Yan Wen1,2, Jonathan W. Weinsaft3, Thanh D. Nguyen2, Jiwon Kim3, Yi Wang4,5, and Pascal Spincemaille2
1Meinig School of Biomedical Engineering, Cornell University, new york, NY, United States, 2Radiology, Weill Cornell Medicine, New York, NY, United States, 3Medicine, Weill Cornell Medicine, New York, NY, United States, 4Meinig School of Biomedical Engineering, Cornell University, New York, NY, United States, 5Radiology, Weill Cornell Medicine, new york, NY, United States
Synopsis
Previous studies have demonstrated noninvasive cardiac
chamber blood oxygenation measurement using T2 and QSM. This work shows the
initial comparison between the two approaches for such measurements in healthy
volunteers.
INTRODUCTION
Differential right ventricle (RV) to left ventricle (LV) oxygen
saturation ($$${\Delta}SaO_2$$$) measures the oxygenation
difference between the blood in RV and LV, which is an important indicator of
cardiopulmonary function for assessing cardiac function in heart failure
patients, and to measure shunt fractions in patients with congenital or acquired
heart disease. Cardiac quantitative susceptibility mapping (QSM) is an emerging
technique for non-invasive quantification of $$${\Delta}SaO_2$$$ (1,2). Here, we present our
initial results comparing QSM based $$${\Delta}SaO_2$$$ measurements with T2 derived $$${\Delta}SaO_2$$$ measurements in healthy subjects.METHODS
Eight healthy volunteers
were scanned on a commercial GE 3.0T scanner with both T2
mapping and QSM to measure $$${\Delta}SaO_2$$$.
T2 approach: A breath-hold ECG-triggered 2D single-shot T2 prepared sequence (3), with typical scan parameters of TE/TR=1.2ms/2.8ms,
3.1x3.1x10 mm3 resolution, 40s scan time was used to acquire a
series T2 weighted images at different inversion spacings (
=12, 15, 20, and 25 ms) and
different number of refocusing pulses (0, 2, 4, 8, 12). ARLO (4) was used to fit T2. The T2 maps were then
fit to the Luz-Meiboom model:
$$\frac{1}{T_{2b}}=\frac{1}{T_{2O}}+(Pa)(1-Pa)\tau_{ex}[(1-\frac{\%SbO_2}{100})\alpha\omega_0]^2\times(1-\frac{2\tau_{ex}}{\tau_{180}}tanh\frac{\tau_{180}}{2\tau_{ex}})\space\space\space\space\space\space\space\space\space\space\space\space\space[1]$$
where $$$T_{2b}$$$ is the measured blood T2, $$$T_{2O}$$$ is the T2 of fully oxygenated
blood, $$$Pa$$$ is approximately hematocrit, $$$\tau_{ex}$$$ is the water proton exchange time
between erythrocytes and plasma, $$$\%SbO_2$$$ is the oxygen saturation, $$$\alpha$$$ is a susceptibility and geometry dependent
parameter, and $$$\omega_0$$$ is the proton resonance frequency.
To obtain the RV oxygen saturation, $$$SaO_{2,RV}$$$,
a joint-processing of all unknown model parameters (
$$$T_{2O}$$$, $$$\tau_{ex}$$$
, $$$\%SbO_2$$$
, and $$$\omega_0$$$
) was used while
assuming a 98% LV oxygen saturation (3), so that $$${\Delta}SaO_2=98%-SaO_{2,RV}$$$.
