High resolution MRI of MCF7 human breast tumors: complemented use of iron oxide microspheres and Gd-DTPA

Relaxometry model of strong dipolar perturbers for balanced-SSFP: application to quantification of SPIO loaded cells

Magnetic resonance microscopy using magnetically labeled cells is an emerging discipline offering the potential for non-destructive studies targeting numerous cellular events in medical research. The present work develops a technique to quantify superparamagnetic iron-oxide (SPIO) loaded cells using fully balanced steady state free precession (b-SSFP) imaging. An analytic model based on phase cancellation was derived for a single particle and extended to predict mono-exponential decay versus echo time in the presence of multiple randomly distributed particles. Numerical models verified phase incoherence as the dominant contrast mechanism and evaluated the model using a full range of tissue decay rates, repetition times, and flip angles. Numerical simulations indicated a relaxation rate enhancement (DeltaR(2b)=0.412 gamma . LMD) proportional to LMD, the local magnetic dose (the additional sample magnetization due to the SPIO particles), a quantity related to the concentration of contrast agent. A phantom model of SPIO loaded cells showed excellent agreement with simulations, demonstrated comparable sensitivity to gradient echo DeltaR(*) (2) enhancements, and 14 times the sensitivity of spin echo DeltaR(2) measurements. We believe this model can be used to facilitate the generation of quantitative maps of targeted cell populations.

Synchrotron microangiography with no contrast agent

Coherent x-rays from synchrotron sources are increasingly used in non-conventional radiological techniques ('phase-contrast' radiology). Our experiments demonstrate that by using white (unmonochromatic) radiation and a time-resolving system, it is possible to image microscopic details of moving blood vessels in different live animals without using any contrast agent. The images have excellent contrast plus unprecedented spatial resolution for microangiography (< 10 microm). This result is likely to impact many different areas of biological and medical research and of diagnostic radiology.

The effects of renal variation upon measurements of perfusion and leakage volume in breast tumours

Dynamic contrast enhanced MRI (DCE-MRI) and pharmacokinetic models have been used to measure tumour permeability (K(trans)) and leakage volume (ve) in numerous studies. The construction of pharmacokinetic models describing such tissue properties relies on defining the blood plasma concentration of contrast agent with respect to time (Cp(t)). When direct measurement is not possible a bi-exponential decay has been applied using data from healthy volunteers. This work investigates, by simulation, the magnitude of errors resulting from this definition with respect to normal variation in renal function and for cases with renal impairment. Errors up to 23% in ve and 28% in K(trans) were found for the normal simulations, and 67% in ve and 61% in K(trans) for the impaired simulations. If this bi-exponential curve is used as an input function to the generalized kinetic model and used in oncology, estimates of tissue permeability and leakage volume will possess large errors due to variation in Cp(t) curves between subjects.

Application of the static dephasing regime theory to superparamagnetic iron-oxide loaded cells

The relaxation rates of iron-oxide nanoparticles compartmentalized within cells were studied and found to satisfy predictions of the static dephasing (SD) regime theory. THP-1 cells in cell culture were loaded using two different iron-oxide nanoparticles (superparamagnetic iron-oxide (SPIO) and ultrasmall SPIO (USPIO)) with four different iron concentrations (0.05, 0.1, 0.2, and 0.3 mg/ml) and for five different incubation times (6, 12, 24, 36, and 48 hr). Cellular iron-oxide uptake was assessed using a newly developed imaging version of MR susceptometry, and was found to be linear with both dose and incubation time. R(2)* sensitivity to iron-oxide loaded cells was found to be 70 times greater than for R(2), and 3100 times greater than for R(1). This differs greatly from uniformly distributed nanoparticles and is consistent with a cellular bulk magnetic susceptibility (BMS) relaxation mechanism. The cellular magnetic moment was large enough that R(2)' relaxivity agreed closely with SD regime theory predictions for all cell samples tested [R(2)'=2 pi/(9 x the square root of 3) x gamma LMD] where the local magnetic dose (LMD) is the sample magnetization due to the presence of iron-oxide particles). Uniform suspensions of SPIO and USPIO produced R(2)' relaxivities that were a factor of 3 and 8 less, respectively, than SD regime theory predictions. These results are consistent with theoretical estimates of the required mass of iron per compartment needed to guarantee SD-regime-dominant relaxivity. For cellular samples, R(2) was shown to be dependent on both the concentration and distribution of iron-oxide particles, while R(2)' was sensitive to iron-oxide concentration alone. This work is an important first step in quantifying cellular iron content and ultimately mapping the density of a targeted cell population.