PurposeThe modulation transfer function (MTF) and detective quantum efficiency (DQE) of x-ray detectors are key Fourier metrics of performance, valid only for linear and shift-invariant (LSI) systems and generally measured following IEC guidelines requiring the use of raw (unprocessed) image data. However, many detectors incorporate processing in the imaging chain that is difficult or impossible to disable, raising questions about the practical relevance of MTF and DQE testing. We investigate the impact of convolution-based embedded processing on MTF and DQE measurements.ApproachWe use an impulse-sampled notation, consistent with a cascaded-systems analysis in spatial and spatial-frequency domains to determine the impact of discrete convolution (DC) on measured MTF and DQE following IEC guidelines.ResultsWe show that digital systems remain LSI if we acknowledge both image pixel values and convolution kernels represent scaled Dirac δ-functions with an implied sinc convolution of image data. This enables use of the Fourier transform (FT) to determine impact on presampling MTF and DQE measurements.ConclusionsIt is concluded that: (i) the MTF of DC is always an unbounded cosine series; (ii) the slanted-edge method yields the true presampling MTF, even when using processed images, with processing appearing as an analytic filter with cosine-series MTF applied to raw presampling image data; (iii) the DQE is unaffected by discrete-convolution-based processing with a possible exception near zero-points in the presampling MTF; and (iv) the FT of the impulse-sampled notation is equivalent to the Z transform of image data.
The MTF and DQE of an x-ray detector are important metrics of performance. They are generally measured following IEC guidelines which requires the use of raw (unprocessed) images. However, many detectors now incorporate processing algorithms that are deeply embedded in the imaging chain and may be difficult or impossible to disable, blurring the line between processed and unprocessed image data, and questioning the relevance of MTF and DQE testing. This study develops a framework to represent the spectral performance of linear and shift-invariant digital-processing algorithms and examines their effects on MTF and DQE measurements with examples. Processing algorithms are represented as a cascade of operations where images are represented using an integrable impulse-sampled notation. This allows the use of Fourier transform theorems and relationships, which differ to a discrete Fourier transform notation, including a specific representation of signal and noise aliasing. It is concluded that: i) digital convolution of image data gives the same result, with the same aliasing artifacts, as a true convolution integral of presampling image data followed by sampling; ii) the slanted-edge method to measure MTF provides the presampling MTF even when processing algorithms operate on aliased image data; iii) the DQE is largely unaffected by LSI post processing, however spectral zero-crossings can suppress specific frequency content in both the MTF and DQE, and unsharp masking algorithms can decrease the DQE at low frequencies.
We demonstrate the feasibility of optical angiography on live mice using a new photoacoustic computed tomography
(PCT) scanner. The scanner uses a sparse array of discrete ultrasound detectors geometrically arranged to capture 128
simultaneous radial "projections" through a 25-mm-diameter volume of interest. Denser sets of interleaved radial
projections are acquired by rotating the sparse array continuously about its vertical axis during data acquisition. The
device has been designed specifically for imaging laboratory mice, which remain stationary during data collection.
Angiographic data are acquired at a rate of 1280 radial projections per second following a bolus injection of 2 mg/mL of
indocyanine green (ICG).
We have designed and built a prototype PCT (photoacoustic CT) scanner suitable for small animal imaging that acquires
a sparse set of 128 photoacoustic, radial "projections" uniformly distributed over the surface of a hemisphere in response
to optical absorption from a tunable, pulsed NIR (near-infrared) laser. Acquisition of a denser set of projections is
achieved by rotating the hemispherical array about its vertical axis and acquiring additional, interleaved projections.
Each detector in the array is a 3-mm diameter, piezo-composite with a center frequency of 5 MHz and 70% bandwidth.
Spatial resolution is < 300 μm and nearly isotropic, owing to the array geometry. Preliminary results acquired at half of
the allowable laser power and with no system optimizations show a low contrast sensitivity sufficient to detect a 350 nM
concentration of a NIR-absorbing organic dye embedded in 12.5 mm of soft tissue. This scanner design will allow our
group to take advantage of HYPR (HighlY constrained backPRojection) reconstruction techniques, which can
significantly improve temporal (or spectral) resolution, without sacrificing signal-to-noise or spatial resolution. We will
report how these accelerated reconstruction techniques can be implemented with this PCT scanner design. Using this
approach, we may be able to achieve 100-ms temporal resolution for dynamic studies throughout a 20-mm-diameter
imaging volume.
Severely premature infants are often at high risk of cerebral hemorrhage or ischemic injury due to their inability to properly regulate blood flow to the brain. If blood flow is too high, the infant is at risk of cerebral hemorrhage, while too little blood flow can result in ischemic injury. The purpose of this research is to design and develop a means of non-invasively measuring cerebral blood flow (CBF) with near infrared spectroscopy (NIRS). Such a device would greatly aid the diagnosis and monitoring of afflicted infants. Previous attempts to measure CBF with NIRS have achieved limited success. In this study we acquired high signal-to-noise NIR spectrum from 600 to 980 nm with a cooled CCD spectrometer. This spectrometer enables the differential path length factor (DPF) to be estimated with accuracy using a second derivative technique described by Matcher et al. The validity of our new approach is determined via direct comparison with a previously validated computed tomography (CT) method. Three newborn piglets were studied. CBF measurements were performed at various partial arterial CO2 tensions (PaCO2) using both the NIRS and CT methods. The results of the two methods correlate well with a relationship of CBFCT equals -4.30 + 1.05 CBFNIRS (r2 equals 0.96).
Stroke is the third leading cause of death in the United States. It is caused by ischemic injury to the brain, usually resulting from emboli from atherosclerotic plaques. The carotid bifurcation in humans is prone to atherosclerotic disease and is a site where emboli may originate. Currently, carotid stenoses are evaluated by non-invasive duplex Doppler ultrasound, with preoperative verification by intra-arterial angiography. We have developed a system that uses a color Doppler ultrasound imaging system to acquire in-vivo 3-D color Doppler images of the human carotid artery, with the aim of increasing the diagnostic accuracy of ultrasound and decreasing the use of angiography for verification. A clinical TL Ultramark 9 color Doppler ultrasound system was modified by mounting the hand-held ultrasound scan head on a motor-driven translation stage. The stage allows planar ultrasound images to be acquired over 45 mm along the neck between the clavicle and the mandible. A 3- D image is acquired by digitizing, in synchrony with the cardiac cycle, successive color ultrasound video images as the scan head is stepped along the neck. A complete volume set of 64 frames, comprising some 15 megabytes of data, requires approximately 2 minutes to acquire. The volume image is reformatted and displayed on a Sun 4/360 workstation equipped with a TAAC-1 graphics accelerator. The 3-D image may be manipulated in real time to yield the best view of blood flow in the bifurcation.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.