Wing K. Luk

Local threshold for segmented attenuation correction of PET imaging of the thorax

Local threshold for segmented attenuation correction technique has been developed for positron emission tomography using short (2-3 minutes) post-injection transmission scans. The technique implements an optimal threshold method on localized histograms to get pseudo-anatomic segmentation on transmission images. Theoretical values of attenuation coefficient are assigned to correspondent anatomic regions. Emission images are reconstructed using attenuation correction factors computed by forward-projecting segmented transmission images. Phantoms and clinical cardiac images are studied using this technique. The technique has been found linearly correlated with the standard method (15 minutes, pre-injection transmission scans), reducing noise in the final emission Images, eliminating process for substraction emission data from transmission scan, cutting down scan time and increasing patient throughput. >

Adaptive, segmented attenuation correction for whole-body PET imaging

A technique is presented for segmented attenuation correction in positron emission tomography (PET) based on the local thresholding technique (LTS) described previously. To accommodate the substantially different body sections encountered in whole-body PET, an adaptive thresholding has been added to yield more uniform results throughout the body. By evaluating the intensity distribution of a set of transverse transmission images, the algorithm determines an optimal threshold for separating two or three different groups or classes of similar pixels. Interclass variance is maximized and intra-class errors are minimized. The algorithm also switches automatically between a three-class mode (background, lungs/air pockets, soft tissue) and a two-class mode (background, soft tissue), thereby achieving more uniform segmentation where lung spaces and bowel air pockets are alternately present then absent from the volume of interest. The addition of adaptive thresholding virtually eliminates the need for operator intervention. The clinical implementation requires short-duration, count-limited, transmission images that would otherwise be too noisy for direct attenuation correction. Emission images corrected with adaptive LTS were shown to be equivalent, both quantitatively and qualitatively, to those corrected using conventional measured attenuation correction.

Characterization of sampling schemes for whole body PET imaging

Whole body PET images suffer from relatively high noise levels due to inherently poor counting statistics in the emission data. Optimization of acquisition parameters is essential, to minimize any additional noise contamination. It has previously been shown that by using a continuous or redundant axial sampling scheme, a reduction in statistical noise and improvement in image quality are possible. In this work, the continuous axial sampling technique is further characterized and compared to the conventional step-and-shoot technique. The main source of additional noise contamination with conventional sampling is the detector sensitivity normalization procedure which is applied to the emission data. With continuous axial sampling and a single normalization matrix for all planes, the statistical noise in the normalization factors is reduced by factor close to the number of planes in the scanner. The continuous sampling technique is shown to be less sensitive to small patient movements ( >

Optimizing rod window width in positron emission tomography

A technique that determines optimal rod window width for rotating rod transmission studies in positron emission tomography (PET) is discussed. Rod windowing reduces noise in rotating rod transmission studies. Lines-of-response (LOR) which intersect the rods generate primarily true coincidence events. LORs which pass far from the rods generate random and scatter events. Since the angular position of the rotating rods is known in real-time, LORs, which produce mostly noise are gated off.<<ETX>>

Beyond list mode: On-line rebinning and histogramming for continuous bed motion in clinical whole-body TOF PET/CT

In this article, methods are described which help move the concept of continuous bed motion (CBM) for TOF PET (i.e. for PET/CT applications) out of the realm of list-mode-only research and into everyday clinical usage. Long proposed for PET, CBM moves the patient horizontally (at a more or less steady rate) through the PET FOV during the acquisition and offers several points of advantage over the more common patient-held-stationary-to-PET-FOV approach. One primary obstacle to frequent clinical application of CBM in PET has been the inability to provide on-line, real-time detector-pair-to-projection-space-bin-address rebinning calculations along with the associated on-line histogramming. Here are presented details for one approach to overcome just such obstacles — significantly allowing the on-line generation of single TOF projection data sets which may each represent an entire whole-body scan. A set of 5 real list-mode data files were collected from a TOF PET/CT — a system which has been outfitted with a patient handling system (PHS or “bed”) which is shown to be largely sufficient for CBM in PET. For an F-18 point source — placed for first a 1cm and then a 10cm transaxial offset, list-mode data was collected for both stationary and horizontal bed motion cases. In addition, a CBM list-mode data set was collected for a custom, F-18, 100cm-long, cylindrical phantom. These data sets were processed in a manner computationally equivalent to that proposed for the realtime, on-line processing case. By comparing stationary to CBM image quality, the resulting analysis strongly suggests these proposed methods will provide real-time CBM processing which is compatible with the needs of clinical-grade TOF PET. As perhaps expected from CBM, the Z-axis FWHM is shown to generally improve over the stationary case via finer axial sampling in the rebinning step. As may be atypical, one specific data point showed a Z-axis improvement of 6.5%. Other examples — which may be more typical — showed little or no Z-axis FWHM improvement when applying CBM.

