Synopsis

T₁ and T₂ maps were acquired at 1.5 T in three cervical cancer patients (52.3±9.0 years) receiving brachytherapy and external beam radiation therapy. Measurements occurred at weeks 1, 3, and 6, where possible. The average T₁ values were 1448.2 ± 209.1ms in the uterus and 1242.9 ± 202.4 ms in the tumor tissue (cervical carcinoma). The average T₂ values were 86.4 ± 15.6 ms in the uterus and 81.1 ± 20.6 ms in the tumor. There was no significant difference in the T₁ and T₂ in the uterus and tumor based on an analysis of variance.

Purpose

Methods

Three radiation therapy patient volunteers with cervical carcinoma (mean age 52.3 ± 9.0 years) received MRI relaximetry during MRI simulations. The patients received external beam radiation and six weekly fractions of tandem/ovoid brachytherapy. Relaximetry measurements were scheduled for simulations for Weeks 1, 3, and 6. However, Volunteers 1 and 2 were only imaged twice. Volunteers were scanned on a 1.5T Philips Ingenia MRI. One petrolatum phantom and three D₂O/H₂O proton density phantoms (100%, 60% and 40% H₂O, doped with 2 mM NiSO₄) were placed inside the field of view.

B₁ fits were acquired using a 2D turbo spin echo (TSE) sequence (TE/TR: 15 ms/4 s, slice thickness/gap: 5/0 mm, Matrix: variable, 50 slices, flip angles (FAs): 45⁰ and 90⁰). The flip angle correction (c) was calculated using the signals from the two flip angle images (S_θ and S_2θ):

$$c(\overrightarrow{r})=\frac{4}{\pi}acos[0.5\cdot\frac{S_{2\theta}(\overrightarrow{r})}{S_{\theta}(\overrightarrow{r})}]$$

T₁ fits were acquired using a variable flip angle 3D FLASH sequence (TE/TR: 2 ms/4 ms, slice thickness/gap: 5/0 mm, Matrix: variable, 40-50 slices, FAs (θ_input): 2°, 5°, 10°, 15°, 20°, and 25°). T₁ was calculated using the actual flip angle $$$\theta_{act}(\overrightarrow{r})=\theta_{input}\cdot c(\overrightarrow{r})$$$, and the FLASH signals (S_FLASH):

$$ S_{FLASH}\left(\overrightarrow{r}\right)=A\left(\overrightarrow{r}\right)\cdot\sin\left[\theta_{act}\left(\overrightarrow{r}\right)\right]\frac{1-\exp\left[-TR/T_{1}\left(\overrightarrow{r}\right)\right]}{1-\cos\left[\theta_{act}\left(\overrightarrow{r}\right)\right]\exp\left[-TR/T_{1}\left(\overrightarrow{r}\right)\right]}+B\left(\overrightarrow{r}\right) $$

T₂ fits were acquired using a multislice 2D Carr Purcell Meiboom Gill (CPMG) sequence (TEs: n·40 ms (n=1-8), TR: variable, slice thickness/gap: 5/0 mm, Matrix: variable, 12, 25 and 30 slices). T₂ and PD (ρ) were calculated using a three-parameter fit to the CPMG signal ($$$S(\overrightarrow{r})$$$):

$$ S\left(\overrightarrow{r}\right)=D\left(\overrightarrow{r}\right)\cdot\exp\left[-\frac{TE}{T_{2}\left(\overrightarrow{r}\right)}\right]+E\left(\overrightarrow{r}\right) $$

The fitting parameter D includes the proton density (ρ), B₁ inhomogeneity (c) and coil/system sensitivity (g):

$$ D\left(\overrightarrow{r}\right)=\rho\left(\overrightarrow{r}\right)\cdot\sin\left[c\left(\overrightarrow{r}\right)\frac{\pi}{2}\right]\cdot g\left(\overrightarrow{r}\right)\approx\rho\left(\overrightarrow{r}\right)\cdot\sin\left[c\left(\overrightarrow{r}\right)\frac{\pi}{2}\right]\cdot g $$

the rightmost equation assumes the prescan normalization removed the spatial dependence of g.

Gross tumor volume (GTV) and healthy tissue regions of interest (ROIs) were extracted from contours registered to high-resolution PD-weighted (PDW) TSE MRIs (TE/TR = 5.5 ms/variable, slice thickness/gap: 2.5/0 mm) during treatment planning, and co-registered to the B₁, T₁, and T₂ images. B₁, T₁, and T₂ were calculated for each tissue and phantom ROI within a single slice, not pixel-by-pixel.

Results

Discussion

For each patient, T₁ values increased for the uterus during treatment and decreased for the GTV. Changes in T₂ values varied across patients. T₁ and T₂ relaxation times measured in the uterus were consistent with a previous study.¹ Only one study on the T₁ and T₂ of cervical cancer in vivo has been published but the measurements were conducted at <0.5 T.² Cervical cancer is hyperintense on PDW and T2W MRI relative to uterus at the beginning of radiotherapy, implying that T₂ and/or PD of cervical cancer is longer than uterus.

We are still recruiting patients for this study but recruitment is challenging due to the discomfort associated with the MRI simulation immediately after implantation of the tandem/ovoids. Another major confounding factor is motion artifacts.

T₁ values were derived only from the central slices to avoid errors from slab selection inhomogeneity.³ However, we can derive and correct the slab excitation profile using the petrolatum phantom to obtain measurements in additional slices.⁴

The T₁ values calculated from FLASH for the D₂O/H₂O phantoms were significantly lower than our inversion recovery TSE (IR-TSE) measurements (T₁ ~1 s) and the T₂. We assume the underestimations are from fluid motion during the FLASH acquisitions since the petrolatum phantom T₁ values were consistent. The PD values were high for the 40% and 60% standards because the phantoms were topped off with distilled water due to evaporation. We estimate the true values to be 58% and 65% assuming an equivalent rate of evaporation, consistent with the MRI fits.

Conclusion

Acknowledgements

References

1. de Bazelaire CM, Duhamel GD, Rofsky NM, Alsop DC. MR imaging relaxation times of abdominal and pelvic tissues measured in vivo at 3.0 T: preliminary results. Radiology 2004;230(3):652-659.

2. Santoni R, Bucciolini M, Chiostrini C, Cionini L, Renzi R. Quantitative magnetic resonance imaging in cervical carcinoma: a report on 30 cases. Br J Radiol 1991;64(762):498-504.

3. Parker GJ, Barker GJ, Tofts PS. Accurate multislice gradient echo T₁ measurement in the presence of non-ideal RF pulse shape and RF field nonuniformity. Magn Reson Med 2001;45(5):838-845.

4. Gach HM, Wang H, Mason NS, Tsien C, Robinson C. B₁ inhomogeneity correction for measuring T₁ from DCE MRI. 2016 July 31-August 4, 2016; Washington D.C. AAPM. p SU-D-207A-204.