Arterial input functions (AIFs) measured from dynamic contrast enhanced (DCE) MRI following low dose (0.015 mmol/kg) contrast media were compared with AIFs from DCE CT as ‘gold standard’. Twenty prostate cancer patients received CT and MRI scans on the same day. To correct for different temporal resolution and sampling periods, an empirical mathematical model was used to fit the AIFs and calculate numerical AIFs. Convolution was performed to correct for differences in CT and MRI injection times (~1.5s vs. 30s). MRI and CT AIFs were very similar. Therefore, AIFs can be accurately measured by MRI following low dose contrast agent injection.
METHODS
This study was approved by the Institutional Review Board and informed consent was obtained from all patients. Twenty prostate cancer patients (58±7 years) were enrolled in this study. DCE-CT scans (120mL Omnipaque350 was injected at 4mL/s with duration of 30 s) were performed on a Philips Brilliance iCT scanner at 120 kVp with 29 dynamics, a temporal resolution of 5s for the first 25 dynamics followed by 2 dynamic scans 1 min apart and, subsequently, 2 dynamics scans 2 min apart. The default scan length was 8 cm with a 128×0.625 mm beam configuration. The DCE-CT temporal resolution and number of scans was limited due to radiation dose considerations and contrast agent volume. The MRI scan was performed ~3 hours after the CT scan on a Philips 3T scanner. After other clinically required scans, variable flip angle (VFA) 3D-FFE-T1 sequences were acquired. Subsequently, 90 ultra-fast DCE-MRI scans were acquired using an mDixon sequence (TE1/TE2/TR=1.5/2.8/4.2 ms, FA=10°, FOV=18×37×8cm3, in-plane resolution=1.5×2.8×3.5mm3, temporal resolution=1.5 s). A low dose (0.015 mmol/kg) of Gadolinium-based contrast agent was injected in ~1.5 s. AIFs were extracted from the iliac arteries. AIF Iodine and Gd concentrations were corrected for the total injected dose (mM/dose). Due to differing temporal resolutions, contrast agent injection durations, and duration of following contrast agent, the AIFs obtained from CT and MRI could not be directly compared. An empirical mathematical model (EMM) was employed to fit the AIFs:
$$AIF(t)=tan^{-1}(t/30)\cdot[1+\sum_{n=1}^2A_{n}exp(-(t-\tau_{n})^{2}/2\sigma_{n}^{2})]\cdot B\cdot exp(-\beta\cdot t)$$
where An and B are scaling constants, $$$\tau_{n}$$$ and σn (n=1,2) are the center and width of a Gaussian function, and β is the decay constant. The resulting EMM’s were used to calculate CT AIF (AIFCT) and MRI AIF (AIFMRI) at temporal resolution of 0.5 s, ending at 8.3 minutes. The AIFMRI was further corrected for the difference in injection times between CT and MR (1.5 s vs. 30 s) by using convolution:
$$AIF_{MRI}^{con}(t)=AIF_{MRI}(t)\otimes\ h(t)$$
where $$h(t)=\frac{M-(t-t_{0})^{2}}{A}$$ is the injection function ( M=196.0,
A=3658.66, t=0 to 28.5 s, t0=14.25 s). The parameters M
and A were determined so that integral
of h(t) is 1 and its width is 28.5 s.
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