Adaptive segmented attenuation correction for whole-body PET imaging

Adaptive segmented attenuation correction using the local thresholding technique (adaptive LTS) has been developed for whole body PET imaging using short (2-3 minutes), post-injection transmission scans. Optimal threshold is derived at every scan position to provide appropriate segmentation on the transmission images, which are forward-projected into the attenuation sinograms. The entire computation of the new attenuation sinograms of a single bed position (47 slices) on ECAT EXACT takes as little as 6 minutes. So far, the technique has found been very successful in 30 /sup 18/FDG whole-body oncologic studies without user interaction. The resulting emission data quantitatively matches and qualitatively surpasses those using the standard method. This method can increase scanner throughput without sacrificing the quality and accuracy of whole-body imaging.<<ETX>>

Effects of breathing motion on PET acquisitions: Step and shoot versus continuous bed motion

Objectives Continuous bed motion (CBM) acquisition recently became available in whole-body PET/CT scanners in addition to the conventional step and shoot (S&S) acquisition. In this work, we compared the image quality between these acquisition methods using a phantom simulating periodic motion to mimic motion from patient breathing in a controlled manner. Methods PET image quality was assessed using the National Electrical Manufacturers Association IQ torso phantom filled with an 18F-FDG solution using a 4 : 1 target-to-background ratio. The phantom was scanned in two states: no motion (stationary) and with periodic motion in the axial direction with a displacement ±10 mm from the initial position. Both S&S and CBM scans were repeated 10 times in an alternating order, whereby the acquisition duration of each scan was adjusted to make the true counts approximately comparable to compensate for the decaying 18F-FDG. Results The recovery coefficient analysis showed that in the stationary state, the 10 mm sphere recovery did not show any difference between S&S and CBM. With motion, the recovery coefficient was lower by ∼40% for both modes of acquisition. In addition, the image-based volume analysis of the 10 mm sphere showed 1.67 (1.57–1.69) cm3 for S&S and 1.73 (1.66–1.83) cm3 for CBM (P=0.13), and there was no difference between two modes. Our study indicated that when the acquisition conditions for S&S and CBM (equivalent net trues, identical phantom motion, and identical CT image used for PET corrections) were controlled carefully, these acquisition modes resulted in equivalent image quality.

Summing of dynamic sinograms

There have been recent growing interests in dynamic FDG whole-body imaging. This current effort was initiated to facilitate and support this emergent trend. Summing dynamic sinograms to create a corresponding static frame may open the possibility of obtaining both dynamic and static data in one scan.

Continuous bed motion in TOF PET: Finer planar sampling and axial image resolution

The impact upon axial image resolution for the TOFPET portion of TOF-PET/CT (time-of-flight positron emission tomography / computed tomography) via continuous bed motion (CBM) due to finer axial/planar sampling in the projection space is examined. We try a 1.6x finer axial sampling pitch in the rebinning step (0.125cm instead of 0.2cm) and show a (slight) 6.5% improvement in full-width-half-maximum axial image resolution (0.447cm instead of 0.448cm) as a result. A 21M event count CBM TOF-PET list-mode study of an F-18 point source placed 1cm radially off from field-of-view (FOV) center was performed on a Siemens 4-Ring Biograph mCT Flow. For this single TOF-PET study, the raw, 64-bit detector-pair coincidence-event list-mode data was recorded. Also for this study, the bed moved horizontally at a constant speed of 1mm/sec with a horizontal bed position reporting (i.e. list-mode tag packets) resolution of 0.05cm. Due to a post-acquisition processing step which added bed position tag packets to the list-mode file, the reported resolution for bed position was effectively extended to 0.001cm via simple linear interpolation. It is noted that this 1.6x finer pitch sampling results in correspondingly larger projection-space data sets - data sets which take longer to reconstruct. It is also noted that such high-counting statistics for each small region of interest in the PET FOV are typically impractical and unobtainable in a normal clinical setting. The significance for clinical use is discussed.

Tracking coincidence events in pet even when count rates are extremely high: The Lost-Event Tally packet concept

We describe techniques that extend the usefulness of real-time data-handling architectures designed for clinical positron emission tomography (PET)-especially for instances of extremely high (>; 10 M events/s) count rate. As is widely known, Rubidium-82 (82Rb) with a 1.3-min half-life is often used in clinical PET. When used, 82Rb is more often applied for dynamic and/or gated studies-typically with little or no delay between tracer injection and start of acquisition. The use of 82Rb for short-duration-framed studies in clinical PET has its own set of challenges. For example, a “too large” dose may temporarily exceed the acquisition throughput. When such saturation occurs, coincidence events are lost. In the case of 82Rb, such loss is typically short-lived and limited to the count-rate peak. Such saturation also leads to a truncation of the count-rate profile-a truncation that often limits curve fitting essential for an accurate compartmental model across the entire study. The techniques offered here help to resolve this issue. By electronically keeping track of those events that are subject to saturation loss in the acquisition channel, the complete count-rate profile itself is preserved. Such tracking is done in real time with an accurate log of loss quantities added to the list-mode stream. This approach enables the clinician to raise the 82Rb dose without a specific concern over count-rate profile truncation. We describe a special 64-bit nonevent (i.e., “tag”) packet that is automatically inserted into the list-mode stream. The “Lost-Event Tally” tag packet stream enables a full recovery of the rate profile even when event packets are lost. During acquisition, this tag packet is repeatedly inserted into the stream to report the counted event-packet loss (up to 1 048 575) since the previous such tag packet was inserted.