Evaluation of magnetic resonance acoustic noise in 1.5 and 3 Tesla scanners

Transcription

1 Evaluation of magnetic resonance acoustic noise in 1.5 and 3 Tesla scanners Poster No.: B-1014 Congress: ECR 2016 Type: Scientific Paper Authors: V. M. F. Silva, I. M. Ramos, J. Moreira, M. Marques; Porto/PT Keywords: Professional issues, MR, Safety Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 13

2 Purpose To perform a magnetic resonance imaging (MRI) exam, a patient is subjected to three types of electromagnetic fields (EMF): A static magnetic field (SMF) (B 0 ) with a given field strength; Time-varying gradient fields (TVGF), with variable intensity; Radiofrequency (RF) (B 1 ) fields, which function as a transmitter and receiver system. There are hazards and effects from all of the three sources listed above, which could affect both patients and workers. One of those hazards is acoustic noise, mainly due to TVGF. As the gradient coils are rapidly switched on and off during image acquisition, a significant amount of acoustic noise will be generated in the core of magnet. This noise occurs during rapid alterations of currents within the gradient coils. Currents, in a presence of a strong SMF produce significant forces, that act upon TVGF, producing acoustic noise. These forces are named Lorenz forces, which deform and vibrate the gradient coils. The latter will impact among their mountings, causing acoustic noise, which may be different from a type of sequence to another, according to the selection of image parameters selected by MRI radiographers, SMF field strength and TVGF slew rate. Depending on noise levels, hearing damage, anxiety, verbal communication and psychological distress can occur, mainly if exposure to MRI examinations is too long. There are noise levels imposed by Directive no. 2003/10/EC of European Parliament transposed to Portuguese Decree Law no. 182/2006. Those levels can be divided into equivalent continuous sound level, averaged over 8 hours - L ex, 8h - expressed in db(a) and peak sound pressure level - L Cpeak - expressed in db(c), differentiating in action (upper and lower) and exposure values (see Table 1). This study aimed to evaluate L ex, 8h - db(a) and L Cpeak - db(c) of different sequences used in brain MRI in two equipments with different SMF strength Tesla (T) and 3T, analyzing if acoustic noise exposure values exceed the recommended limits set by the Directive and Portuguese legislation. Images for this section: Page 2 of 13

3 Table 1: Exposure limit values and upper and lower action values from Directive 2003/10/ EC transposed to Portuguese Decree Law no. 182/2006. Page 3 of 13

4 Methods and materials This study was performed in MRI Department of Centro Hospitalar São João, EPE, Oporto, Portugal. Two MRI scanners were used: A Siemens Magnetom Symphony with a SMF of 1.5 Tesla (T), with a slew rate up to 125 T/m/s and a gradient field strength up to 30 mt/m; A Siemens Magnetom Trio with a 3T SMF, with a slew rate up to 200 T/m/s and a gradient field strength up to 45 mt/m. A Bruel & Kaejer sound level meter, model 2250, was used to determine L ex, 8h - db(a) and L Cpeak - db(c) exposure values using ten types of MRI brain sequences: T1-weighted spin echo (SE), spoiled gradient echo (SPGR) and inversion-recovery (IR) sequences; proton density (PD) and T2-weighted turbo spin echo (TSE), gradient echo (GE), FLAIR (Fluid Attenuated Inversion Recovery) sequences; echo planar imaging (EPI) sequences used in diffusion and perfusion images and 3D time-of-flight (TOF) sequences. All of sequences were acquired on z-axis (axial or transversal images). Parameters of each sequence are displayed on Table 2 for both scanners used. It was used ISO 9612:2009 methodology (Determination of occupational noise exposure - Engineering method) to determine noise exposure. This standard suggests that the duration of each measurement shall be long enough to represent the average equivalent continuous sound pressure level for the task. The duration of each measurement shall at least be 5 minutes. In order to prove the reproducibility of data, one must perform 3 different measurements (cycles of acoustic noise) of each sequence to be representative of the whole task. Therefore, it was made three measuremens with a minimum of 5 minutes duration for each sequence listed above. Any difference #3 db from a sample to another, in the same sequence, required the repetition of three more measurements (not found in any of the measurements made in this study). A spherical phantom simulating patient's head and a phased array head coil were used to simulate MRI brain acquisition of a patient. The microphone's sound meter level was positioned at the distal end of patient's table connected to the sound level meter via a double shielded compatible cable, placed on the direction of z-axis. Due to ferromagnetic and electrical components, the sound level meter was out of examinations room (see figure 1). After data collection, they were analyzed through BZ-5503 Multifunctional Program for Acoustic Noise Analysers (see figure 2). Page 4 of 13

