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S J Riederer

Publications and source records attributed to S J Riederer.

At least 127 records · Page 7Linked to original sources
PubMed1987

The contrast-to-noise in relaxation time, synthetic, and weighted-sum MR images.

The contrast-to-noise ratio (CNR) in three types of computed MR images is compared in a computer simulation. The original data consist of two spin-echo or two saturation-recovery images. Each pair of images is used to generate a relaxation time image, a synthetic image at arbitrary echo or repetition time, and an image which is a weighted sum of the original images. The CNR produced by these three methods is compared for signals spanning a wide range of relaxation times. In every comparison an optimally weighted sum produces the highest CNR that is statistically attainable. The CNR in the optimum synthetic image equals this bound only if contrast reversal does not occur in the original images. The CNR in relaxation time images is always less than the statistical bound and can be less than the CNR in the original images.

Brain Diseases↗
PubMed1987

Optimizing the precision in T1 relaxation estimation using limited flip angles.

In this article we describe the precision in the estimation of the spin-lattice relaxation time T1 from MRI signals acquired for various flip angles with the repetition time TR held constant. We review the estimation procedure itself and present a model for the propagation of noise in the signal into the calculated T1. This model is verified by both Monte Carlo simulations and experimental data taken on image phantoms. Based on this model, we find that for a given TR/T1 there exist two optimal flip angles that will minimize the uncertainty in the estimated T1. We also show how two optimal angles can be selected for a given range of TR/T1 values. In addition, T1 estimation using the two optimal angles can be comparable to or better than using multiple evenly spaced angles. Finally, in an initial comparison with the two-point saturation recovery method of calculating T1, results for equal total scanning time TRTot suggest that for T1 greater than 0.5 TRTot, the limited flip angle approach gives better T1 precision whereas for T1 less than 0.5 TRTot the saturation recovery approach is better.

Humans↗
PubMed1986

Instrumentation for rapid MR image synthesis.

MR image synthesis has previously been developed as a means of retrospectively optimizing contrast of arbitrary materials in MR images. The first step of this process is to form computed N(H), T1, and T2 images from source images acquired at a variety of echo delay and repetition times. The second step is to take these computed images, along with operator-selected timing parameters, and mathematically generate a synthesized image. Computation is carried out pixel by pixel according to the equation describing the chosen pulse sequence. This paper presents a study of design considerations for a digital image processor capable of rapidly performing the second step, the actual synthesis. In this work the computations inherent to image synthesis are identified, and the feasibility of performing them in high-speed hardware examined. An analysis of the imprecision due to bit-limited calculations shows that an error bound of 0.4% is possible with a 16-bit processor design. A method is described which uses a commercially available image processor by which images can be synthesized according to any of the standard pulse sequences in less than 600 ms.

Computers↗
PubMed1986

A modified saturation-recovery approximation for multiple spin-echo pulse sequences.

In equations describing multiple spin-echo pulse sequences, the T1 dependence is often approximated by a saturation-recovery expression, which is valid only when the repetition time TR is much greater than the time required for the echoes. A slight modification of this approximation greatly increases its accuracy for small TR values.

Humans↗
PubMed1986

Optimum acquisition times of two spin echoes for MR image synthesis.

Spin-echo images can be synthesized at arbitrary values of echo time TEs if two images are acquired at the same repetition time and two different echo times TE1 and TE2. Depending on the value of TEs, the noise in the synthetic images can either be greater or less than the acquisition noise. This note shows that if the time between the acquired echoes TE2 - TE1 is equal to T2, the noise level in the synthetic images is no larger than the acquisition noise for TEs greater than or equal to TE1. This is the lowest possible noise bound for two-echo acquisition. Also, the noise bound for images synthesized with O less than or equal to TEs less than or equal to TE1 is minimized by making TE1 as short as possible.

Biometry↗
PubMed1986

Pulse sequence extrapolation with MR image synthesis.

Previous reports have presented validation studies of magnetic resonance (MR) image synthesis in which multiple spin-echo (MSE) source data were used to generate spin-echo images for various echo times and repetition times (TRs). A new method-"pulse sequence extrapolation" -synthesizes images for pulse sequences different from that of the acquisition. MSE data acquired in a time equivalent to a TR of 2,000 msec can be used to generate inversion-recovery (IR) images for arbitrarily chosen TI inversion times. Other combinations of pulse sequences were also studied, and synthetic images were compared visually and quantitatively to directly acquired images with corresponding parameters. Synthetic IR signals of the brain parenchyma consistently matched directly acquired signals to within 6%, with respect to the full magnetization signal. The noise level of synthetic signals was generally no more than twice that of direct acquisition signals, as predicted. This method can achieve selective fat suppression and enhancement in IR imaging.

Biophysical Phenomena↗
PubMed1986

T2 estimates in healthy and diseased brain tissue: a comparison using various MR pulse sequences.

