Karl Hittmair

Accuracy of predicting and controlling time-dependent aortic enhancement from a test bolus injection.

Purpose The purpose of this work was to determine the accuracy of predicting arterial enhancement from peripheral versus central venous test bolus injections at CT angiography (CTA). Method In 40 patients with abdominal aortic aneurysms, aortoiliac enhancement profiles were predicted by mathematical deconvolution of the time-attenuation response to a 16 ml test bolus injection. Injection sites were either a cubital vein (n = 20) or a central venous injection site (n = 20). The accuracy of predicting enhancement was quantified as the “off-predicted deviation” (calculated as mean squared differences between observed minus predicted enhancement values) in all patients. Results Off-predicted deviation was significantly smaller in the central venous injection group (17 ± 6 HU) than the peripheral injection group (33 ± 18 HU) (p < 0.001). Conclusion Arterial enhancement at CTA can be mathematically predicted and controlled more accurately if a central venous injection site is used. Automated saline flushing of the veins might improve the accuracy of the mathematical model for peripheral injections.

Double Inversion Recovery Imaging of the Brain: Initial Experience and Comparison with Fluid Attenuated Inversion Recovery Imaging

The purpose of this study was to evaluate the diagnostic efficacy of the double inversion recovery fast spin echo (DIR-FSE) sequence for brain imaging compared to the fluid-attenuated inversion recovery (FLAIR) sequence. DIR-FSE and FLAIR-FSE sequences were obtained and compared side by side. Image assessment criteria included lesion conspicuity, contrast between different types of normal tissue, image quality, and artifacts. In addition, contrast ratios and contrast-to-noise ratios were determined. Scan time of DIR-FSE was 33% longer than scan time of FLAIR-FSE. Overall lesion conspicuity was equal on DIR-FSE and FLAIR-FSE; however, DIR-FSE showed particular advantages for infratentorial lesions and lesions with only poor contrast on T2-weighted images, whereas FLAIR was slightly superior for small superficial cerebral abnormalities. Gray-white differentiation was better with DIR-FSE. Cerebrospinal fluid suppression was equal on both sequences; cerebrospinal fluid pulsation artifacts were more pronounced on DIR-FSE but did not cause diagnostic difficulties on these images. We conclude that DIR-FSE might be obtained if infratentorial lesions and abnormalities with only slightly prolonged T2 relaxation times are suspected. Otherwise, FLAIR-FSE seems preferable.

A possible role of in-flow effects in functional MR-imaging

The potential of functional MR-imaging at 1.5 T was studied using optical stimulation of the human brain. Differences in the image intensity of up to 30% were obtained in the visual cortex area, correlating with the time course of the stimuli. A simplified semi-quantitative model describing the role of in-flow effects in brain vessels on signal increase during the stimulation period is presented.

MRI in a case of Sandhoff's disease

An 18-month-old girl was examined by MRI for progressive psychomotor retardation. T2-weighted images demonstrated abnormal high signal in the putamina and low signal in the thalamus (due probably to calcification). Although the cerebral cortex was markedly atrophic, there were signs of brain enlargement because of swelling of the extensively diseased white matter. The diagnosis of Sandhoffs disease was established by low serum levels of hexosaminidase A and B.

Comparison between Conventional and Fast Spin-Echo Stir Sequences

Purpose: To evaluate the common characteristics and differences in contrast behavior of short-TI-inversion-recovery (STIR) and short-TI-inversion-recovery fast spin-echo (TurboSTIR) sequences. Material and Methods: Phantoms doped with increasing doses of Gd-DTPA and a pork-fat phantom were used to evaluate the dependence of the STIR and TurboSTIR signals on the T1 relaxation time. Clinical TurboSTIR images were obtained from 30 patients with musculoskeletal abnormalities and compared to conventional STIR images in 15 cases and to postcontrast TurboSTIR images in another 15 cases. Results: In the phantom measurements, a significantly shorter inversion time (TI) was needed to achieve fat suppression on TurboSTIR images, and, with an identical number of signal averages, contrast-to-noise ratios were lower on TurboSTIR images. These differences between STIR and TurboSTIR can be attributed to the contribution of stimulated echoes to overall TurboSTIR signal and can be compensated by a shorter TI and a higher number of signal averages for TurboSTIR, respectively. With these adaptations, clinical TurboSTIR and STIR images showed an identical contrast behavior with fat suppression and a high sensitivity to pathological lesions but TurboSTIR saved a significant amount of scan time and reduced some types of artifacts. Contrast uptake impaired lesion conspicuity on TurboSTIR images. Conclusion: TurboSTIR sequences should replace conventional STIR sequences and should be performed before contrast administration.

