An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (> or = 1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B(1) field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.Peer reviewed: YesNRC publication: Ye

A multifrequency narrow band-pass filter

NRC publication: Ye

A comparison of MR imaging of a mouse model of glioma at 0.2T and 9.4T

Both 0.2 T and 9.4 T MRI systems were used to image a mouse model of glioma. RF coils were designed for both fields. A spin-echo, multi-echo pulse sequence was used to determine T2 relaxation times of both brain and tumor tissues. Contrast-to-noise ratio was calculated based on the selected echo time. The results showed that 0.2 T is suitable for mouse model imaging, however total scan time must be long to achieve high enough SNR. T2 relaxation times of the tumor and brain tissues can be measured at 0.2 T and are 2.1 and 1.8 times respectively longer at 0.2 T than at 9.4 T. Contrast to noise ratio for tumor and brain was better at high field than at the low field. We concluded that 0.2 T may be used to study mouse model of glioma using spin echo pulse sequence, yet the total scan time is long (about 40 min), resolution is lower ( 3c250 \u3bcm 7 250 \u3bcm) and slice thickness (3 mm) must be large enough to obtain sufficient SNR.Peer reviewed: YesNRC publication: Ye

Applications of Nanoparticles for MRI Cancer Diagnosis and Therapy

Recent technological advances in nanotechnology, molecular biology, and imaging technology allow the application of nanomaterials for early and specific cancer detection and therapy. As early detection is a prerequisite for successful treatment, this area of research has been rapidly growing. This paper provides an overview of recent advances in production, functionalization, toxicity reduction, and application of nanoparticles to cancer diagnosis, treatment, and treatment monitoring. This review focuses on superparamagnetic nanoparticles used as targeted contrast agents in MRI, but it also describes nanoparticles applied as contrasts in CT and PET. A very recent development of core/shell nanoparticles that promises to provide positive contrast in MRI of cancer is provided. The authors concluded that despite unenviable obstacles, the progress in the area will lead to rapidly approaching applications of nanotechnology to medicine enabling patient-specific diagnosis and treatment

Applications of Nanoparticles for MRI Cancer Diagnosis and Therapy

Recent technological advances in nanotechnology, molecular biology, and imaging technology allow the application of nanomaterials for early and specific cancer detection and therapy. As early detection is a prerequisite for successful treatment, this area of research has been rapidly growing. This paper provides an overview of recent advances in production, functionalization, toxicity reduction, and application of nanoparticles to cancer diagnosis, treatment, and treatment monitoring. This review focuses on superparamagnetic nanoparticles used as targeted contrast agents in MRI, but it also describes nanoparticles applied as contrasts in CT and PET. A very recent development of core/shell nanoparticles that promises to provide positive contrast in MRI of cancer is provided. The authors concluded that despite unenviable obstacles, the progress in the area will lead to rapidly approaching applications of nanotechnology to medicine enabling patient-specific diagnosis and treatment.Peer Reviewe

Contrast Enhancement in MRI Using Combined Double Action Contrast Agents and Image Post-Processing in the Breast Cancer Model

Gd- and Fe-based contrast agents reduce T1 and T2 relaxation times, respectively, are frequently used in MRI, providing improved cancer detection. Recently, contrast agents changing both T1/T2 times, based on core/shell nanoparticles, have been introduced. Although advantages of the T1/T2 agents were shown, MR image contrast of cancerous versus normal adjacent tissue induced by these agents has not yet been analyzed in detail as authors considered changes in cancer MR signal or signal-to-noise ratio after contrast injection rather than changes in signal differences between cancer and normal adjacent tissue. Furthermore, the potential advantages of T1/T2 contrast agents using image manipulation such as subtraction or addition have not been yet discussed in detail. Therefore, we performed theoretical calculations of MR signal in a tumor model using T1-weighted, T2-weighted, and combined images for T1-, T2-, and T1/T2-targeted contrast agents. The results from the tumor model are followed by in vivo experiments using core/shell NaDyF4/NaGdF4 nanoparticles as T1/T2 non-targeted contrast agent in the animal model of triple negative breast cancer. The results show that subtraction of T2-weighted from T1-weighted MR images provides additional increase in the tumor contrast: over two-fold in the tumor model and 12% in the in vivo experiment

Validation of Inner, Second, and Outer Sphere Contributions to T₁ and T₂ Relaxation in Gd³⁺-Based Nanoparticles Using Eu³⁺ Lifetime Decay as a Probe

