An MRI perfusion model incorporating nonequilibrium exchange between vascular and extravascular compartments

Magnetic resonance imaging (MRI) controlled outcome of side effects caused by ionizing radiation, treated with 780 nm-diode laser -- preliminary results

BACKGROUND AND OBJECTIVE

Ionizing radiation therapy by way of various beams such as electron, photon and neutron is an established method in tumor treatment. The side effects caused by this treatment such as ulcer, painful mastitis and delay of wound healing are well known, too. Biomodulation by low level laser therapy (LLLT) has become popular as a therapeutic modality for the acceleration of wound healing and the treatment of inflammation. Evidence for this kind of application, however, is not fully understood yet. This study intends to demonstrate the response of biomodulative laser treatment on the side effects caused by ionizing radiation by means of magnetic resonance imaging (MRI).

STUDY DESIGN/PATIENTS AND METHODS

Six female patients suffering from painful mastitis after breast ionizing irradiation and one man suffering from radiogenic ulcer were treated with lambda=780 nm diode laser irradiation at a fluence rate of 5 J/cm2. LLLT was performed for a period of 4-6 weeks (mean sessions: 25 per patient, range 19-35). The tissue response was determined by means of MRI after laser treatment in comparison to MRI prior to the beginning of the LLLT.

RESULTS

All patients showed complete clinical remission. The time-dependent contrast enhancement curve obtained by the evaluation of MR images demonstrated a significant decrease of enhancement features typical for inflammation in the affected area.

CONCLUSION

Biomodulation by LLLT seems to be a promising treatment modality for side effects induced by ionizing radiation.

Detection of areas with viable remnant tumor in postchemotherapy patients with Ewing's sarcoma by dynamic contrast-enhanced MRI using pharmacokinetic modeling

An approach is presented for monitoring the effects of neoadjuvant chemotherapy in patients with Ewing's sarcoma using dynamic contrast-enhanced perfusion magnetic resonance (MR) images. For that purpose, we modify the three-compartment pharmacokinetic permeability model introduced by Tofts et al. (Magn Reson Med 1991;17:357-67) to a two-compartment model. Perfusion MR images acquired using an intravenous injection with Gadolinium (Gd-DTPA) are analyzed with this two-compartment pharmacokinetic model as well as the with an extended pharmacokinetic model that includes the (local) arrival time t(0) of the tracer as an endogenous (estimated) parameter. For each MR section, a wash-in parameter associated with each voxel is estimated twice by fitting each of the two pharmacokinetic models to the dynamic MR signal. A comparison of the two wash-in parametric images (global versus local arrival time) with matched histologic macroslices demonstrates a good correspondence between areas with viable remnant tumor and a high wash-in rate. This can be explained by the high number and permeability of the (leaking) capillaries in viable tumor tissue. The novel pharmacokinetic model based on a local arrival time of tracer results in the best fit of the wash-in rate, the most important factor discerning viable from nonviable tumor components. However, parameter estimates obtained with this model are also more sensitive to noise in the MR signal. The novel pharmacokinetic model resulted in a sensitivity between 0.22 and 0.60 and a specificity between 0.61 and 1. The model based on a global arrival time gave sensitivities between 0.33 and 0.77 and specificities between 0.58 and 0.99. Both statistics are computed as the fraction of correctly labeled voxels (viable or nonviable tumor) within a specified ROI, which delineates the tumor. We conclude that the added value of estimating the local arrival time of tracer first manifests itself for moderate noise levels in the MR signal. The novel pharmacokinetic model should moreover be preferred when pharmacokinetic modeling is applied on the average signal intensity within a ROI, where noise has less effect on the fitted parameters.

Effects of 780 nm diode laser irradiation on blood microcirculation: preliminary findings on time-dependent T1-weighted contrast-enhanced magnetic resonance imaging (MRI)

Laser therapy by low light doses shows promising results in the modulation of some cell functions. Various clinical studies indicate that laser therapy is a valuable method for pain treatment and the acceleration of wound healing. However, the mechanism behind it is still not completely understood. To explore the effect of a low-power diode laser (lambda = 780 nm) on normal skin tissue, time-dependent contrast enhancement has been determined by magnetic resonance imaging (MRI). In the examinations, six healthy volunteers (four male and two female) have been irradiated on their right planta pedis (sole of foot) with 5 J/cm2 at a fluence rate of 100 mW/cm2. T1-weighted magnetic resonance imaging is used to quantify the time-dependent local accumulation of Gadolinium-DPTA, its actual content in the local current blood volume as well as its distribution to the extracellular space. Images are obtained before and after the application of laser light. When laser light is applied the signal to noise ratio increases by more than 0.35 +/- 0.15 (range 0.23-0.63) after irradiation according to contrast-enhanced MRI. It can be observed that, after biomodulation with light of low energy and low power, wound healing improves and pain is reduced. This effect might be explained by an increased blood flow in this area. Therefore, the use of this kind of laser treatment might improve the outcome of other therapeutic modalities such as tumour ionizing radiation therapy and local chemotherapy.