Fast flair imaging of the brain using the fast spin-echo and gradient spin-echo technique

The purpose of this study was to compare the gradient spin-echo (GRASE) to the fast spin-echo (FSE) implementation of fast fluid-attenuated inversion recovery (FLAIR) sequences for brain imaging. Thirty patients with high signal intensity lesions on T2-weighted images were examined on a 1.5 T MR system. Scan time-minimized thin-section FLAIR-FSE and FLAIR-GRASE sequences were obtained and compared side by side. Image assessment criteria were lesion conspicuity, contrast between different types of normal tissue, image quality, and artifacts. In addition, contrast ratios and contrast-to-noise ratios were determined. Compared to FSF, the GRASE technique allowed a 17% reduction in scan time but conspicuity of small lesions in particular was significantly lower on FLAIR-GRASE images because of higher image noise and increased artifacts. Gray-white differentiation was slightly worse on FLAIR-GRASE. Physiological ferritin deposition appeared slightly darker on FLAIR-GRASE images and susceptibility artifacts were stronger. Fatty tissue was less bright with FLAIR-GRASE. With current standard hardware equipment, the GRASE technique is not an adequate alternative to FSE for the implementation of fast FLAIR sequences in routine clinical MR brain imaging.

Imaging of the brain using the fast-spin-echo and gradient-spin-echo techniques.

Abstract. The aim of our study was to compare gradient-spin-echo (GRASE) to fast-spin-echo (FSE) sequences for fast T2-weighted MR imaging of the brain. Thirty-one patients with high-signal-intensity lesions on T2-weighted images were examined on a 1.5-T MR system. The FSE and GRASE sequences with identical sequence parameters were obtained and compared side by side. Image assessment criteria included lesion conspicuity, contrast between different types of normal tissue, and image artifacts. In addition, signal-to-noise, contrast-to-noise, and contrast ratios and were determined. The FSE technique demonstrated more lesions than GRASE and with generally better conspicuity. Smaller lesions in particular were better demonstrated on FSE because of lower image noise and slightly weaker image artifacts. Gray–white differentiation was better on FSE. Ferritin and hemosiderin depositions appeared darker on GRASE, which resulted in better contrast. Fatty tissue was less bright on GRASE. With current standard hardware equipment, the FSE technique seems preferable to GRASE for fast T2-weighted routine MR imaging of the brain. For the assessment of hemosiderin or ferritin depositions, GRASE might be considered.

Infratentorial brain maturation: a comparison of MRI at 0.5 and 1.5T

Our purpose was to establish parameters for normal infratentorial brain maturation at 0.5 and 1.5T and to evaluate the field strength criteria for the assessment of infratentorial brain maturation with MRI. We examined 27 children with normal psychomotor development (3 days to 24 months) with a 1.5T system and 22 (4 days to 29 months) with a 0.5T system; standard T2-weighted spin-echo sequences (TR/TE 2500/90 ms at 1.5T and TR/TE 2200/90 ms at 0.5T) were obtained. The signal intensity of infratentorial anatomical structures compared to their surroundings was classified as high, isointense or low by three neuroradiologists. For anatomical structures with age-related contrast changes, the time of these changes was determined statistically for the 0.5T and 1.5T system independently. The delineation of the structures without age-related contrast changes at the two field strengths was compared using a χ2 test. Age-related contrast changed were found in the same anatomical structures (“marker sites”) at 0.5 and 1.5T. Generally, these changes were apparent in larger structures (pons, middle cerebellar peduncles, medulla, cerebellar folia, red nuclei, cerebral peduncles), with only slight field-strength-dependent differences in the time of the contrast changes. Contrast changes from high to isointense signal were observed slightly earlier at 0.5T and changes from isointense to low signal slightly later at 0.5T. The delineation of the smaller anatomical structures was significantly better at 1.5T but these structures did not show age-related contrast changes. The differences in the assessment of infratentorial brain maturation between 0.5 and 1.5T can be attributed to a lower signal-to-noise ratio at lower magnetic field strengths. These differences do not complicate temporal classification of the stage of infratentorial brain maturation using the same “marker sites” and the same temporal criteria at 0.5 or 1.5T. However, higher field strengths are preferable for the assessment of smaller structures with physiological signal differences; this may imply better detection of small lesions at higher field strengths.