5 Images for this section: Fig. 1: Positioning of the sound level meter (via a double shielded compatible cable) in a MRI scanning room (in this case, a MRI Siemens Magnetom Symphony equipment). Note the spherical phantom into a phased-array head coil. Page 5 of 13

6 Fig. 2: Screenshot of BZ-5503 Multifunctional Program for Acoustic Noise Analysers used in this study. Page 6 of 13

7 Table 2: Main parameters of image acquisition on the two scanners used in this study for the ten sequences used. Page 7 of 13

8 Results To analyzed data collected, two different parameters were used as indicators of exposure to acosustic noise in MRI scanners: equivalent continuous sound level (L Aeq ), calculating the L ex, 8h in db(a) through the L Aeq obtained. Peak sound pressure level - L Cpeak was collected in each measurement. Obtained data was compared to those recommended in the Directive 2003/10/EC transposed to Portuguese legislation (see Table 1). There is only occupational legislation to acoustic noise exposure, not for common people, such as patients. So, acquired data was compared to legal occupational data. According to Table 3 and Table 4, we see that L Aeq values are very high in most of cases, but none of them exceed imposed limits of EC Directive and Portuguese legislation for L ex, 8h because values are influenced by exposure time (in this case, time duration of each sequence) and calculated by an average of 8 hours exposure. Looking to time exposure of each sequence and L ex, 8h values acquired in each measurement, we can assume that none of the studied sequences exceed legal values. Analyzing both tables (3 and 4) simultaneously, we can see that: 3D sequences, such as 3D TOF and T1 SPGR, have higher exposure level (L ex, 8h and L cpeak ) values when compared to other brain sequences, such as 2D sequences; EPI sequences have higher values (L ex, 8h and L cpeak ) when compared to other conventional sequences used on brain MRI, such as SE and IR sequences; Perfusion and diffusion (EPI) sequences have higher values of L Aeq when we compare to 3D TOF, although 3D TOF have higher L ex, 8h values. This fact is explained by the duration time of the sequence; IR sequences have lower L ex, 8h and L cpeak values; 3D TOF sequence has higher L ex, 8h values, while EPI sequences have higher L cpeak values. In Siemens Magnetom Symphony 1.5T, L ex, 8h ranged from 58.6 db(a) in T2 FLAIR to 78.7 db(a) in 3D TOF sequence. L Cpeak ranged from db(c) in T2 FLAIR to db(c) in EPI sequence used in brain perfusion. Analyzing Table 3, we can see that 3D sequences, such as 3D TOF and T1 spoiled gradient echo (SPGR), and EPI sequences have higher values of L ex, 8h and L cpeak, instead of IR and TSE sequences, such as T2 FLAIR, T1 IR, T2 TSE and PD+T2 (dual echo sequence) TSE, respectively. Page 8 of 13

9 In Siemens Magnetom Trio 3T, L ex, 8h ranged from 47.4 db(a) in T2-weighted TSE to 63.8 db(a) in in 3D TOF sequence. L Cpeak ranged from db(c) in T1 IR to db(c) in EPI sequence used in diffusion-weighted brain imaging. Analyzing Table 4, we can see that 3D and EPI sequences have higher values of L ex, 8h and L cpeak, the same happened in 1.5T MRI scanner. Lower values of L ex, 8h can be seen in IR and TSE sequences. A different result found in 3T scanner in lower L ex, 8h and L cpeak values is what we found in T2 weighted gradient echo (GE) sequence. Data obtained in the 3T scanner showed that L ex, 8h and L cpeak values are lower by about 15 db in most of sequences. Images for this section: Table 1: Exposure limit values and upper and lower action values from Directive 2003/10/ EC transposed to Portuguese Decree Law no. 182/2006. Page 9 of 13