Fourteen patients and five healthy individuals underwent magnetic resonance (MR) imaging to determine an effective multiple spin echo pulse sequence for estimating T2. Lesions examined included infarction, glioma, multiple sclerosis, and acute hematoma. A pulse repetition time (TR) of 1,500 msec and echo delays (TEs) of 25, 50, 75, and 100 msec were used. Computed T2 images were derived from all four echoes, the first two echoes, and the first and fourth echoes. T2 values were obtained from specific brain locales using region-of-interest analysis. Use of either the first two echoes or the first and fourth in the T2 fit provided T2 estimates which closely correlated with that of the four-echo analysis. The noise level in T2 maps constructed from the 25- and 100-msec echoes was modestly (typically 10%) higher than that from four echoes; noise level from the 25- and 50-msec echoes was markedly higher, typically 60%. This behavior is remarkably consistent with that predicted from theory. All 19 subjects displayed consistent relative T2 values for specific brain structures; in 13, the absolute T2 values fell within a limited range. Despite the high sensitivity of T2 images, their specificity in the detection of most brain disease appears limited except in acute intracerebral hematoma, which exhibited a decreased T2 relaxation time using high-field-strength MR imaging.

Adolescent↗
PubMed1986

Magnetic resonance image synthesis. Clinical implementation.

Magnetic resonance (MR) image synthesis is a technique enabling the retrospective optimization of scanning parameters. This paper describes the methods used to enable the clinical implementation of this technique. It is shown that effective synthetic images can be generated from only three acquired images per slice. Data for 16 slices and TR times of 500 and 2000 ms can be efficiently acquired in a multi-slice multi-echo dual-TR pulse sequence. Computation of T1, T2, and density images and the subsequent synthesis of images for arbitrary TR and TE times can be performed at high speed with dedicated hardware. The method is seen as one of standardizing acquisition protocols and thereby improving patient throughput. Specific clinical applications are discussed.

Humans↗
PubMed1985

Synthesized MR images: comparison with acquired images.

Synthesized and directly acquired spin-echo images were compared in order to assess the validity of magnetic resonance (MR) image synthesis as a method enabling retrospective formation of images by interactive manipulation of scan parameters. Synthetic images subjectively compared favorably in both accuracy and precision with acquired images when formed for the same values of echo (TE) and repetition times (TR) and for interpolated and extrapolated values of both TE and TR. Plots of synthetic and acquired signals within the same pixel sectors quantitatively showed comparable values for several regions of interest in the brain. Percent error and noise-normalized differences between acquired and synthetic images were tested as a quantitative measure of accuracy. Percent error was consistently less than 5% for brain parenchyma, and synthetic signals were accurate to within four times the noise level at acquisition. The apparent signal-to-noise ratio of synthetic images was comparable, superior, or inferior to similar acquired images, depending on the values of TE and TR. Total acquisition time required for synthetic formation of images for arbitrary values of TE and TR was equivalent to that of a single direct acquisition with a TR of 2,500 msec.

Brain↗
PubMed1985

Digital radiography.

This paper is a review of digital radiography, the application of digital electronic technology to medical X-ray imaging. At some stage in the formation of an image the detected X-ray fluence is converted into a digital format for processing and display. In many X-ray procedures digital methods offer considerable advantages over previously used techniques in speed, precision, flexibility, sophistication of processing, or other characteristics. These in turn can lead to reductions in degree of invasiveness, X-ray exposure, or contrast material dose or certain other advantages such as reduced time per X-ray procedure. The major digital X-ray imaging techniques are discussed, ranging from contrast enhancement to image subtraction schemes to more recently developed methods such as temporal filtering of image sequences at realtime video rates. Digital imaging systems are categorized according to detector geometry, and the relative merits of each are discussed. Examples of clinical applications are given.

Analog-Digital Conversion↗
PubMed1984

Automated MR image synthesis: feasibility studies.

The authors describe an automated technique of magnetic resonance (MR) image synthesis. Given a specific pulse sequence, MR signals are acquired for several pulse delay and/or repetition times and used to compute images of intrinsic parameters T1, T2, and N(H). Both the computed images and operator-specified pulse delay and repetition times are then used to "synthesize" a new image based on equations descriptive of MR signal behavior and comparable to that acquired by using the operator-specified parameters in an actual MR study. Instrumentation enabling rapid operator-interactive generation of synthesized images is described and initial results presented, allowing for dependence of the signal on T2 in spin echo images. Extension to full T1, T2, and N(H) dependence for arbitrary pulse sequences is described. Major advantages of this technique include retrospective optimization of contrast between arbitrary materials, rapid and systematic image analysis, and reduced scanning time; potential limitations include accuracy, noise, motion artifacts, and multicomponent behavior.

Computers↗
PubMed1983

The application of matched filtering to x-ray exposure reduction in digital subtraction angiography: clinical results.

