Feathering: Vertebral artery pseudostenosis with elliptical centric contrast-enhanced MR angiography.

Elliptical centric contrast-enhanced MR angiography of the cervical vasculature is a well-established technique that in many practices has replaced conventional angiography for several clinical indications, including atherosclerotic disease and dissections. Occasionally blurring or loss of signal intensity occurs in the vertebral arteries, especially in young patients with rapid circulation times. This ringing artifact, which we termed "feathering," results from rapidly changing signal intensity in small vascular structures during the sampling of the center of k-space.

Contrast-enhanced magnetic resonance angiography of the cervical vessels: experience with 422 patients.

BACKGROUND AND PURPOSE: Contrast-enhanced magnetic resonance angiography (CEMRA) permits acquisition of high-spatial-resolution, venous-suppressed, 3D MR angiograms of the cervical carotid and vertebral arteries. In this study, an elliptic centric-view ordering with either MR fluoroscopic triggering or test bolus timing was used. The use of CEMRA of the cervical vessels has changed our clinical practice and is replacing conventional angiography for the evaluation of most carotid and vertebral artery diseases. METHODS: We retrospectively reviewed our experience with the use of CEMRA performed in 422 patients from January through December 1999. RESULTS: CEMRA was performed to evaluate transient ischemic attack and ischemic stroke in 239 patients, asymptomatic carotid bruit in 88 patients, and other neurological symptoms in 95 patients. Carotid endarterectomies were performed in 97 patients (103 procedures), and conventional angiography was performed in 12 of these patients. CEMRA was used to evaluate for the presence of an arterial dissection in 85 of the 239 patients with transient ischemic attack and ischemic stroke. Of this group, 32 patients had cervical arterial dissection, and pseudoaneurysm was detected in 11 of these patients. Compared with ultrasonography of the cervical vessels, CEMRA provided additional information in 43 of 422 patients and led to changes in the decision as to whether to perform carotid endarterectomy in 5 patients. CONCLUSIONS: Use of CEMRA permits noninvasive evaluation of patients suspected of having carotid or vertebral disease and avoids the potential complications of conventional angiography.

Some useful concepts of matched filtering in intravenous digital subtraction angiography.

Using computer calculations and assumed contrast bolus curves, several aspects of the application of temporal integration methods and matched filtering to intravenous digital subtraction angiography (IV-DSA) were studied. The topics included the improvement in signal-to-noise ratio (SNR) of the final image provided by simple integration, a comparison of the SNR performance of matched filtering and extensive integration, the degradation of SNR caused by the motion of noniodinated objects and the sensitivity of SNR to variations in DSA bolus dynamics from patient to patient. Additionally the dependence of matched filter SNR on exposure position and duration was both estimated and demonstrated with clinical DSA images. The results indicate that a substantial improvement in SNR can be obtained with only moderate integration increasing to a two X improvement for longer durations. Integration methods are able to withstand moderate durations (2 seconds) of motion and still provide image quality superior to more conventional DSA results.

Real-time digital K-edge subtraction fluoroscopy.

We report in vitro and in vivo trials of K-edge fluoroscopy, by which iodine contrast concentration is displayed live, with tissue and bone images suppressed, free of patient-motion artifacts. Iodine and cerium, 125 and 225 mg/cm2 respectively, filter alternate TV fields of cine-pulsed 50 KVP x-rays. Weighted subtraction of successive TV fields isolates the iodine image and simultaneously minimizes artifacts. Digital techniques are used in real time. At our present x-ray tube limit, 500 mA instantaneous current, the patient exposure is 180 mR/sec and quantum mottle limits the image quality. Integrating four successive difference images provides a compromise between mottle and smoothly moving displays. Cardiovascular images of a 17-kg dog, using 1 ml/kg Renografin-60 injected into a foreleg vein, show that a 15-cm chest thickness is our present practical maximum. This method may be useful in diagnosing cardiovascular anomalies in infants without catheterization or suspension of breathing.

Computerized fluoroscopy techniques for intravenous study of cardiac chamber dynamics.

A computerized fluoroscopy system which was recently developed in our laboratories permits image contrast increases of 8-16 relative to conventional image intensifer fluoroscopy and permits study of canine and human ventricular wall motion using peripheral intravenous injections. Two time-dependent image subtraction algorithms are illustrated in connection with observation of artificially infarcted dog hearts. The first algorithm produces a display analogous to direct ventriculography using catheterization. The second displays regions of dyskinetic motion as anomalous image grey shades.

Magnetic resonance imaging of pelvic floor relaxation: dynamic analysis and evaluation of patients before and after surgical repair.