Paramagnetic lanthanide-based NPs are currently designed as magnetic resonance imaging (MRI) contrast agents to obtain optimal relaxivities at high magnetic fields of 7, 9.4, and 11.7 T where human imaging has been possible yielding high contrast to noise ratio in the MR images compared to the clinical field of 3 T. However, the underlying longitudinal (T₁) and transverse (T₂) relaxation mechanisms of the NP-based contrast agents based on the spatial motion and proximity of water protons with respect to the paramagnetic ions on the surface of NPs are still not well understood, specifically, in terms of contributions from inner, second, and outer spheres of coordination of water molecules to the NPs. Gd³⁺-based NPs, e.g., NaGdF₄, are promising T₁ contrast agents owing to the paramagnetic Gd³⁺ possessing a symmetric ⁸S_7/2-state and slow electronic relaxation relevant to its efficiency to produce a positive (T₁) contrast. Here, water-dispersed NaGdF₄:Eu³⁺ (3 nm diameter, TEM) and NaYF₄–NaGdF₄:Eu³⁺ core–shell NPs (18.3 nm core diameter with 0.5 nm thick shell, TEM) were studied for their <i>r</i>₁ and <i>r</i>₂ relaxivities at 9.4 T. Excited state lifetime decays of Eu³⁺ dopants, which are highly sensitive to proximate water molecules, were analyzed, demonstrating a dominance of inner and second sphere contribution over outer sphere to the T₁ and T₂ relaxations in smaller NaGdF₄:Eu³⁺ NPs while exclusively outer sphere in NaYF₄–NaGdF₄:Eu³⁺ core–shell NPs

NaDyF4 nanoparticles as T2 contrast agents for ultrahigh field magnetic resonance imaging

A major limitation of the commonly used clinical MRI contrast agents (CAs) suitable at lower magnetic field strengths (<3.0 T) is their inefficiency at higher fields (>7 T), where next-generation MRI scanners are going. We present dysprosium nanoparticles (\u3b2-NaDyF4 NPs) as T2 CAs suitable at ultrahigh fields (9.4 T). These NPs effectively enhance T2 contrast at 9.4 T, which is 10-fold higher than the clinically used T2 CA (Resovist). Evaluation of the relaxivities at 3 and 9.4 T show that the T2 contrast enhances with an increase in NP size and field strength. Specifically, the transverse relaxivity (r2) values at 9.4 T were 64 times higher per NP (20.3 nm) and 6 times higher per Dy3+ ion compared to that at 3 T, which is attributed to the Curie spin relaxation mechanism. These results and confirming phantom MR images demonstrate their effectiveness as T2 CAs in ultrahigh field MRIs.Peer reviewed: YesNRC publication: Ye

Cation Exchange: A Facile Method To Make NaYF₄:Yb,Tm-NaGdF₄ Core–Shell Nanoparticles with a Thin, Tunable, and Uniform Shell

Cation exchange was performed on up-conversion NaYF₄:Yb,Tm nanoparticles, resulting in NaYF₄:Yb,Tm-NaGdF₄ core–shell nanoparticles as indicated by electron energy-loss spectroscopy 2D mapping. Results show that core–shell nanoparticles with a thin, tunable, and uniform shell of subnanometer thickness can be made via this cation exchange process. The resulting NaYF₄:Yb,Tm-NaGdF₄ core–shell nanoparticles have an enhanced up-conversion intensity relative to the initial core nanoparticles. As potential magnetic resonance imaging (MRI) contrast agents, they were tested for their proton relaxivities. The r₁ relaxivity per Gd³⁺ ion of the nanoparticles with a thin NaGdF₄ shell (ca. 0.6 nm thick) measured at 9.4 T was found to be 2.33 mM^–1·s^–1. This r₁ relaxivity is among the highest in all the reported NaYF₄–NaGdF₄ core–shell nanoparticles. The r₁ relaxivity per nanoparticle is 1.56 × 10⁴ mM^–1·s^–1, which is over 4000 times higher than commercial Gd³⁺-complexes. The very high proton relaxivity per nanoparticle is critical for targeted MRI as such nanoparticles provide strong contrast even in low concentrations. The presented cation exchange method is a promising way to manufacture core–shell nanoparticles with up-conversion NaYF₄:Yb,Tm core and paramagnetic NaGdF₄ shell for bimodal imaging, i.e. MR and optical imaging

Detection of T2 changes in an early mouse brain tumor

The aim of the study was to determine the effect of early tumor growth on T2 relaxation times in an experimental glioma model. A 9.4-T magnetic resonance imaging (MRI) system was used for the investigations. An animal model (n=12) of glioma was established using an intracranial inoculation of U87MGdEGFRvIII cells. The imaging studies were performed from Day 10 through Day 13 following tumor inoculation. Tumor blood vessel density was determined using quantitative immunochemistry. Tumor volume was measured daily using MR images. T2 values of the tumor were measured in five areas across the tumor and calculated using a single exponential fitting of the echo train. The measurements on Days 10 and 13 after tumor inoculation showed a 20% increase in T2. The changes in T2 correlated with the size of the tumor. Statistically significant differences in T2 values were observed between the edge of the tumor and the brain tissue on Days 11, 12 and 13 (P=.014, .008, .001, respectively), but not on Day 10 (P=.364). The results show that T2-weighted MRI may not detect glioma during an early phase of growth. T2 increases in growing glioma and varies heterogenously across the tumor.Peer reviewed: YesNRC publication: Ye