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A theoretical algorithm for shaping the signal response during the approach to steady state in three-dimensional magnetization-prepared rapid gradient-echo (3D MP-RAGE) pulse sequences has been developed and implemented. This algorithm derives the flip angle series required to produce specifically chosen time evolutions of the signal intensities during the data acquisition segment of 3D MP-RAGE sequences. Theoretical predictions for the cases of unshaped, uniform, and mono-exponential decay signal responses were quantitatively validated with a doped-water phantom on a 1.5-T whole-body imager and in all cases there was excellent agreement between the theoretical and experimental values. The effects of RF inhomogeneities and eddy currents on the signal response shaping were also investigated. To demonstrate the potential utility of the technique, the signal response shaping algorithm was applied to a T1-weighted 3D MP-RAGE sequence to derive the acquisition flip angle series which theoretically yields the maximum white matter/gray matter signal difference (WGSD) consistent with the chosen response shape. Images obtained from a healthy volunteer using this variable flip angle sequence were compared with 3D RF-spoiled steady-state gradient-echo images obtained in the same total imaging time. The 3D MP-RAGE images demonstrated a 41% increase in the WGSD-to-noise ratio. These initial very promising results indicate that with further refinement to eliminate some intensity artifacts, the variable flip angle 3D MP-RAGE technique may, with respect to certain image properties, provide considerable improvements over currently available 3D gradient-echo imaging techniques. 相似文献
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Y Leskinen JP Salenius T Lehtim?ki H Huhtala H Saha 《American journal of kidney diseases》2002,40(3):472-479
BACKGROUND: Knowledge of the prevalence of peripheral arterial disease (PAD) in patients with chronic renal failure (CRF) is limited because of a lack of uniformity in disease definition and recognition. Furthermore, little is known of the prevalence of medial arterial calcification (MAC) in patients with CRF. Our goal is to study the prevalence of PAD and MAC defined by ankle brachial index (ABI) or toe brachial index (TBI) measurements in a Finnish population of patients with CRF consisting of predialysis and dialysis patients, as well as renal transplant recipients. METHODS: We examined 136 patients with CRF and 59 control subjects. Fifty-nine of the patients with CRF had moderate to severe predialysis CRF, 36 patients were on dialysis treatment, and 41 were renal transplant recipients. Mean age of patients was 51.9 +/- 11.5 years, and 39 patients (29%) had diabetes. ABI and TBI were measured by means of photoplethysmography. The definition of PAD required an ABI value of 0.90 or less, a TBI value of 0.60 or less, or a previous positive lower-extremity angiogram result. ABI values of 1.3 or greater or incompressible arteries at ankle level indicated MAC. The presence of claudication was determined by an interview. RESULTS: Prevalences of PAD on this study were 22.0% in patients with predialysis CRF, 30.6% in patients on dialysis treatment, 14.6% in renal transplant recipients, and 1.7% in the control group (P = 0.001). Prevalences of MAC were 23.7%, 41.7%, 23.1%, and 3.4% (P < 0.001), respectively. Only 9 patients had claudication, and 6 of those patients had PAD. CONCLUSION: Both asymptomatic PAD and MAC are common in patients with CRF. Therefore, we recommend the use of both ABI and TBI measurements in the evaluation of PAD in patients with CRF. 相似文献
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We used arthrotomography to study the glenoid labrum in 114 patients. Sixty-nine of the patients had anatomic instability of the shoulder (including recurrent dislocation and subluxation of the shoulder), and 45 patients had functional instability of the shoulder (denoted by chronic pain, clicking of the joint, and the sensation that an unstable condition exists without the objective signs of it). Labral tears were revealed arthrotomographically in 86% of the patients with anatomic instability, while only 40% of the patients with functional instability had labral abnormalities, and these were primarily of minor severity. Fifty-six patients (44 of whom had anatomic instability; 12, functional instability) required surgery. The surgical findings were correlated with the arthrotomographic findings, and no false-positive results were revealed. However, arthrotomography demonstrated only part of the pathologic condition of two patients. These results confirm that there is a strong correlation between labral pathologic conditions and anatomic instability of the shoulder. Arthrotomographic studies have a great impact on the selection of therapy in cases of both anatomic and functional instability of the shoulder. 相似文献
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