HD Contrast HD Uniformity
Images are distortion-free throughout the entire field of view, from center to edges, enabling more accurate visualization of fine detail no matter where you look.

HD Resolution
Because images are reproduced with the highest level of detail for enhanced detectability, HD•PET allows visualization of lesions smaller than 3 millimeters.

HD Contrast
With an unprecedented 2X improvement in signal-to-noise ratio, HD•PET reveals sharper images, as well as greater distinctness within the image.

HD Clarity
Greater specificity and accuracy deliver crystal-clear results for more confident diagnoses, and earlier, more targeted treatment.

PET: The Innovation Chronicle HD•PET builds upon 10 years of PET/CT engineering research. The first PET scanning systems were plagued by incongruities between the initial dataset and the physician’s need for a viewable image. That is, when a PET scanner makes a measurement, it produces a dataset that is not viewable. The resulting dataset must be converted into a viewable image dataset.

In the mid-1990s, engineers solved the dilemma with a new technique — iterative reconstruction. Iterative reconstruction employs an algorithm to produce an image to compare against the original dataset. The image is progressively refined until the algorithm produces an image that correlates to the original image dataset. Although iterative reconstruction produced tremendous improvements in PET image quality, Siemens engineers realized that the technique required additional innovations before it could be deployed in clinical applications.  

Conventional PET (top) and HD•PET (bottom) images of a 66-year-old female show bony and pelvic lymph node metastases. Two separate nodes are identified by HD•PET, while standard reconstruction shows one large extended node. Images courtesy of the University of Erlangen, Erlangen, Germany.Hypermetabolic cervical lymph node metastases are visible within FDG avid brown fat. Sharper delineation of nodes is shown by the HD•PET images in the bottom row. Standard reconstruction shows increased uptake in the region without sharp delineation of individual nodes. The study was acquired on Siemens Biograph 64 at the University of Erlangen, Erlangen, Germany. Over the last decade, three developments emerged to make iterative reconstruction clinically feasible. The first development, the Ordered Set Expectation Maximization (OSEM) algorithm, simulates, projects and compares PET measurements to produce a clinical image. The algorithm repeats the image production loop until the final image correlates with the initial PET dataset. The next development was the discovery of a second algorithm — the Fourier rebinning algorithm. The Fourier algorithm converts a three-dimensional reconstruction into a 2D problem to deliver a faster reconstruction process. Finally, computer processing capabilities have advanced significantly. Improved processing not only further increases speed but also improves image quality by reducing image noise while maintaining the integrity of the actual initial dataset.

Over time, these three developments to PET have advanced the technology — but now, HD•PET promises to reinvent PET imaging. Now, with enhancements to computer speed, this enables even further improvements in image quality, such as improved uniformity, resolution and contrast in HD•PET for enhanced clarity.

At the first level, reducing the image noise and retaining real information produces more uniform resolution. HD Uniformity, in turn, yields a distortion-free image across the entire field of view, enabling physicians to accurately visualize fine details. Siemens new PET reconstruction processes reproduce images with the highest level of detail. This technology also delivers significant improvements in contrast. HD Contrast provides a 2X improvement in the signal-to-noise ratio, resulting in sharper, more distinct images. The final benefit is HD Clarity. That is, HD•PET increases specificity and accuracy of PET images to increase clinical confidence and provide physicians with the data needed to develop earlier, more targeted treatment.

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