SU-E-J-55: Development of a Novel Cableless Radiolucent RF Coil for MR-Guided Radiation Therapy (MRgRT™).

PURPOSE: To demonstrate the feasibility of a novel radiotherapy-compatible cableless radiofrequency (RF) coil for an MR-guided radiation therapy(MRgRT™) system that employs a movable MRI system. This coil technology will expedite clinical workflow by eliminating need for coil connections and cables, allowing RF coils to remain in place for treatment. METHODS: We quantified radiation transmission factors and surface dose changes for aluminum (Al), copper (Cu) and FR4 substrate typical of RF coils using a Varian Trilogy linear accelerator. Ion chamber measurements were performed by applying 6 MV fields of various sizes through sheets of each material and metal-substrate combination. These measurements were repeated with the materials on a standard treatment couch. Material optimization provided input to build a prototype cableless coil and representative coil segment for surface dose measurements. RESULTS: We observed expected patterns for radiation field sizes between 1 cm × 1 cm and 25 cm × 25 cm and expected variation with off-axis distances < 12 cm. All metal-substrate combinations had transmission factors > 0.996, the lowest being Cu-FR4. Relative surface dose increases were similar for Cu-FR4 (3.9) and Al-FR4 (3.3) combinations. Couchtop relative surface dose increases were much greater than coil materials alone and did not increase substantially with the addition of coil materials (couch-only = 6.6, couch with Cu-FR4 = 7.3). Relative surface dose increase was 6.23 for prototype coil segment with capacitor, but the capacitors are not in the primary beam path for the coil design. CONCLUSIONS: Results indicate surface dose effectsare the dominant consideration in RF coil design for MRgRT. Similar Cu and Al surface dose effects suggest Cu is a viable coil inductor material for this application. Given that coil material contribution to surface dose is small compared to the couchtop material it is feasible to keep this cablelesscoil in place during radiation treatment. This work has received research personnel support from IMRIS.

Application of the SUSHI method to the design of gradient coils.

An approach to potential improvements in magnetic field shielding for a gradient coil system with cylindrical geometry is presented, utilizing "supershielding" conditions for the currents on both the primary and the secondary coils. It is demonstrated that the field can be strongly suppressed everywhere outside a cylindrical shield coil radius, even though the finite-length active shield only partially surrounds a primary coil. The supershielding method, which is aimed at controlling eddy currents, still has sufficient freedom to maintain the desired magnetic field behavior inside the imaging volume. The trade-off is an additional primary current oscillation and increased current peaks and field energy. This method has been applied to design short transverse and axial gradient coils, giving substantially improved shielding compared to an apodization method. Magn Reson Med 45:147-155, 2001.

Activation and regulation of interferon regulatory factor 4 in HTLV type 1-infected T lymphocytes.

The human T cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T cell leukemia (ATL), an aggressive and fatal leukemia of CD4(+) T lymphocytes, and is also associated with a neurological demyelinating disease, tropical spastic paraparesis. The oncogenic potential of HTLV-1 resides in the 353-aa, 40-kDa viral Tax oncoprotein, a positive regulator of viral gene transcription. A novel member of the interferon regulatory factor (IRF) family of transcription factors, IRF-4, was shown to be constitutively produced in HTLV-1-infected cells. IRF-4 is transiently expressed in anti-CD3 and PMA/ionomycin-stimulated T lymphocytes but not in continuous non-Tax-expressing T cell lines. In transient coexpression assays, HTLV-1 Tax protein induced the 1. 2-kb IRF-4 promoter, indicating that Tax functions as an indirect trans-activator of the IRF-4 gene. Furthermore, IRF-4 levels in HTLV-1-infected cells appear to be proportional to the level of Tax expression, suggesting a role for IRF-4 in T cell transformation. In an effort to further characterize IRF-4 function, we identified a novel interaction between IRF-4 and FKBP52, a 59-kDa member of the immunophilin family with peptidyl-prolyl isomerase activity (PPIase). IRF-4-FKBP52 association inhibited the interaction between IRF-4 and its DNA-binding partner PU.1, as well as the trans-activation function of IRF-4/PU.1. FKBP52 association resulted in a structural modification of IRF-4, detectable by immunoblot analysis and by IRF-4 partial proteolysis. These results demonstrate a novel posttranslational mechanism of transcriptional control, mediated through the interaction of an immunophilin with a transcriptional regulator.

