Focal spot size is one of the crucial factors that affect the image quality of any x-ray imaging system. It is, therefore,
important to measure the focal spot size accurately. In the past, pinhole and slit measurements of x-ray focal spots were
obtained using direct exposure film. At present, digital detectors are replacing film in medical imaging so that, although
focal spot measurements can be made quickly with such detectors, one must be careful to account for the generally
poorer spatial resolution of the detector and the limited usable magnification. For this study, the focal spots of a
diagnostic x-ray tube were measured with a 10-μm pinhole using a 194-μm pixel flat panel detector (FPD). The twodimensional
MTF, measured with the Noise Response (NR) Method was used for the correction for the detector blurring.
The resulting focal spot sizes based on the FWTM (Full Width at Tenth Maxima) were compared with those obtained
with a very high resolution detector with 8-μm pixels. This study demonstrates the possible effect of detector blurring on
the focal spot size measurements with digital detectors with poor resolution and the improvement obtained by
deconvolution. Additionally, using the NR method for measuring the two-dimensional MTF, any non-isotropies in
detector resolution can be accurately corrected for, enabling routine measurement of non-isotropic x-ray focal spots. This
work presents a simple, accurate and quick quality assurance procedure for measurements of both digital detector
properties and x-ray focal spot size and distribution in modern x-ray imaging systems.
We demonstrate a novel approach for achieving patient dose savings during image-guided neurovascular
interventions, involving a combination of a material x-ray region of interest (ROI) attenuator and a spatially different
ROI temporal filtering technique. The part of the image under the attenuator is reduced in dose but noisy and less bright
due to fewer x-ray quanta reaching the detector, as compared to the non-attenuating (or less attenuating) region. First the
brightness is equalized throughout the image by post processing and then a temporal filter with higher weights is applied
to the high attenuating region to reduce the noise, at the cost of increased lag; however, in the regions where less
attenuation is present, a lower temporal weight is needed and is applied to preserve temporal resolution.
A simulation of the technique is first presented on an actual image sequence obtained from an endovascular
image guided interventional (EIGI) procedure. Then the actual implementation of the technique with a physical ROI
attenuator is presented. Quantitative analysis including noise analysis and integral dose calculations are presented to
validate the proposed technique.
Due to the need for high-resolution angiographic and interventional vascular imaging, a Micro-Angiographic
Fluoroscope (MAF) detector with a Control, Acquisition, Processing, and Image Display System (CAPIDS) was
installed on a detector changer, which was attached to the C-arm of a clinical angiographic unit at a local hospital. The
MAF detector provides high-resolution, high-sensitivity, and
real-time imaging capabilities and consists of a 300 μm thick
CsI phosphor, a dual stage micro-channel plate light image intensifier (LII) coupled to a fiber optic taper (FOT),
and a scientific grade frame-transfer CCD camera, providing an image matrix of 1024×1024 35 μm effective square
pixels with 12 bit depth. The changer allows the MAF
region-of-interest (ROI) detector to be inserted in front of the
Image Intensifier (II) when higher resolution is needed during angiographic or interventional vascular imaging
procedures, e.g. endovascular stent deployment. The CAPIDS was developed and implemented using Laboratory
Virtual Instrumentation Engineering Workbench (LabVIEW) software and provides a user-friendly interface that enables
control of several clinical radiographic imaging modes of the MAF including: fluoroscopy, roadmapping, radiography,
and digital-subtraction-angiography (DSA). The total system has been used for image guidance during endovascular
image-guided interventions (EIGI) for diagnosing and treating artery stenoses and aneurysms using self-expanding
endovascular stents and coils in fifteen patient cases, which have demonstrated benefits of using the ROI detector. The
visualization of the fine detail of the endovascular devices and the vessels generally gave the clinicians confidence on
performing neurovascular interventions and in some instances contributed to improved interventions.
Region-of-interest (ROI) fluoroscopy takes advantage of the fact that most neurovascular interventional activity is
performed in only a small portion of an x-ray imaging field of view (FOV). The ROI beam filter is an attenuating
material that reduces patient dose in the area peripheral to the object of interest. This project explores a method of
moving the beam-attenuator aperture with the object of interest such that it always remains in the ROI. In this study, the
ROI attenuator, which reduces the dose by 80% in the peripheral region, is mounted on a linear stage placed near the xray
tube. Fluoroscopy is performed using the Microangiographic Fluoroscope (MAF) which is a high-resolution, CCD-based
x-ray detector. A stainless-steel stent is selected as the object of interest, and is moved across the FOV and
localized using an object-detection algorithm available in the IMAQ Vision package of LabVIEW. The ROI is moved to
follow the stent motion. The pixel intensities are equalized in both FOV regions and an adaptive temporal filter
dependent on the motion of the object of interest is implemented inside the ROI. With a temporal filter weight of 5% for
the current image in the peripheral region, the SNR measured is 47.8. The weights inside the ROI vary between 10% and
33% with a measured SNR of 57.9 and 35.3 when the object is stationary and moving, respectively. This method allows
patient dose reduction as well as maintenance of superior image quality in the ROI while tracking the object.
KEYWORDS: Point spread functions, Positron emission tomography, Sensors, Scanners, 3D modeling, Monte Carlo methods, Animal model studies, Reconstruction algorithms, Manufacturing, Data modeling
We previously designed a component based 3-D PSF model to obtain a compact yet accurate system matrix for a
dedicated human brain PET scanner. In this work, we adapted the model to a small animal PET scanner. Based on the
model, we derived the system matrix for back-to-back gamma source in air, fluorine-18 and iodine-124 source in water
by Monte Carlo simulation. The characteristics of the PSF model were evaluated and the performance of the newly
derived system matrix was assessed by comparing its reconstructed images with the established reconstruction program
provided on the animal PET scanner. The new system matrix showed strong PSF dependency on the line-of-response
(LOR) incident angle and LOR depth. This confirmed the validity of the two components selected for the model. The
effect of positron range on the system matrix was observed by comparing the PSFs of different isotopes. A simulated
and an experimental hot-rod phantom study showed that the reconstruction with the proposed system matrix achieved
better resolution recovery as compared to the algorithm provided by the manufacturer. Quantitative evaluation also
showed better convergence to the expected contrast value at similar noise level. In conclusion, it has been shown that the
system matrix derivation method is applicable to the animal PET system studied, suggesting that the method may be
used for other PET systems and different isotope applications.
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