Modulation transfer function assessment in parallel beam and fan beam collimators with square and cylindrical holes

Area of Science:

• Nuclear medicine physics
• Medical imaging technology
• Radiation detection and measurement

Background:

• Collimators are crucial components in SPECT imaging, influencing image quality by defining the field of view and reducing scattered radiation.
• Parallel beam (PB) and fan beam (FB) collimators are commonly used, each with distinct geometric properties affecting performance.
• Understanding the modulation transfer function (MTF) and scatter/penetration (S+P) components is essential for optimizing collimator design.

Purpose of the Study:

• To investigate and compare the modulation transfer function (MTF) of parallel beam (PB) and fan beam (FB) collimators.
• To evaluate the impact of different hole shapes (square and circular) on collimator performance, including full width at half maximum (FWHM) and scatter/penetration (S+P) components.
• To explore the feasibility of estimating fan beam (FB) collimator performance using parallel beam (PB) parameters.

Main Methods:

• Utilized the Monte Carlo method (MCNP5 code) to simulate collimator performance.
• Simulated radioactive sources in air and within a water phantom.
• Employed a lead-to-air ratio regulation for output data estimation and compared hole patterns between PB and FB collimators.

Main Results:

• Square holes in PB collimators (PBs) showed better FWHM than cylindrical PB collimators (PBc).
• Cylindrical holes in FB collimators (FBc) exhibited better FWHM than square FB collimators (FBs).
• FBc generally provided better resolution than PBc in both air and scatter conditions. S+P decreased with increasing distance (z) from the source. FBc had lower S+P than FBs, while PBc had higher S+P than PBs. FB collimators demonstrated higher MTF than PB collimators, particularly at increased spatial frequencies.

Conclusions:

• Fan beam collimators, especially those with cylindrical holes, generally outperform parallel beam collimators in terms of resolution and MTF.
• The choice of hole shape significantly impacts collimator performance, with cylindrical holes favoring FB and square holes favoring PB for FWHM.
• Estimating FB collimator characteristics using PB parameters and considering diverse hole shapes can aid in designing more efficient SPECT imaging systems.