Finite size disc gradient coil set for open vertical field magnets.

A new analytical approach is used in the design of disc-like gradient coils suitable for magnet geometries with main field direction perpendicular to the surface of the disc. An inverse procedure is used to optimize the coil's characteristics, subject to the restrictions imposed by the desired field behavior over a certain set of constraint points inside a predetermined imaging volume. Excellent agreement between the expected values of the gradient magnetic field and the numerical values generated by applying the Biot-Savart law to a discrete current pattern of the perspective disc coil was found. A Finite Element Analysis package was used to predict the fringe gradient field levels for a non-shielded axial disc coil and for a self-shielded transverse disc coil in the vicinity of the magnet poles. The numerical results indicate that for the self-shielded design the gradient fringe field is 1000 times smaller than the corresponding fringe field for the non-shielded disc case. Also no significant spatial dependence was noticed for the shielded coil's fringe field.

Posttranslational regulation of IRF-4 activity by the immunophilin FKBP52.

Interferon regulatory factor-4 (IRF-4) plays an important role in immunoregulatory gene expression in B and T lymphocytes and is also highly expressed in human T cell leukemia virus type 1 infected cells. In this study, we characterize a novel interaction between IRF-4 and the FK506-binding protein 52 (FKBP52), a 59 kDa member of the immunophilin family with peptidyl-prolyl isomerase activity (PPIase). IRF-4-FKBP52 association inhibited IRF4-PU.1 binding to the immunoglobulin light chain enhancer E(lambda2-4) as well as IRF-4-PU.1 transactivation, effects that were dependent on functional PPIase activity. FKBP52 association also resulted in a structural modification of IRF-4, detectable by immunoblot analysis and by IRF-4 partial proteolysis. These results demonstrate a novel posttranslational mechanism of transcriptional control, mediated through the interaction of an immunophilin with a transcriptional regulator.

Nuclear IkappaBbeta maintains persistent NF-kappaB activation in HIV-1-infected myeloid cells.

Monocytic cells exhibit constitutive NF-kappaB activation upon infection with human immunodeficiency virus-1 (HIV-1). Because IkappaBbeta has been implicated in maintaining NF-kappaB.DNA binding, we sought to investigate whether IkappaBbeta was involved in maintaining persistent NF-kappaB activation in HIV-1-infected monocytic cell lines. IkappaBbeta was present in the nucleus of HIV-1-infected cells and participated in the ternary complex formation with NF-kappaB and DNA. In contrast to uninfected cells, the addition of recombinant glutathione S-transferase-IkappaBalpha protein to preformed NF-kappaB.DNA complexes from HIV-1-infected cell extracts did not completely dissociate the complexes, suggesting that IkappaBbeta may protect NF-kappaB complexes from IkappaBalpha-mediated dissociation. Immunodepletion of IkappaBbeta resulted in an NF-kappaB.DNA binding complex that was sensitive to IkappaBalpha-mediated dissociation, thus demonstrating the protective role of IkappaBbeta. In addition, co-transfection studies with an NF-kappaB-dependent reporter construct demonstrated that IkappaBbeta co-expression partially alleviated inhibition of NF-kappaB-mediated gene expression by IkappaBalpha, implying that IkappaBbeta can maintain transcriptionally active NF-kappaB.DNA complexes. Furthermore, constitutive phosphorylation of IkappaBalpha was observed. Immunoprecipitation of the IkappaB kinase (IKK) complex followed by in vitro analysis of kinase activity demonstrated that IKK was constitutively activated in HIV-1-infected myeloid cells. Thus, virus-induced constitutive IKK activation, coupled with the maintenance of a ternary NF-kappaB.DNA complex by IkappaBbeta, maintains persistent NF-kappaB activity in HIV-1-infected myeloid cells.

A hybrid inverse approach applied to the design of lumped-element RF coils.