OBJECTIVE: To evaluate structures involved in pelvic support using conventional and snapshot magnetic resonance imaging (MRI). METHODS: We used conventional spin-echo MRI and dynamic snapshot GRASS MRI at various levels of the Valsalva maneuver to describe and quantitate the anatomy of pelvic floor relaxation and to assess anatomical changes produced by surgical repair. Ten female volunteers were evaluated to define normal anatomy and reference measurements. Five women with pelvic floor relaxation were evaluated before and after surgical repair. RESULTS: Static and dynamic MRI were more sensitive than clinical pelvic examination in assessing and grading pelvic floor relaxation. Quantitative results showed widening of the levator hiatus and more vertical lie of the levator plate postoperatively. Descent of the pelvic organs on maximal straining postoperatively was the same as that in normal volunteers. The posterior urethrovesical angle on MRI was more than 110 degrees in 14 of 15 continent subjects. CONCLUSIONS: Magnetic resonance imaging may be valuable in analyzing and assessing pelvic floor relaxation and in understanding anatomical changes occurring before and after surgical repair. The increased sensitivity of MRI in grading prolapse may make it useful in evaluating women with symptoms of pelvic floor relaxation but who have negative findings on clinical examination.

Mapping of the central sulcus with functional MR: active versus passive activation tasks.

PURPOSE: Our purpose was to assess the pattern of functional MR activation obtained with a passive sensory versus an active sensorimotor hand stimulus paradigm. METHODS: Eight functional MR runs, four with an active sensorimotor (sponge-squeezing) task and four with a passive sensory (palm-finger brushing) reception, were acquired for each of 10 healthy volunteers. Activation maps were generated by thresholding cross-correlation maps. Regions of interests (ROIs) were drawn around the precentral and postcentral gyri on T1-weighted images according to established anatomic criteria, and the number of activated pixels inside the ROIs was ascertained. Displacement of the sensorimotor and sensory activation centroids within the ROIs from the central sulcus as well as from each other was measured. RESULTS: Active sensorimotor stimulation produced a significantly greater number of activation pixels than did passive sensory stimulation. Run-to-run variability was equivalent between sensorimotor and sensory activation tasks. On average, the sensorimotor and sensory activation centroids were located in the postcentral gyrus, and their spatial locations were not significantly different. CONCLUSION: Active and passive activation tasks produce largely equivalent results. Presurgical mapping of the sensorimotor area can be performed with functional MR imaging using a passive palm-finger brushing task in patients who are physically unable to perform active finger-tapping or hand-squeezing sensorimotor activation tasks.

Real-time reconstruction and high-speed processing in functional MR imaging.

Access to fully processed activation maps in near real time during a functional MR examination enables run-to-run assessment of results. This is particularly useful in clinical studies, since the results of the functional MR examination can be ascertained before the patient leaves the MR suite, permitting interactive tailoring of the functional MR study. We describe how a real-time MR system can be customized to complete the following tasks in less than 3 minutes: obtain an 81-second acquisition of a multisection functional MR imaging time series using single-shot echo-planar imaging, perform image reconstruction, extract functional MR activation maps using cross-correlation and thresholding, and superimpose activation maps on previously acquired T1-weighted anatomic images.

Cerebral magnetic resonance image synthesis.

The authors previously described magnetic resonance (MR) image synthesis, a process that enables the investigator to manipulate imaging parameters retrospectively and generate or "synthesize" the image that corresponds to various arbitrary scanning factors. They demonstrate the validity and utility of synthetic spin-echo images in cerebral imaging. As a test of their method, spin-echo images are synthesized for echo times identical to those of the original acquired images as well as for alternate values. Subjectively, the quality of synthetic and acquired images is comparable. It is shown quantitatively for several tissue types that the reconstructed synthetic signal matches the acquired signal within the uncertainty of the acquired images. Observed and measured noise levels in the acquired and synthetic images are comparable. Because of a signal-averaging effect, the synthetic images can have a higher signal-to-noise ratio than the source images, thereby providing improved boundary definition. Applications of MR image synthesis are discussed with respect to potential reduction in scanning time. The advantages of image synthesis versus analysis of computed images are discussed.

Assessment of functional MR imaging in neurosurgical planning.

