Gary Sylvester Keyes

Intracranial intravenous digital subtraction angiography.

SummaryIntravenous digital subtraction angiography (iDSA) promises to significantly alter the use of conventional cerebral angiography in the workup of neurological patients. Understanding its diagnostic potential and its limitations are important in incorporating this new examination into the diagnostic thought process of neuroradiologic tests. Different image processing techniques such as integration of mask and contrast images promise to improve image quality for neuroradiologic application. At present, iDSA is suitable for the diagnosis and follow-up of vascular lesions (atherosclerosis, aneurysms, arteriovenous malformations, venous sinus occlusion), and tumor (meningioma). Although limited, the spatial resolution of iDSA studies is capable of demonstrating diffuse vascular disease such as arteritis and vasospasm after subarachnoid hemorrhage. In some patients in conjunction with the CT scan, iDSA may prove sufficient as the primary and only diagnostic angiographic test necessary, supplanting conventional angiography.

IODINE SENSITIVITY OF DIGITAL IMAGING SYSTEMS.

One of the most successful applications of digital radiography systems thusfar has been the imaging of vasculature opacified by intravenously administered contrast agents using temporal subtraction. Because the contrast in the arterial structures of interest is small as a consequence of the dilution due to the intravenous injection, it is essential that such imaging systems have high contrast sensitivity. This paper discusses means to assess the contrast sensitivity of imaging systems using iodinated phantoms and temporal subtraction. A difference image is shown demonstrating a 1 mm vessel containing 0.5 mg/cm2 of iodine, i.e. less than 1% radiographic contrast. Initial CDD results are presented. Dependence of CDD analysis on video signal level is demonstrated. The effects of x-ray scatter and image intensifier veiling glare are predicted and measured experimentally. Contrast levels in two digital fluorographic clinical procedures are estimated.

Clinical application of hybrid subtraction digital angiography: Preliminary results

The clinical application of hybrid subtraction in digital fluoroscopy of the vasculature is reported in our first 30 patients studied. Hybrid subtraction combines the advantages of temporal and dual energy subtraction techniques to achieve simultaneous elimination of overlying bone, soft tissue, and motion induced artifacts. Hybrid subtraction improved the subjective appearance of an image in 19 of 30 (63%) studies but additional diagnostic information was only revealed in 11 of 30 (37%) patients.

A QUANTITATIVE IN VIVO COMPARISON OF SIX CONTRAST AGENTS BY DIGITAL SUBTRACTION ANGIOGRAPHY

Burbank FH, Brody WR, Hall A, Keyes G. A quantitative in vivo comparison of six contrast agents by digital subtraction angiography. Invest Radio! 1982; 17:610–616. Digital subtraction angiography (DSA) technology can now visualize many significant arterial structures from intravenous injections of contrast media. Image quality of these DSA studies is related to contrast agent enhancement. This study compares contrast agents of differing iodine concentration, viscosity, and osmolarity. A technique is described that utilizes a scanned projection digital radiographic system to compare quantitatively degrees of intra-arterial opacification after the administration of six intravenous contrast agents: iothalamate (at four different concentrations and in combinations with two different cations), diatrizoate, and ioxaglate. The quantitative arterial enhancement was compared in dogs utilizing an extra- period latin-square multiple change-over clinical trial design. The contrast agents span a range of organically bound iodine from 282 mg I/ml to 480 mg I/ml. When rate and volume of contrast agent injected are held constant, intra-arterial opacification is directly a function of the iodine concentration (mg I/ml) of the agent injected, while osmolarity and viscosity have no effect on opacification. These studies support the use of agents with high iodine concentration for maximum vascular visualization.

Performance Characteristics Of A Digital Fluorographic System

Several technical characteristics of digital fluorography are discussed. A derivation is made for the noise levels in the acquisition of digital fluorographic video images. The importance of high video signal-to-noise ratio is demonstrated and discussed. A noise analysis is made for reprocessing amplified difference images from analog video disk. It is shown that under the most stringent circumstances a disk with a signal-to-noise ratio of 200:1 increases the rms noise by at most 7%. Potential artifacts associated with the measurement of iodine sensitivity for digital radiographic systems are discussed. A phantom is presented which eliminates such artifacts. Experimental digital fluorographic phantom images are shown.

Hybrid Subtraction In Digital Fluorography

Digital fluorography using temporal subtraction techniques has achieved excellent visualization of the vascular anatomy with intravenous injections of contrast medium. However, clinical studies have demonstrated that a major limitation to this method is patient motion. Movements such as swallowing, breathing, peristalsis, cardiac motion, and arterial pulsations cause artifacts in difference images which can reduce the diagnostic accuracy of the procedure. Many of these artifacts can be eliminated by the use of hybrid subtraction, a second-order technique which combines temporal and dual-energy subtraction. This procedure is described and compared to both temporal and dual energy subtraction methods in terms of data acquisition, iodine signal-to-noise ratios, dose, and x-ray tube loading. Images are shown to demonstrate the viability of this new technique.

Beam Hardening, Noise, and Contrast Considerations in Selective Iodine Digital Radiography

Temporal and dual beam energy subtraction are discussed in conjunction with selective imaging of iodine. Several different kinds of energy subtraction are described and their relative sensitivities to noise are predicted. Effects due to beam hardening are demonstrated for temporal and energy subtraction. Hardening errors are more severe for the latter case by about a factor of three. A possible hardening correction is presented for energy subtraction and an experimental example is provided. Techniques for choosing optimized spectra and data handling for dual beam energy subtraction are presented. These include qualitative principles for spectral selection, identification of the individual noise contributions from the two beams, use of weighted least squares fits, and beam hardening and exposure reduction through the use of filtration.

Chest Radiography: The Effect Of Varying Spatial Resolution On The Detection And Discrimination Of Diffuse Small Pulmonary Nodules

Clinical evaluation of CT localization devices for chest imaging have demonstrated the advantages of wide dynamic range and edge enhancement produced by image processing. Major disadvantages in relation to film screen radiography are inferior spatial and temporal resolution. An observer detection study utilizing digitized film screen radiographs of patients with diffuse small pulmonary nodules were performed. Relatively low levels of spatial resolution are required for the detection/discrimination of these abnormal pattern types. More complex observer detection studies are required to determine the spatial resolution requirements for electronic digital radiography of the chest.

Contrast Sensitivity Of Digital Fluorographic Systems

Several aspects of iodine sensitivity of digital fluorographic systems are discussed as applied to digital subtraction angiography. Expected iodine signal magnitudes are reviewed and shown to be in the range of 1-10 mg/cm2. An experimental example of spurious contrast in iodine sensitivity measurements is2 provided. Imaging of a 1 mm wide 0.25 mg/cm simulated iodinated vessel is demonstrated. Such iodine amounts are thought to be near the limit of the sensitivity of digital fluorographic systems. The concept of matched filtering is proposed for generating the equivalent of temporal subtraction images but using low intensity continuous x-ray exposures. The first experimental matched filter canine image is presented using a total exposure for the procedure about one-sixth of that used for pulsed exposure images of comparable quality.

Carotid Stenosis And Ulcer Detectability As A Function Of Pixel Size

Digital radiography, in conjunction with digital subtraction methods can provide high quality images of the vascular system,1-4 Spatial resolution is one important limiting factor of this imaging technique. Since spatial resolution of a digital image is a function of pixel size, it is important to determine the pixel size threshold necessary to provide information comparable to that of conventional angiograms. This study was designed to establish the pixel size necessary to identify accurately stenotic and ulcerative lesions of the carotid artery.