提高PET扫描器空间分辨率的采样技术研究

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本文档深入探讨了"泰森多边形在PET扫描器中的应用——提高空间分辨率的采样技术"。PET（正电子发射断层成像）是医学影像领域的重要工具，其性能的提升在很大程度上依赖于探测器的改进，特别是在空间分辨率方面。传统的PET探测器设计中，小型晶体和光共享技术被采用，以实现更高的灵敏度和图像质量。研究者们针对像素化探测器设计，提出了两种关键的采样技术来改善重建的空间分辨率。首先，"inter-crystal positioning technique"（晶间定位技术）通过在晶体洪水图（表示探测器内所有晶体对事件的响应）中进行子采样，更好地捕捉到散射事件。这种技术旨在减少因晶体间的相对位置不确定性导致的分辨率损失，通过对事件分布进行更精确的模拟和测量，优化重建过程。另一种技术是"Compton scatter rejection technique"（康普顿散射拒绝技术），它通过分析洪水图中事件与晶体中心的距离，识别并剔除那些远离晶体质心的事件，从而降低由康普顿散射带来的伪影，进一步提高空间分辨率。康普顿散射是PET成像中的主要干扰源，因为它可能导致能量损失和位置模糊。为了验证这些技术的有效性，研究者进行了大规模的蒙特卡洛模拟，模拟了使用不同晶体材料（如LaBr3和LYSO，其能量阈值范围在46.9至63keV之间）的全身体检PET扫描器。实际的实验数据测量也同时进行，这包括对来自点源的数据进行处理和分析，以评估新技术对真实成像性能的影响。这篇论文为提高PET扫描器的空间分辨率提供了一种创新的方法，通过结合理论模拟和实际测试，展示了采样技术在优化探测器性能方面的潜力。这对于整个医疗成像领域的进步具有重要意义，特别是在肿瘤检测和定量分析中，更精确的图像有助于提高诊断准确性和治疗决策。

596 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009

Improved Spatial Resolution in PET Scanners

Using Sampling Techniques

Suleman Surti, Senior Member, IEEE, Ryan Scheuermann, Matthew E. Werner, and

Joel S. Karp, Senior Member, IEEE

Abstract—Increased focus towards improved detector spatial

resolution in PET has led to the use of smaller crystals in some

form of light sharing detector design. In this work we evaluate two

sampling techniques that can be applied during calibrations for

pixelated detector designs in order to improve the reconstructed

spatial resolution. The inter-crystal positioning technique uti-

lizes sub-sampling in the crystal ﬂood map to better sample the

Compton scatter events in the detector. The Compton scatter

rejection technique, on the other hand, rejects those events that

are located further from individual crystal centers in the ﬂood

map. We performed Monte Carlo simulations followed by mea-

surements on two whole-body scanners for point source data.

The simulations and measurements were performed for scanners

using scintillators with

e

ranging from 46.9 to 63 for LaBr

and LYSO, respectively. Our results show that near the center

of the scanner, inter-crystal positioning technique leads to a gain

of about 0.5-mm in reconstructed spatial resolution (FWHM)

for both scanner designs. In a small animal LYSO scanner the

resolution improves from 1.9-mm to 1.6-mm with the inter-crystal

technique. The Compton scatter rejection technique shows higher

gains in spatial resolution but at the cost of reduction in scanner

sensitivity. The inter-crystal positioning technique represents a

modest acquisition software modiﬁcation for an improvement

in spatial resolution, but at a cost of potentially longer data cor-

rection and reconstruction times. The Compton scatter rejection

technique, while also requiring a modest acquisition software

change with no increased data correction and reconstruction

times, will be useful in applications where the scanner sensitivity

is very high and larger improvements in spatial resolution are

desirable.

Index Terms—PET, pixelated detector, positioning, sampling,

spatial resolution.

I. INTRODUCTION

HE intrinsic physics limitations on reconstructed spatial

resolution in a PET scanner are determined by the positron

range which is a function of the positron energy [1], and the

non-collinearity of the two annihilation photons [2], [3]. For the

most common radio-isotope used in routine clinical imaging,

F, the positron range leads to a degradation of about 0.5-mm in

the system spatial resolution. Besides these two intrinsic physics

Manuscript received February 06, 2008; revised April 08, 2008. Current ver-

sion published June 10, 2009. This work was supported by the National Insti-

tutes of Health under Grants R33-EB001684 and R01-CA113941

S. Surti, M. E. Werner, and J. S. Karp are with the Department of

Radiology, University of Pennsylvania, Philadelphia, PA 19104 USA

(e-mail: surti@mail.med.upenn.edu; joelkarp@mail.med.upenn.edu;

matt.werner@uphs.upenn.edu).

R. Scheuermann is with the Department of Physics, University of Pennsyl-

vania, Philadelphia, PA 19104 USA (e-mail: rscheuermann@xrt.upenn.edu).

Digital Object Identiﬁer 10.1109/TNS.2009.2013389

effects, a third factor in the system spatial resolution of a PET

scanner is the impact of the detector in localizing the inter-

actions of the annihilation photons within it. In recent years

there has been an increased focus in PET detector design to-

wards the use of small cross-section crystals in some form of a

light-sharing detector. Theoretically, assuming very high scin-

tillation photon statistics, the intrinsic resolution due to detector

effects is d/2 for a crystal of cross-section size, d. However,

the discrete (under) sampling due to ﬁxed-size pixilated de-

tectors, leads to a convolution of the two effects [4]. The de-

tector sampling effect can be minimized with continuous scin-

tillation detectors that have very ﬁne sampling within the de-

tector, and hence the detector resolution in this case is deter-

mined only by the uncertainty in positioning of the event within

the detector. This uncertainty is determined by the amount of

Compton scatter in the detector as well as the scintillation light

statistics. In order to improve the sampling in pixelated detec-

tors and achieve results closer to the theoretical estimates, re-

searchers in the past developed techniques such as “wobbling”

which involves a mechanical movement of the detector ring

while performing patient scans [5]–[7]. Our aim in this work is

to investigate two techniques that can be applied to the calibra-

tions performed in a pixelated detector design and potentially

lead to improved system spatial resolution in a PET scanner.

II. C

ALIBRATION TECHNIQUES FOR

POSITIONING EVENTS IN

ARRAYS OF SMALL

CRYSALS

In modern PET scanners based on pixelated detectors a

common technique is to use Anger logic positioning with en-

coding of the crystal to a PMT [8]–[10]. As a calibration step,

boundaries are drawn between all individual crystals in a ﬂood

map and a look-up table generated which assigns all events

within a region (region-of-interest, ROI) to the center position

of that crystal [see Fig. 1(a) and (b)]. The center position is

the real physical position of that crystal on the scanner surface.

Fig. 1(b) shows schematically the crystal space partitioned into

ROIs where all points within each ROI are located closest to

that ROI peak.

We have performed simulations for a pixelated detector with

-mm LaBr crystals. For these simulations, only

the

-ray interactions were tracked and the interaction position

calculated as an energy-weighted mean of the

-ray interaction

points (total deposited energy

keV). The -ray entry

points were uniformly distributed over a single crystal surface

area (4

4-mm ). The energy weighted interaction position of

the

-rays was mapped onto a 2D array of ROIs, each of which

was initially 4

4-mm in area. The measured position of each

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提高PET扫描器空间分辨率的采样技术研究

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