Table 3.

Technicalities in MRI ocular drug-delivery study and their comparison: a summary

MRI pulse sequence	Spin-echo (SE) straightforward T1 and T2 contrast relatively poor SNR long imaging time	Gradient-echo (GRE) fast imaging using steady-state and low flip angle local non-uniformity of magnetic field mixed with T1 and T2* contrast
Signal to concentration calculation	T1 and T2 mappings to determine concentration require model fitting of the data not practical because of the long scan time independent of hardware variability such as RF field uniformity	MR signal to concentration direct conversion via calibration possible errors due to RF coil positioning signal depends on both T1 and T2 affected by hardware variability direct approach
MRI parameters related to spatial resolution	Imaging field of view, readout matrix, slice thickness determine spatial resolution vs SNR	Number of image slices increases MRI coverage too many slices will increase scan time
MRI parameters directly related to signal	TR, TE determine T1 or T2 contrast affect signal intensity need TR and TE for optimal contrasts and acquisition efficiency	Signal averages increase SNR increase scan time and decrease temporal resolution
Coil	Surface coil higher SNR lacks signal homogeneity variation due to inhomogeneous spatial sensitivity of coil	Volume coil increased RF field homogeneity increased signal uniformity lower SNR from eye
MRI system	Clinical whole-body scanner fits large animals stable and well-tuned imaging sequences limitation for high-resolution imaging less friendly to custom-made hardware, such as RF coil for non-proton MRI/MRS can be used for humans	Animal scanner suitable for small animals usually higher magnetic field more hardware flexibility
Animal model	Large animals may not fit into the bore of a scanner can be difficult to handle eye dimensions are more representative of human eye	Small animals such as rodents easy to handle small eyes not representative of human eye
Contrast agent considerations	Physicochemical properties relative to the drug of interest molecular size molecular charge lipophilicity	MR properties and biological effects MR signal enhancement specific tissue interactions tissue binding tissue MR signal and contrast concentration to MR signal calibration