A combination of inverse procedures is employed in the design of radio-frequency (RF) coils with specific examples in, but not restricted to, magnetic resonance imaging. The first inverse procedure is the use of functional methods for the optimization of coil characteristics subject to restrictions on the field behavior. Continuous current distributions are derived from analysis of the fields they are required to produce. To make use of these distributions at a desired frequency, the method of moments is applied as a second inverse procedure to a discretized version of the current distribution. The advantage of this hybrid technique is that it provides a computational algorithm for optimization of feeding, tuning, impedance matching and other aspects of RF coil design. A prototype RF coil has been built using the engineering values predicted by the theory. Experimental results including images acquired from the prototype coil are presented.

Association between HTLV-1 Tax and I kappa B alpha is dependent on the I kappa B alpha phosphorylation state.

Biological, molecular, and epidemiological data have demonstrated that human T cell leukemia virus type 1 (HTLV-1) encoded Tax protein plays a central role in the initiation of T cell malignancy. The 40-kDa Tax oncoprotein serves as a potent transcriptional activator that induces viral gene expression driven by the HTLV-1 long terminal repeats and also stimulates multiple cellular genes involved in T cell activation, cell cycle regulation, and gene activation. Since Tax has been shown to interact directly and indirectly with the NF-kappa B/I kappa B regulatory proteins, we examined the significance of an in vivo association between Tax and the I kappa B alpha inhibitor. Using GST affinity chromatography, Tax was shown to interact with the I kappa B alpha ankyrin repeats which are essential for interaction with the NF-kappa B/Rel proteins. In vivo, using I kappa B alpha mutants and co-immunoprecipitation, a preferential interaction between HTLV-1 Tax and N-terminally hypophosphorylated I kappa B alpha was detected. Tax also enhanced binding of I kappa B alpha to the proteasome subunit HsN3, resulting in a Tax-enhanced, constitutive degradation of wild-type and mutated forms of I kappa B alpha in the absence of phosphorylation and ubiquitination. Binding of I kappa B alpha to proteasome subunit HC9 was also observed, but this interaction occurred independently of Tax. Taken together, these results suggest a role for Tax as a viral chaperone resulting in the enhanced constitutive turnover of I kappa B alpha. The association of Tax with hypophosphorylated I kappa B alpha may prevent I kappa B alpha from binding to NF-kappa B and also target I kappa B alpha to the proteasome for degradation via a phosphorylation-independent pathway.

Cellular and viral protein interactions regulating I kappa B alpha activity during human retrovirus infection.

NF-kappa B/Rel transcription factors participate in the activation of numerous genes involved in immune regulation/inflammation including cytokines, cell surface receptors, adhesion molecules, and acute phase proteins. NF-kappa B activity is controlled by inhibitory proteins, I kappa Bs, that maintain the DNA-binding forms of NF-kappa B in an inactive state in the cytoplasm. Many viruses, including the human retroviruses HIV-1 and HTLV-1, also utilize the NF-kappa B/I kappa B pathway to their transcriptional advantage during viral infection. Our recent studies have focused on the I kappa B alpha inhibitor and have characterized several protein interactions that modulate the functional activity of I kappa B alpha during human retrovirus infection. In this article, we summarise recent studies demonstrating that (1) chronic HIV-1 infection of human myelomonoblastic PLB-985 cells leads to constitutive NF-kappa B activity, activated in part due to enhanced I kappa B alpha turnover and increased NF-kappa B/Rel production; (2) HTLV-1 Tax protein physically associates with the I kappa B alpha protein in vivo and in vitro and also mediates a 20- to 40-fold stimulation of NF-kappa B DNA binding activity mediated via an enhancement of NF-kappa B dimer formation; (3) casein kinase II phosphorylates I kappa B alpha at multiple sites in the C-terminal PEST domains and regulates I kappa B alpha function; (4) transdominant forms of I kappa B alpha, mutated in critical Ser or Thr residues required for inducer-mediated (S32A,S36A) and/or constitutive phosphorylation block HIV LTR trans-activation and also effectively inhibit HIV-1 multiplication in a single cycle infection model; and (5) the amino-terminal 55aa of I kappa B alpha (NIK) interacts with the human homologue of dynein light chain 1, a small 9-kDa human homologue of the dynein light chain protein involved in microtubule and cytoskeletal dynamics. Together, our results highlight a number of intriguing molecular interactions between I kappa B alpha and cellular or viral proteins that modulate transcription factor activity and nuclear-cytoplasmic flow of host proteins.

