Dielectric shading artifacts in high field Magnetic Tomography systems

Dielectric shading artifacts in high field Magnetic Tomography systems

Poster No.: C-0027

Congress: ECR 2018

Type: Educational Exhibit

Authors: V. G. Syrgiamiotis1, V. Maliakas2, T. Thomas2, L. G. Astrakas2, M. Argyropoulou2, A. Plousi1; 1Athens/GR, 2Ioannina/GR

Keywords: Radiographers, MR physics, MR, Education, Physics, Technical

aspects, Education and training, Artifacts

DOI: 10.1594/ecr2018/C-0027

Learning objectives

Interpretation of dielectric shadow artifact - occurring due to the dielectric phenomenon and one of the disadvantages of imaging in high field magnetic resonance imaging systems (# 3.0 Tesla). The term dielectric effect refers to the interaction of matter – and in this case of biological tissues and materials - with the coexisting electric field (E1) of an electromagnetic field.

Background

Magnetic and Electrical Fields in MRI

In the MRI, together with the B1 of the RF coexists an electric field (E1). As described by the Maxwell equations, the fields B1 and E1 oscillate vertically with each other in the direction of wave propagation. When electromagnetic waves meet the human body, various phenomena occur

The main ones are the following:

(i) wavelength reduction

(ii) electrical current generation and

(iii) somewhat reflection / refraction of wave during the transition between different tissues.

In all of the images a central area of reduced intensity ("dark area") is observed due to the dielectric phenomenon, which develops strongly due to the strong magnetic field (3T) and the presence of the cystic large structure, which is a tissue with high conductivity, the creation of stagnant waves and the intense appearance of the phenomenon.

vii, viii and ix: T2 images TSE sequences at transverse, sagittal and frontal plane. (MT 1.5 Tesla University hospital of Ioannina)

Images for this section:

Fig. 1: The diagram shows three phases of a transmitted linearly polarized wave emitted from left to right with the same wave equations, where E = E0 sin (-#t + k#r) and B = B0sin (-#t + k#r). © https://en.wikipedia.org – modified

Fig. 2: #n magnetic fields of intensity #1.5 T, RF wavelengths are larger compared to body size. As the intensity of B0 increases, the resonance frequency of B1 increases and wavelengths become equal or smaller than the anatomical regions we wish to depict. © https://en.wikipedia.org – modified

Fig. 3: i, ii and iii: T2 images TSE sequences at transverse, sagittal and frontal levels. (MT 3.0 Tesla University Hospital of Ioannina) iv, v and vi: T1 DIXON images in the transverse plane without suppressing the fat signal and suppressing the fat signal before and after the i.v. the granting of NSA. (MT 3.0 Tesla University hospital of Ioannina). © University Hospital of Ioannina

Fig. 4: x, xi and xii: T1 TSE images at transverse level without suppressing the fat signal, T1 TSE on sagittal and transverse level by suppressing the fat signal after i.v. administration of a NSA (MT 1.5 Tesla WNI). In all of the images there is a central area of reduced intensity ("dark area") due to the dielectric phenomenon, which develops due to the presence of the cystic large structure, which is a tissue with high conductivity, which intensifies the creation of stagnant waves and the intense appearance of the phenomenon. The dielectric effect on 1.5 T magnetic resonance imaging is significantly milder compared to the corresponding effect in the 3.0 T magnetic resonance imaging. © University Hospital of Ioannina.

Findings and procedure details

The degree to which matter interacts with electric and magnetic fields can be described by three parameters: i) Magnetic permeability (μ).(ii) Electrical permeability (e).iii) Electrical conductivity (#).For weakly conductive dielectric materials such as those of the human body, the internal RF field is disturbed by a jump current (JC) and displacement current flux (JD) that can be described by Ampère's law with the Maxwell correction # X B = μJC + μJD = μ#E + i##EIn human tissues, the RF frequencies used in JC and JD are

of the same order of magnitude. As the frequency increases, the dielectric term (JD) becomes more important. Even in 7.0T (300 MHz) static field magnetic field tomography, the conduction / displacement current ratios for fat, gray matter, muscles and CSF are about 0.4, 0.6, 0.7 and 1.7 respectively.

To avoid the heterogeneous distribution of RF in the anatomical area under consideration, many #3.0T magnetic resonance imaging systems use the "MultiTransmit" technology, which catalyzes the prevention of dielectric phenomenon expression as a dielectric shadow

In 3.0T systems without Multi Transmit technology, dielectric shading effects may occur depending onthe patient, particularly in body imaging. To avoid this, it is recommended to use the body-coordinated CLEAR technique - a B1 special filter designed to eliminate the inconsistencies in the body imaging.

Images for this section:

Fig. 5: MultiTransmit" technology. Left without and Right with "MultiTransmit" to avoid dielectric shading. © https://en.wikipedia.org – modified.

Conclusion

Dielectric phenomena and associated pseudo-images are progressively more important and more complex as the field strength increases. With MultiTransmit technology, the power, range, phase and waveform of all RF sources are automatically adjusted for optimal homogeneity in each

patient's unique anatomy. In this way, MultiTransmit technology provides optimal signal homogeneity and stability

Personal information

V.G.Syrgiamiotis RT,BSc,MSc,PhD(c) ,EFRS Treasurer, Lab assistant ATEI of Athens, General Childrens hospital of Athens Agia Sophia CT-MRI Department

Thomas Theofanis RT,BSc,MSc (c) ,University hospital of Ioannina

Maliakas Vasileios RT,BSc,MSc,PhD(c) ,University hopsital of Ioannina

Loukas Astrakas Assistant Professor Medical Physists ,University hospital of Ioannina

Argyropoulou Maria Professor Radiology , University hospital of Ioannina

Ploussi Agapi Medical Physist ,BSc,MSc,PhD(c) University hospital of Athens , Attiko

References

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