This article presents a small-flip-angle, three-dimensional tailored RF pulse that excites thin slices with an adjustable quadratic in-plane spatial variation. The quadratic spatial variation helps to compensate for the loss in image uniformity using a volume coil at 3 T due to the wavelike properties of the RF field. The pulse is based on a novel "fast-k z " design that uses a series of slice-select subpulses along k z and phase encoding "blips" along k x
This work presents a small tip-angle 3D tailored RF slab-select pulse for reducing the B 1 field inhomogeneity at 3T. The compensated slice profile was determined from a B 1 inhomogeneity map. SNR improvement and degree of artifact reduction were evaluated in a NiCl 2 doped phantom and human brains. The technique was found to reduce inhomogeneities as large as 30% of the peak image magnitude in all three spatial directions in the brain using a standard head coil. (4,5). This is a result of the decreased RF wavelength, coupled with the dielectric and conductive properties of the body, producing spatially dependent flip angles and receiver sensitivities. Methods have been proposed to mitigate B 1 inhomogeneity including coil designs (6,7), adiabatic pulses (8), active transmit power modulation (9), and impulse 2D pulses (10). Although improved coils are necessary, they alone may not be able to remove all inhomogeneity because the effect also depends on sample geometry and its physical properties. Adiabatic pulses are problematic because of increased SAR. Active transmit power modulation works well if coil geometry is the dominant source of inhomogeneity. Impulse 2D pulses are also effective; however, they are not slab selective. Furthermore, a 3D correction may be useful because the B 1 variation has been shown to be larger than 10% over a 10-cm diameter in a birdcage coil at 3 T (11). We propose using 3D tailored RF (TRF) pulses (12-15) to reduce B 1 inhomogeneity at 3T. The pulses use a map of the inhomogeneity and are implemented for slab selection in 3D imaging. We demonstrate the technique using a NiCl 2 doped phantom and normal human brains. THEORYIgnoring relaxation and off-resonance, the signal s(t) at time t for a gradient echo and small tip angles ␣(r) can be written as[1]Here V is the coil volume, G acq (s) are the acquisition gradients, and C(r) is the receive sensitivity. The spatial dependencies of ␣(r) and C(r) reflect the effect of an inhomogeneous B 1 field and will be present in the reconstructed image I(r) of the spin density (r):The small tip approximation (16) equates the pulse waveform P(t) to the Fourier transform of the slice profile, W(k(t)), weighted by the k-space velocity produced by the excitation gradients G(t):This equation is valid for tip angles less than 30°. In the above equation, W(k(t)) is defined as W͑k͑t͒͒ ϭ ͐W͑r͒e ik͑t͒⅐r dr, where k͑t͒ ϭ Ϫ ␥ ͵ t T G͑s͒ds. [4]The trajectory k(t) is equal to the area remaining under the gradients at time t until the end of the pulse T. The term ⌬(k(t)) is one over the sampling density (17 In this equation W 0 (r) is the desired uniform 3D slice profile.
BackgroundIron overload cardiomyopathy remains the major cause of death in patients with transfusion-dependent thalassemia. Cardiac T2* magnetic resonance imaging is costly yet effective in detecting cardiac iron accumulation in the heart. Heart rate variability (HRV) has been used to evaluate cardiac autonomic function and is depressed in cases of thalassemia. We evaluated whether HRV could be used as an indicator for early identification of cardiac iron deposition.MethodsOne hundred and one patients with transfusion-dependent thalassemia were enrolled in this study. The correlation between recorded HRV and hemoglobin, non-transferrin bound iron (NTBI), serum ferritin and cardiac T2* were evaluated.ResultsThe median age was 18 years (range 8–59 years). The patient group with a 5-year mean serum ferritin >5,000 ng/mL included significantly more homozygous β-thalassemia and splenectomized patients, had lower hemoglobin levels, and had more cardiac iron deposit than all other groups. Anemia strongly influenced all domains of HRV. After adjusting for anemia, neither serum ferritin nor NTBI impacted the HRV. However cardiac T2* was an independent predictor of HRV, even after adjusting for anemia. For receiver operative characteristic (ROC) curve analysis of cardiac T2* ≤20 ms, only mean ferritin in the last 12 months and the average of the standard deviation of all R-R intervals for all five-minute segments in the 24-hour recording were predictors for cardiac T2* ≤20 ms, with area under the ROC curve of 0.961 (p<0.0001) and 0.701 (p = 0.05), respectively.ConclusionsHemoglobin and cardiac T2* as significant predictors for HRV indicate that anemia and cardiac iron deposition result in cardiac autonomic imbalance. The mean ferritin in the last 12 months could be useful as the best indicator for further evaluation of cardiac risk. The ability of serum ferritin to predict cardiac risk is stronger than observed in other thalassemia cohorts. HRV might be a stronger predictor of cardiac iron in study populations with lower somatic iron burdens and greater prevalence of cardiac iron deposition.
Iron overload cardiomyopathy remains the major cause of death in β-thalassemia (β-thal). Conventional routine screening parameters such as serum ferritin and echocardiogram (ECG) do not permit early detection of this condition. Although non-transferrin-bound iron (NTBI) is a reliable indicator for iron overload, it is still not universally available. Recently, heart rate variability (HRV), representing cardiac autonomic function, was found to be depressed in thalassemia patients. We hypothesized that HRV can be used for early detection of iron overload cardiomyopathy. Fifty patients (aged 29 ± 11 years; 31 females and 19 males) with β-thal were enrolled. The 24-hour Holter monitoring for HRV, serum ferritin, NTBI, hematological values and ECG were performed for each patient. Of the 50 patients, 29 carried β-thal major (β-TM). Non-transferrin-bound iron was weakly correlated to all time-domain HRV parameters. Low- and high-frequency domain HRV parameters were also inversely weakly correlated with NTBI. Neither HRV nor NTBI was correlated with serum ferritin. With its weak but significant correlation with NTBI, HRV may be considered to be used as a potential indicator of an iron overload condition and an early marker of cardiac involvement in patients with β-thal.
Short stature is one of the most common endocrinopathies in transfusion-dependent thalassemia (TDT). This study aimed to determine the longitudinal pattern of growth in pediatric patients with TDT and study the relationship between growth and hemoglobin level, serum ferritin level/iron overload parameters, and other clinical factors. The interval height-for-age Z-scores (HAZ) of 50 patients with TDT, of a mean age of 13.3±2.8 years, were analyzed using linear mixed model analysis. Nineteen patients (38%) had short stature with HAZ≤-2.0. The prevalence of short stature increased with age. The estimated mean HAZ decreased by 0.19 SD per year from the age of 5 years until approximately 14 years (95% confidence interval [CI], -0.22 to -0.16, P<0.001). Male sex (estimate, -0.28; 95% CI, -0.43 to -0.14; P<0.001), mean 3-year hemoglobin level ≤8 g/dL (estimate, -0.36; 95% CI, -0.53 to -0.19; P<0.001), mean 3-year ferritin level ≥1800 ng/mL (estimate, -0.44; 95% CI, -0.59 to -0.29; P<0.001), and cardiac T2* ≤20 ms (estimate, -1.05; 95% CI, -1.34 to -0.77; P<0.001) were significantly associated with short stature. In conclusion, short stature in patients with TDT is common and relates significantly with increasing age, male sex, hemoglobin level, and iron overload status.
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