Bolusoptimierte CT-Angiographie

ZusammenfassungDiese Arbeit stellt verschiedene Techniken der Kontrastmittelinjektion zur Optimierung der Gefäßdarstellung bei der CT-Angiographie (CTA) gegenüber. Einleitend wird als angestrebtes Ziel der Kontrastmittelgabe eine gleichbleibend hohe und auf die Zeit der CT-Messung beschränkte Gefäßkontrastierung definiert. Diese „Plateauanfärbung” garantiert hochqualitative CTA-Bilder bei minimalem Kontrastmittelverbrauch und soll deshalb bei jeder CTA-Untersuchung durch eine geeignete Art der Kontrastmittelgabe angestrebt werden. Bei etablierten Injektionstechniken wie der Standardbolustechnik (gleichbleibendes uniphasisches Injektionsprotokoll für alle Patienten) oder der Anpassung der CT-Verzögerung allerdings werden beträchtliche individuelle Unterschiede in der Gefäßanfärbung beobachtet, und das Resultat kann im Einzelfall eine unzureichende Untersuchungsqualität oder eine ineffiziente Kontrastmittelausnutzung sein. Es wurden deshalb kürzlich 2 mathematische Modelle zur Analyse der individuellen Anfärbecharakteristik entwickelt, welche die arterielle Anfärbung für einen gegebenen Kontrastmittelbolus beim jeweiligen Patienten vorhersagen können und so eine individuelle Bolusoptimierung im Hinblick auf die gewünschte Plateauanfärbung erlauben. Diese Techniken müssen allerdings noch ihre Effektivität in größeren klinischen Studien beweisen.SummaryThis paper compares different contrast injection techniques for optimizing vessel contrast in CT angiography (CTA). The optimal vessel contrast shall be defined as constant strong enhancement confined to the scanning interval. This ”plateau enhancement” guarantees high-quality CTA images and should therefore be approximated during every CTA examination by an appropriate contrast injection protocol. With well-established injection techniques such as the standard bolus technique (constant uniphasic contrast bolus for all patients) or adjustment of the scan delay, considerable individual differences in the arterial enhancement can be observed, and a nondiagnostic examination or an inefficient use of contrast agent might be the result in a particular patient. Therefore, two sophisticated mathematical models have recently been developed for analyzing the individual enhancement characteristics. These models can be exploited to predict the arterial enhancement for any given intravenous contrast bolus in any patient and to optimize the contrast bolus in order to approach the ideal ”plateau enhancement”. These techniques have to prove their effectiveness in larger clinical series.

Vegetable oil as an MR contrast agent for rectal applications

The purpose of this study was to evaluate the feasibility of pure vegetable oil as an MR contrast agent for rectal applications. The hypothesis was that vegetable oil highlights the lumen of the rectum after rectal application as a positive contrast medium and offers additional contrast qualities using fat suppression techniques. Eleven MRI examinations were performed on 11 subjects (five healthy volunteers, all males, mean age 35 yr; and six patients, three males, three females, mean age 49 yr). Peanut oil, 200 ml, was applied rectally. In addition, 0.1 mmol/kg GD-DTPA was administered intravenously to the six patients only. Conventional T1-weighted SE sequences and T1-weighted SE images with fat suppression were obtained. Criteria for image evaluation were: overall image quality; uniformity of contrast distribution; chemical shift artifact; and delineation of the rectal wall. Side effects were assessed. There were no complaints reported by the 11 subjects. The image quality was sufficient in all studies. In all five of the volunteers and five of the six patients, the distribution of oil was uniform. Chemical shift artifacts did not deteriorate image quality. After rectal application of vegetable oil, the delineation of the rectal wall was sufficient with and without fat suppression techniques. Vegetable oil highlights the lumen of the rectum in MRI studies and offers additional contrast qualities with fat suppression techniques, acting as a positive as well as a negative contrast agent, depending on the chosen sequence.