This new digital fluorography processing method of matched filtering generates a set of images that have been acquired at continuous video-frame rates (30 per second) over the temporal extent of the bolus (10 seconds), and it combines them to produce the equivalent of a single digital subtraction angiography (DSA) image. Because of the extensive temporal averaging used, the method can provide substantial x-ray exposure reduction per run as compared with conventional techniques for an equivalent signal-to-noise ratio in the final image. The matched filtering technique was compared with conventional pulsed (one per second) DSA images of both extracranial and intracranial arteries, and the results are presented. Matched filtered images provided image quality that was equivalent to that of conventional DSA images at about one-fourth the patient exposure per run for both carotid artery and cerebral vessel studies. Despite long integration times, patient motion irretrievably corrupted image quality in only two of five carotid artery studies and in none of three intracranial studies. Compensation for patient motion is demonstrated, and additional applications and limitations of the technique are discussed.

Angiography↗
PubMed1983

Low-dose, high-frame-rate versus regular-dose, low-frame-rate digital subtraction angiography.

Using intravenous digital subtraction angiography for carotid, cerebral, and renal arteries, a comparison was made between a single pulsed image and an integrated image composed of several pulsed images. The single image was generated with a conventional detected dose per frame while the frames composing the integrated image were generated at a lower dose per frame but at a higher frame rate. Comparisons were made for an equivalent detected dose per image. For relatively stationary arteries, the integrated images were equivalent or superior to the single images. When patient motion was present, or if pulsatile vessel motion was significant, the single image was better than the integrated image. A low-dose, high-frame-rate technique has the potential for furnishing physiologic data in addition to high-quality morphologic information.

Angiography↗
PubMed1983

Evaluation of peripheral vascular disease using digital subtraction angiography.

Intravenous digital subtraction angiography (DSA) facilitates imaging of peripheral arteries and bypass grafts on an outpatient basis. Forty-five patients underwent intravenous peripheral angiography, 16 following placement of bypass grafts. Complex oblique and sagittal angulations optimize visualization of bifurcations and overlapping vascular structures. Reprocessing techniques using integrated masks and contrast images allow for diagnostic delineation of fine vascular detail. This is a practical, noninvasive, well tolerated method for both diagnostic screening and follow-up evaluation of patients with peripheral vascular disease.

Angiography↗
PubMed1983

Intravenous digital subtraction: a summary of recent developments.

Recent developments in digital subtraction angiographic imaging are discussed. The technical characteristics of several alternate subtraction methods are reviewed, including integrated remasking, matched filtering, recursive filtering, hybrid subtraction, and tomographic digital subtraction angiography (DSA). The properties that each exhibits are compared with conventional DSA imaging, and the major expected limitations and applications of each technique are discussed.

Angiography↗
PubMed1983

The application of matched filtering to contrast material dose reduction in digital subtraction angiography.

Temporal filtering methods were applied to a 50% reduction of contrast material dose in intravenous digital subtraction angiography (DSA). The performance of matched filtering was reviewed, indicating a 50% reduction of noise through the integration of an entire (greater than 10 sec) DSA image sequence spanning the contrast material bolus arrival and washout. Used in conjunction with 50% contrast material dose reduction, matched filtering could provide final iodine signal-to-noise ratios (SNR) comparable to those of conventional DSA. Half-contrast-material-dose (20 ml) filtered DSA runs were compared with full-contrast-material-dose (40 ml) unfiltered runs for 5 carotid, 2 cerebral, and 4 peripheral studies. In all but 2 studies, the half-contrast-material-dose runs were judged to be diagnostic. In all 4 peripheral studies and in some of the 7 carotid and cerebral studies the 20-ml results were judged to be equivalent to the 40-ml results. It is concluded that the half-contrast-material-dose temporally filtered approach is appropriate for peripheral runoff studies. Matched filtering was experimentally compared with simple integrated remasking. If the identical image sequence was used for each, the two had virtually identical performance for integration times less than 10 sec, consistent with theoretical predictions. Two other applications of matched filtering, SNR enhancement and x-ray exposure reduction, are discussed schematically.

Angiography↗
PubMed1983

Work in progress: the application of temporal filtering techniques to hybrid subtraction in digital subtraction angiography.

Temporal filtering methods were applied to iodine signal-to-noise ratio (SNR) restoration in intravenous hybrid subtraction digital subtraction angiography (DSA). For equal detected exposure rates hybrid subtraction had approximately 35% of the SNR of temporal subtraction. When matched filtering was applied to a DSA run, the filtered result had approximately two times higher SNR than the peak contrast image in the run. Thus, when matched filtering techniques were applied to the hybrid image sequence, the resultant SNR increased to about 70% of that of temporal subtraction. With an additional factor-of-two increase in exposure rate for the hybrid run, SNR parity with temporal subtraction could be achieved. This compared with a factor-of-nine increase in exposure that would be required if no filtering were performed. Experimental hybrid matched filter results, generated with intravenous canine DSA studies, supported the predictions in SNR performance.

Angiography↗