10 Table 3: Equivalent continuous sound level - LEX,8h db (A) - and peak sound pressure level - LC peak db (C) -, in a 1.5T MRI equipment. Red values indicate the higher values for each parameter and green values indicate lower values. Page 10 of 13

11 Table 4: Equivalent continuous sound level - LEX,8h db (A) - and peak sound pressure level - LC peak db (C) -, in a 3T MRI equipment. Red values indicate the higher values for each parameter and green values indicate lower values. Page 11 of 13

12 Conclusion We can conclude that L Ex, 8h and L cpeak values do not exceed the limits imposed by EC Directive and Portuguese legislation. Although the values do not exceed legislation, there are values that are close to the recommended lower action values (for example in a 3D TOF sequence in 1.5T scanner). IR sequences are "quieter" when compared to other types of sequences, such as SE, TSE and EPI. 3D sequences have higher values comparing to 2D conventional sequences because they apply, simultaneously, multiple gradient coils for image acquisition. Obtained data showed that on the 3T scanner are lower comparing to 1.5T scanner. This fact can be explained by intrinsic hardware and software parameters of Magnetom Siemens Trio for acoustic noise reduction as compared to other conventional scanners. Nowadays, this a fact, since manufacturers are increasingly concerned with hardware concerning with acoustic noise to be a greater comfort both for patients and workers. Sequences' duration time influences directly L Ex, 8H values. For example, in 3D TOF if radiographer slightly increase sequence's duration, L Ex, 8H values can reach and/or exceed lower and upper action values displayed on EC Directive. This study have shown that patients are exposed to relatively high levels of acoustic noise. However, those levels of acoustic noise do not exceed exposure limits and action (lower and upper) values of legislation, but several values are near Directive's stipulated values. So, radiographers have to take into account some safety decisons. In all MRI examinations, radiographers must provide patients with earplugs or headphones (or both) during scanning, to avoid discomfort, anxiety and even temporary hearing loss and damage. Personal information We like to thank Occupational Health Department of Centro Hospitalar São João, EPE, Oporto, Portugal for helping to obtain data for this study. Page 12 of 13

13 References 1. Comissão Europeia - Direcção-Geral do Emprego, Assuntos Sociais e Igualdade de Oportunidades Unidade F4. Guia Indicativo de Boas Práticas para a Aplicação da Directiva 2003/10/CE "Ruído no Trabalho" Shellock et. al. Acoustic Noise and MRI Procedures. MRIsafety.com. [Online] Accessed in January SafetyInfov.asp?SafetyInfoID= Hattori, Y., Ishigaki, T., Measurement and evaluation of the acoustic noise of a 3 Tesla MR scanner, Nagoya J Med Sci, 69, 23-28, McNulty, J.P., McNulty, S. Acoustic noise in magnetic resonance imaging: An ongoing issue. Radiography, 15, , Crook N, Robinson, L.,. A review of the safety implications of magnetic resonance imaging at field strengths of 3 Tesla and above. Radiography. 15:351-6, Coskun O. Magnetic resonance imaging and safety aspects. Toxicology and Industrial Health. 27(4):307-13, Crozier S, Wang, H., Trakic, A., Liu, F.,. Exposure of Workers to Pulsed Gradients in MRI. Journal of Magnetic Resonance Imaging. 26: , Nakai T, Kamiya, N., Sone, M., Muranaka, H., Tsuchihashi, T., Yamada, N., Yamaguchi, S. A Survey Analysis of Acoustic Trauma Related to MR Scans. Magn Reson Med Sci. 11(4):253-64, Swinborne University of Technology. MRI-09 Health & Safety Risk Assessment. Hawthorn: Swinborne University of Techonology; Lauer, AM, eral. MRI acoustic noise can harm experimental and companion animals. J Magn Reson Imag. 36:743, Rondinoni C, et al. Effect of scanner acoustic background noise on strict resting-state fmri. Braz J Med Biol Res 46:359-67, Page 13 of 13