BACKGROUND AND PURPOSE: Presurgical sensorimotor mapping with functional MR imaging is gaining acceptance in clinical practice; however, to our knowledge, its therapeutic efficacy has not been assessed in a sizable group of patients. Our goal was to identify how preoperative sensorimotor functional studies were used to guide the treatment of neuro-oncologic and epilepsy surgery patients. METHODS: We retrospectively reviewed the medical records of 46 patients who had undergone preoperative sensorimotor functional MR imaging to document how often and in what ways the imaging studies had influenced their management. Clinical management decisions were grouped into three categories: for assessing the feasibility of surgical resection, for surgical planning, and for selecting patients for invasive functional mapping procedures. RESULTS: Functional MR imaging studies successfully identified the functional central sulcus ipsilateral to the abnormality in 32 of the 46 patients, and these 32 patients are the focus of this report. In epilepsy surgery candidates, the functional MR imaging results were used to determine in part the feasibility of a proposed surgical resection in 70% of patients, to aid in surgical planning in 43%, and to select patients for invasive surgical functional mapping in 52%. In tumor patients, the functional MR imaging results were used to determine in part the feasibility of surgical resection in 55%, to aid in surgical planning in 22%, and to select patients for invasive surgical functional mapping in 78%. Overall, functional MR imaging studies were used in one or more of the three clinical decision-making categories in 89% of tumor patients and 91% of epilepsy surgery patients. CONCLUSION: Preoperative functional MR imaging is useful to clinicians at three key stages in the preoperative clinical management paradigm of a substantial percentage of patients who are being considered for resective tumor or epilepsy surgery.

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.

Application of the noise power spectrum to positron emission CT self-absorption correction.

Two methods are compared in correcting for self-absorption in positron emission CT scans, or equivalently, in determining the integral of attenuation along a strip in a cross section. These are CT reconstruction and direct measurement. It is shown that the former method is slightly more precise than the latter with the degree of improvement proportional to the number of angles used. Additionally, it is demonstrated that the CT approach is statistically very inefficient in its use of detected events in comparison to the direct approach. For the CT case, it is shown using the noise power spectrum that two-thirds of the variance in the attenuation integral is from one projection alone, namely that along the direction of the strip. The remaining one-third is primarily from adjacent projections. For the problem of determining the attenuation in a single strip within the slice, the CT approach is not recommended. However, if the problem is extended to a complete set of strips within the slice, as is desired in positron emission CT, the CT approach may require considerably less counting time than the direct approach for comparable precision.

Selective iodine imaging using K-edge energies in computerized x-ray tomography.

Iodine is commonly used as a contrast material in computerized x-ray tomography. In some cases the determination of the iodine distribution in the image may be prevented by the presence of bone or tissue variations within the tomographic slice. This paper describes a method for quantitative selective imaging of the iodine concentration in the slice. The method employs scans using three heavily filtered x-ray beams, two having mean energies which straddle the iodine K edge (33 keV) and another at a slightly higher energy. The results are independent of tissue and bone over a broad range of projection path lengths. It is shown that, for separation of iodine from one other material, a two-beam K-edge approach requires less integral dose than a two-beam technique at conventional CT energies for slice diameters up to 30 cm. For selective iodine imaging in the presence of more than one other material, the three-spectrum K-edge technique is a necessity. Exposure requirements and beam-hardening corrections are discussed in detail and a computer-simulated CT image generated by the proposed scheme is presented.

Spectral considerations for absorption-edge fluoroscopy.

In our previous reports on absorption-edge fluoroscopy, it was not possible to relate fully the subtleties involved in the selection of spectral parameters. This paper is intended as an overview of this important aspect of the technique. It is shown that, by using the 1-kVp, 2-filter technique, it is possible to image certain elements (e.g., iodine and xenon) in the presence of tissue variations of +/-2 cm about the thickness at which perfect tissue cancellation takes place. Use of logarithmic signal processing extends this range, but bone thickness variations may not be accomodated because only two x-ray energies are involved in the imaging process. Use of a 3-kVp, 3-filter technique with logarithmic signal processing is shown to solve this problem. Computer simulations show that 1-mg/cm2 iodine may be imaged in the presence of 10 cm or more tissue variations and 2000-mg/cm2 bone variations.

An analysis of noise propagation in computed T2, pseudodensity, and synthetic spin-echo images.

Methods are reviewed for estimating the transverse relaxation time T2 and the pseudodensity (PD) from spin-echo measurements acquired at an arbitrary set of echo times [TEi]. Least-squares fitting is applied to the logarithmically processed signals for the case in which the weights are proportional to the inverse of the logarithmically transformed signal variances (the minimum variance case). General formulas are derived for the estimated noise levels in the PD and T2 estimates due to the propagation of uncertainties in the original measurements. It is shown that the T2 and PD estimates are anticorrelated. Additionally, an expression is derived for the variance in a synthetic spin-echo signal subsequently formed from the PD and T2 estimates. It is shown that under many circumstances a signal synthesized at some echo time can have a signal-to-noise ratio superior to that in a signal directly acquired at that time. Experimental measurements made on phantoms match the theoretical predictions to a high degree.