QSM approach: A free-breathing ECG-triggered Navigator gated
3D multi-echo GRE sequence with full first order flow compensation was used for
QSM data acquisition. Typical scan parameters were: TE1/$$$\Delta$$$TE=2.3ms/3.6ms, #TE=5, TR=20ms, 2x2x5mm3, scan
time 4.7±1.1minutes. The QSM maps were reconstructed using the
preconditioned total field inversion algorithm with imposed blood pool
uniformity regularizations as follows:
$$y^*=argmin_y\frac{1}{2}{\parallel}w(f-d*Py){\parallel}^2_2+{\lambda\parallel}M_G{\triangledown}Py{\parallel}_1+{\lambda_{RV}\parallel}M_{RV}P(y-\overline{y}^{RV}){\parallel}^2_2+{\lambda_{LV}\parallel}M_{LV}P(y-\overline{y}^{LV}){\parallel}^2_2$$
The first two terms are the data fidelity term and structure consistency
regularization term, respectively, where $$$w$$$ is the SNR weighting, $$$f$$$ is the total field, $$$D$$$ is the dipole kernel, $$$*$$$ is the convolution operator, $$$P$$$ is the preconditioner, $$$\lambda$$$ is the regularization parameter,
$$$M_G$$$ is a
binary edge mask constructed, $$$\triangledown$$$ is the gradient operator.
The final QSM map, $$$\chi$$$, is then $$$\chi=Py^*$$$. The last two terms constrain the susceptibility variation within the
RV and the LV blood pools, where $$$\lambda_{RV}$$$ and $$$\lambda_{LV}$$$ are regularization parameters, $$$M_{RV}$$$ and $$$M_{LV}$$$ are the mask for RV and LV
obtained through manual segmentation on the GRE images. $$$y-\overline{y}^{RV}$$$ and $$$y-\overline{y}^{LV}$$$ are the average susceptibility over the RV and LV blood pools,
respectively. The differential susceptibility between RV and LV blood pools
) was then converted
to blood oxygenation difference ($$${\Delta}SaO_2$$$) using $$${\Delta}SaO_2=\frac{-\Delta\chi}{4H\chi_{eoxyheme}}$$$.
Statistical
methods: Pearson correlation coefficients, Deming linear regression, and the
Bland Altman plots were used to test the difference between the T2 and QSM $$${\Delta}SaO_2$$$ measurements. RESULTS
T2 maps and free-breathing QSM were successfully acquired in all 8
healthy volunteers. Figure 1 shows the T2 and the QSM maps in a representative
case. On the T2 maps, the T2 values in both RV and LV decrease with increasing
, as the Luz-Meiboom model predicts. Note that the T2 values in the
heart chambers are not homogenous. On QSM, susceptibility is higher in the RV
than in the LV due to higher deoxyheme concentration in the RV. Figure 2 shows
the resulting oxygen saturation maps from the two approaches. As shown in Figure
3, linear regression between the $$${\Delta}SaO_2$$$ measured with T2 and with QSM
returned a slope=0.84 and R2=0.82. Bland Altman plot shows a bias=2.9%
and limits of agreement=4.2%.Point
out image quality issuesDISCUSSION
In
the 8 healthy volunteers, $$${\Delta}SaO_2$$$ measured using T2 linearly
correlated with the $$${\Delta}SaO_2$$$ from QSM (slope=0.84, r=0.82), with a
small bias (2.9%). The
known LV oxygenation decrease during a 40 seconds breath hold (5) compared to the assumed 98% oxygen saturation
level may have contributed to this bias. A limitation for the T2 approach is that the Luz-Meiboom
model fitting requires careful empirical selections of initial guesses and
bounds for the fitting parameters, as it is mathematically impossible to
separate
and
in Eq.1. In this study, we followed the
recommendations from a previous study on 1.5T (3),
but these initial guesses and bounds may not be optimal for our 3T
experiments. As the result, at least 1 bound was active for all 8 datasets. In
addition, B1 inhomogeneity and blood flow artifacts may also have affected our
T2 measurements, as observed by the inhomogeneous T2 values in the heart
chamber blood. However, the scan time for the T2 sequence is shorter than the QSM
sequence.
[yw1]Pro
and cons for each methodCONCLUSION
This
initial comparison shows that the $$${\Delta}SaO_2$$$ measurements from T2 and QSM agree reasonably well in healthy
volunteers and QSM represents a more rigorous approach to $$${\Delta}SaO_2$$$ quantification.Acknowledgements
This work was supported in part from NIH grant R01HL128278, R01NS072370,
R01NS090464, R01NS095562, and R01CA181566.References
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