Subject‐specific models of susceptibility‐induced B0 field variations in breast MRI

Purpose:

To rapidly calculate and validate subject‐specific field maps based on the three‐dimensional shape of the bilateral breast volume.

Materials and Methods:

Ten healthy female volunteers were scanned at 3 Tesla using a multi‐echo sequence that provides water, fat, in‐phase, out‐of‐phase, and field map images. A shape‐specific binary mask was automatically generated to calculate a computed field map using a dipole field model. The measured and computed field maps were compared by visualizing the spatial distribution of the difference field map, the mean absolute error, and the 80% distribution widths of frequency histograms.

Results:

The 10 computed field maps had a mean absolute error of 38 Hz (0.29 ppm) compared with the measured field maps. The average 80% distribution widths for the histograms of all of the computed, measured, and difference field maps are 205 Hz, 233 Hz, and 120 Hz, respectively.

Conclusion:

The computed field maps had substantial overall agreement with the measured field maps, indicating that breast MRI field maps can be computed based on the air–tissue interfaces. These estimates may provide a predictive model for field variations and thus have the potential to improve applications in breast MRI. J. Magn. Reson. Imaging 2013;37:227–232. © 2012 Wiley Periodicals, Inc.     

Simultaneous field and R2 mapping to quantify liver iron content using autoregressive moving average modeling

Purpose:

To investigate the use of a complex multigradient echo (mGRE) acquisition and an autoregressive moving average (ARMA) model for simultaneous susceptibility and R measurements for the assessment of liver iron content (LIC) in patients with iron overload.

Materials and Methods:

Fifty magnetic resonance imaging (MRI) exams with magnitude and phase mGRE images were processed using the ARMA model, which provides fat‐separated field maps, R maps, and T₁‐W imaging. The LIC was calculated by measuring the susceptibility between the liver and the right transverse abdominal muscle from the field maps. The relationship between LIC derived from susceptibility measurements and LIC from R measurements was determined using linear least‐squares regression analysis.

Results:

LIC measured from R is highly correlated to the LIC with the susceptibility method (mg/g dry = 8.99 ± 0.15 × [mg Fe/mL of wet liver] ?2.38 ± 0.29, R² = 0.94). The field inhomogeneity in the liver is correlated with R (R² = 0.85).

Conclusion:

By using the ARMA model on complex mGRE images, both susceptibility and R‐based LIC measurements can be made simultaneously. The susceptibility measurement can be used to help verify R measurements in the assessment of iron overload. J. Magn. Reson. Imaging 2012;35:1125‐1132. © 2011 Wiley Periodicals, Inc.

     

Patient-to-patient variation of susceptibility-induced B(0) field in bilateral breast MRI

Purpose:

To evaluate intersubject variability of susceptibility‐induced static field inhomogeneity in breast and to assess effectiveness of whole‐body high‐order shimming applied to bilateral breast.

Materials and Methods:

A fast, computationally efficient method to calculate susceptibility‐induced static field from anatomical images was developed. The method was validated against the conventional multiecho B₀ mapping method and was used to generate data for linear and higher‐order shim simulation on 13 volunteers.

Results:

Most volunteers showed a significant anterior–posterior B₀ gradient. The majority of the subjects also exhibited a statistically significant left–right gradient. The second‐ and third‐order shimming provided only minor (<5% each) improvement in B₀ homogeneity.

Conclusion:

The shape of the air–tissue boundary determines most of the observed B₀ distribution in bilateral breast. Despite significant variability among subjects, a common feature traceable to generic anatomy exists in the linear gradient. Nonlinear variation of susceptibility‐induced B₀ field occurs over a relatively short length scale and is likely best shimmed by slice‐dependent or localized shimming. J. Magn. Reson. Imaging 2012;36:873–880. © 2012 Wiley Periodicals, Inc.     

The effect of reducing repetition time TR on the measurement of liver R2 for the purpose of measuring liver iron concentration

The effects of reducing the pulse repetition time from 2500 ms to 1000 ms when using spin‐density‐projection‐assisted R2‐magnetic resonance imaging for the purpose of measuring liver iron concentration were evaluated. Repeated liver R2 measurements were made using both protocols on 60 subjects with liver iron concentrations ranging from 0.5 to 48.6 mg Fe (g dry tissue)^?1. The mean total scan time at repetition time 1000 ms was 42% of that at repetition time 2500 ms. The repeatability coefficients for the two protocols were not significantly different from each other. A systematic difference in the measured R2 using each protocol was found indicating that an adjustment factor is required when one protocol is used to replace the other. The 95% limits of agreement between the two protocols were not significantly different from their repeatability coefficients indicating that the protocols can be interchanged without any significant change in accuracy or precision of liver iron concentration measurement. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Blipped multi gradient-echo slice excitation profile imaging (bmGESEPI) for fast T2 * measurements with macroscopic B0 inhomogeneity compensation.

With the rapid development of human MRI at field strengths > or = 7 T, knowledge of T(2) (*) relaxation times at such field strengths is needed to optimize acquisition parameters and understand relaxation mechanisms in many applications. However, standard T(2) (*) measurements (e.g., using conventional multiecho gradient-echo (GE) sequences) are affected by macroscopic static magnetic field (B(0)) inhomogeneities, which are particularly severe at high field strength. The multi-GE slice excitation profile imaging (mGESEPI) method was developed for T(2) (*) measurements in the presence of macroscopic B(0) inhomogeneity, but it requires excessive acquisition times at field strengths > or = 7 T. In this paper a more efficient technique, named blipped mGESEPI (bmGESEPI), is proposed. To demonstrate its advantages, T(2) (*) maps were acquired using a conventional multiecho GE method, the mGESEPI method, and the bmGESEPI method in postmortem and in vivo human brains at 8 T.