Relative properties of tomography, K-edge imaging, and K-edge tomography.

The properties of tomography, K-edge imaging, and K-edge tomography are discussed in relation to the imaging of small concentrations of elements such as iodine and xenon and are compared by means of phantom images. It is demonstrated that the complementary selectivities provided by depth and energy subtraction are combined in K-edge tomography. Using a three-spectrum subtraction technique, the iodine difference signal predicted by computer calculations is on the order of 8000 times that of an equal concentration fo bone. The corresponding ratio in tomography without energy subtraction is 20:1. It is argued that K-edge tomography can successfully eliminate artifacts due to tissue inhomogeneities which presently enable 0.6% variations in tissue attenuation to mimic minimum detectable iodine signals in conventional computed tomography. Various instrumentation possibilities and energy subtraction techniques are discussed.

Limitations to iodine isolation using a dual beam non-K-edge approach.

In dual-beam selective iodine imaging, images of an object are made with each of two spectrally different x-ray beams. The mean beam energies may either straddle the 33 keV iodine K-edge or both lie above the K-edge. Both patient exposure considerations and the availability of sufficient x-ray flux make the latter approach favorable for tissue thicknesses exceeding 5 cm. Consider such an approach in which image contrast from tissue is suppressed in the difference image. It is proven theoretically that the residual bone-to-iodine contrast is a constant independent of the two mean beam energies used. This invariance principle is demonstrated experimentally by comparing images made from different pairs of x-ray spectra. Observed contrast ratios match the predicted value very well. In dual-beam imaging, contrast from only one material may be suppressed. Other substances yield residual signals which compete with the iodine. Subtleties of this incomplete cancellation are demonstrated, discussed, and quantitated. A contrast enhancement factor (CEF) is defined as the factor by which iodine contrast is enhanced in a multiple beam subtraction technique relative to monoenergetic imaging at 40 keV. CEFs are determined for tissue and bone cancellation separately and their limits are discussed. Images of a simulated artery containing iodine superimposed over a Rando head and neck phantom show that the CEF limitation for dual beam imaging is quite severe compared to a time dependent mask mode imaging approach. Finally, optimum, energies for dual beam images are discussed.

Three-beam K-edge imaging of iodine using differences between fluoroscopic video images: theoretical considerations.

Our lab has previously generated selective iodine images with an image intensifier fluoroscopic system using a three-beam K-edge approach. Logarithmically amplified video images Li were linearly combined to yield the final image k1L1 + k2L2 + k3L3. This paper discusses refinements of the K-edge technique. A study is made of the manner in which contrast-reducing effects such as x-ray scatter and image intensifier veiling glare enter into the final image. If such biases can be approximated as multiplicative and independent of the x-ray spectrum, and if the sum of the ki is zero, then the biases are canceled. Experimental data is presented which demonstrates that the inaccuracy due to such biases can be reduced by a factor as large as 10. The theorem that K1 + K2 + K3 is approximately equal to 0 is proven rigorously and discussed. Because the ki add to zero, the final image can be expressed as a linear combination of two of the differences between the Li. A difference-based algorithm which reduces biases and make allowance for nonlinearities such as beam hardening is proposed and discussed.

The precision of TR extrapolation in magnetic resonance image synthesis.

We present a model of noise propagation from acquired magnetic resonance (MR) images to TR-extrapolated synthetic images. This model assumes that images acquired at two repetition times TR1 and TR2 are used to generate synthetic images at arbitrary repetition times TR. The predictions of the model are compared with experimentally acquired phantom data, and show excellent agreement. The model is utilized in an analysis of two applications of MR image synthesis: scan time reduction and multiple-image synthesis. Scan time is reduced by acquiring data at two short repetition times, and synthesizing at a longer repetition time, with TR1 + TR2 less than TR. For T1 = 800 ms, a reduction of 20% in scan time results in a 45% reduction in signal-to-noise ratio SNR, when compared to direct acquisition. Reducing scan time by much more than 20% produces large noise levels in the synthetic image, and is unlikely to be useful. In multiple-image synthesis, images are synthesized at any repetition time in the range 0 to TR1 + TR2, for contrast optimization. If T1 = 800 ms, and TR1 + TR2 = 2000 ms, the optimum combination of TR1, TR2 results in synthetic images whose SNR is at worst 22% less than the SNR of directly acquired images. For many values of TR, the synthetic images have SNR superior to that obtainable by direct acquisition.