Human T cell leukemia virus type 1 tax protein increases NF-kappa B dimer formation and antagonizes the inhibitory activity of the I kappa B alpha regulatory protein.

Human T cell leukemia virus type 1 (HTLV-1) encodes a strong transcriptional transactivator, the Tax protein, that stimulates viral transcription through the long terminal repeat and also stimulates many cellular genes via the activation of host transcription factors. Previous studies have demonstrated that Tax activates NF-kappa B through binding to the Rel homology domain of NF-kappa B proteins. Tax was also shown to increase degradation of I kappa B alpha resulting in the induction of NF-kappa B DNA binding activity. We addressed the specificity and function of Tax interaction with members of the NF-kappa B/I kappa B alpha family by using EMSA, protein affinity chromatography, protein-protein crosslinking and co-immunoprecipitation assays. The results of the present study demonstrate that: (1) Tax enhances NF-kappa B binding to DNA 40- to 100-fold by increasing NF-kappa B dimer formation which can be detected in the absence of DNA; (2) Tax binds to all NF-kappa B DNA binding subunits in vitro and to I kappa B alpha; (3) Tax physically associates with I kappa B alpha in vivo; and (4) Tax and I kappa B alpha have antagonistic effects on NF-kappa B binding and gene activity. These results suggest that Tax interaction with I kappa B alpha interferes with the formation of NF-kappa B-I kappa B alpha complexes and may play a role in targeting I kappa B alpha for degradation.

Constitutive phosphorylation and turnover of I kappa B alpha in human T-cell leukemia virus type I-infected and Tax-expressing T cells.

Human T-cell leukemia virus type I (HTLV-I) encodes a strong transcriptional activator, Tax, that stimulates transcription indirectly through the viral long terminal repeat and also activates a number of cellular genes via association with host transcription factors. The NF-kappa B/Rel pathway is a target for Tax trans-activation, and Tax has been correlated with increased NF-kappa B-binding activity and NF-kappa B-dependent gene expression in HTLV-I-infected cells. In this study we demonstrate that constitutive phosphorylation and increased turnover of the regulatory I kappa B alpha protein in HTLV-I-infected MT-2 and C8166 cells and Tax-expressing 19D cells contribute to constitutive NF-kappa B-binding activity, which consists primarily of c-Rel, p52(NFKB2), and p50(NFKB1). I kappa B alpha mRNA expression is also increased 7- to 20-fold in these cells, although the steady-state level of I kappa B alpha protein is reduced in HTLV-I-infected and Tax-expressing T cells. These results indicate that the viral Tax protein, by indirectly mediating phosphorylation of I kappa B, may target I kappa B alpha for rapid degradation, thus leading to constitutive NF-kappa B activity.

A simple assay for monitoring the mutagenic effects of 5-methylcytosine deamination in Escherichia coli.

We have developed and tested a simple phenotypic assay which monitors C to T transition mutations at the second C of a CCAGG sequence in the lacZ gene of Escherichia coli. The assay is based on new data concerning amino acid requirements on either side of a crucial active site residue in beta-galactosidase, glutamic acid 461. We show that the frequency of occurrence of the mutation is influenced by two genes: dcm, the cytosine methylase gene, and vsr, one of the genes involved in very short patch repair. The assay has been used to evaluate the function of vsr cloned from a potential very short patch repair mutant.

Predicting RF field penetration in heterogeneous bodies using a 3-D finite element approach: preliminary results.

We present preliminary results for a 3D finite element calculation to evaluate RF penetration in conducting dielectric materials at high field strengths. A tetrahedral mesh is used along with a Coulomb gauge constraint in a finite element method that yields excellent numerical stability at high frequencies. Accuracy is verified by comparisons with analytic solutions for single-layer and multiple-layer heterogeneous systems and for a 3D spherical model. We have also compared the finite element model with experimental results presented by Foo et al., Magn. Reson. Med. 23, (1992). Agreement is very good and argues for the usefulness of the method in the calculation of RF penetration and RF power deposition effects in heterogeneous objects.