Magnetic resonance imaging verification of a multi-compartment perfusion model for a chromatography gel phantom

A model of MRI signal intensity was developed for chromatography column phantoms containing Sephadex gels, which were used to simulate tissue perfusion and the exchange of protons between extravascular and intravascular tissue compartments. Computer simulations of two-compartment and three-compartment models were compared in experiments that used two chromatography columns. One column contained a Sephadex G-25 gel that had a smaller pore size and a slower rate of exchange between extrabead and intrabead compartments than did a second column, which contained a G-50 gel with a larger pore size. In both columns the beads were of approximately the same size to simulate the same ratio of intrabead volume to extrabead volume. Slice-selective spin-echo experiments were performed with the columns oriented parallel to each other in a 1.5 T imager and parallel to the magnetic field, with water flowing through each column perpendicular to the slice at mean flow velocities ranging between 0 and 0.66 mm/s. The results of the experiments agreed with computer simulations, which showed that the MRI signal intensity in the perfused columns is a function of the rate of exchange between extrabead and intrabead compartments. The exchange process modifies the transit time of protons passing through an excited region. The simulations also showed that both two-compartment and three-compartment models could be used to fit the experimental data. Experiments with chromatography gel columns are a preliminary investigation into the potential of using MRI to measure the exchange between extravascular and intravascular compartments of endogenous protons during blood perfusion of biological tissue.

Inversion recovery EPI of bolus transit in rat myocardium using intravascular and extravascular gadolinium-based MR contrast media: dose effects on peak signal enhancement

Inversion recovery gradient recalled echo planar imaging (TI/TR/TE = 700/2000/10 ms) was used to dynamically monitor the first pass of an intravascular (GdDOTA-polylysine) and an extravascular (GdDTPA-BMA) contrast agent through normal rat myocardium. It was found that myocardial enhancement increased with dose of the intravascular agent to a limiting value of approximately 50% of fully relaxed intensity, consistent with enhancement of 40% of myocardial water content during the first pass. Larger doses produced no further increase in peak response. On the other hand, the extravascular agent caused incrementally increased enhancement throughout the dose range examined to a final value of 68 +/- 2% of fully relaxed intensity. The profile of dose dependence for both agents was inconsistent with monoexponential T1 relaxation. It was concluded that: (a) compartmentalization of myocardial water combined with restricted myocardial water diffusion limits the peak response during bolus transit; (b) extraction of the extravascular agent during transit elevates the peak response over that obtained from agent confined to the vascular volume; and (c) models that assume simple monoexponential T1 relaxation to derive time-density curves do not adequately describe the relationship between changes in signal intensity, R1 and contrast concentration.

An evaluation of the sensitivity of the intravoxel incoherent motion (IVIM) method of blood flow measurement to changes in cerebral blood flow

To evaluate the sensitivity of the intravoxel incoherent motion (IVIM) technique to changes in cerebral blood flow, we made measurements of IVIM parameters in rat brain under conditions of altered arterial pCO2. The arterial pCO2 was varied over a range which would be expected to change cerebral blood flow from roughly 50 to 500 ml/(100 g.min). The IVIM measurements were made with suppression of extravascular water signal. The parameters f' (the apparent fraction of spins which have "fast" pseudodiffusion), D1* (the "fast-pseudodiffusion") coefficient), and D2* (the "slow-pseudodiffusion" coefficient) all showed statistically significant positive linear correlations with arterial pCO2. These results suggest that the IVIM method, when used with suppression of extravascular water signal, is sensitive to changes in blood flow.

The capillary network: a link between IVIM and classical perfusion

MR measurements based on motion encoding gradients, such as intravoxel incoherent motion imaging, could provide, in principle, information on flowing blood volume and blood velocity. This note shows that, in addition, the knowledge of the capillary network organization may provide a link between these measurements and those obtained by conventional and MR perfusion techniques based on tracer uptake by tissues.