MR engineers who specialize in acoustic noise control, gradient pulse design and hybrid system development.To reduce the acoustic noise level in a magnetic resonance imaging and linear accelerator (MRI-LINAC) hybrid system by active gradient pulse design.In an MRI scanner, if the main frequency of a gradient pulse is close to the natural frequency of the system, the gradient assembly will vibrate significantly and emit loud noise. However, by avoiding the natural frequencies of the system through gradient pulse alterations, it is expected that the overall noise level will be attenuated [1]. Taking an echo planar imaging (EPI) as an example, its basic pulse pattern is a trapezoidal form, as is shown in Fig. 1. Its time-domain periodic function can be expressed as a Fourier series shown in Eq. (1),$$ \begin{cases}f(t)=\sum\limits_{n=1}^\infty b_n\sin\frac{2\pi nt}{4t_1+2t_2} \\b_n=\frac{2}{4t_1+2t_2}\cdot\frac{2A}{t_1}(\frac{4t_1+2t_2}{2\pi n})^2[\sin\frac{(2\pi nt_1)}{4t_1+2t_2}+\sin\frac{2\pi n(t_1+t_2)}{4t_1+2t_2}] \end{cases}\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad(1)$$where A is the plateau current value, t₁ is the rising or falling time and t₂ is the duration of the plateau. We investigated the acoustic responses of a simulated 2D simplified axisymmetric model of a split MRI scanner, an element of an MRI-LINAC system (only the split gradient assembly including the z coils were simulated and the assembly ends were fixed). A series of sinusoidal gradient pulses were used to energize the z coils from 20 Hz to 20000 Hz and the average sound pressure levels (SPL) in the central gap were calculated to find the system’s resonant frequencies. The peak current value of the sinusoidal pulses was 600 A and the static magnetic strength was 1 T. The system is still currently being built. An acoustic model is shown in Fig. 2. After the resonant frequencies were identified, we removed the sinusoidal components of the pulse’s Fourier series, which were identical or close to the system’s resonant frequencies. During the removal process of the sinusoidal components, we set two principles, expressed as Eq. (2),$$ \begin{cases}SPL(f)>SPL_0 \\(A_{plateau-max}-A_{plateau-min})*0.5/A<3\%\end{cases}\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad(2) $$where SPL(f) is the acoustic response with respect to frequency, SPL₀ is a designated SPL value, A_plateau-max is the maximum current value of the pulse plateau and A_plateau-min is the minimum current value of the pulse plateau. Namely, we removed the sinusoidal components of those SPLs which were higher than a designated value and the pulse plateau part of the remaining sinusoidal components which had a deviation less than 3% [2, 3], which is an acceptably small deviation from a perfect plateau for an EPI pulse. The pulse alteration process balances these two principles in order to achieve an optimal pulse form. Assuming the system is linear, the sound pressure of the EPI pulse is a linear superposition of its sinusoidal components. The ultimate SPL can be easily acquired from Eq. (3),$$ SPL=10\lg\left(\sum_{n=1}^{\infty}\frac{b_n}{A}\cdot\frac{p_n}{p_0}\right)^2=10\lg\sum_{n=1}^{\infty}\left(\frac{b_n}{A}\cdot\frac{p_n}{p_0}\right)^2=10\lg\sum_{n=1}^{\infty}\left(\frac{b_n}{A}\right)^2\cdot10^{0.1*SPL\left(\frac{n}{4t_1+2t_2}\right)}\quad(3)$$ where p_n is the effective value of pressure and p₀ is the referential sound pressure.The acoustic responses of the split MRI system were plotted and are shown in Fig. 3. Here exemplifying t₁=237 μs, t₂=474 μs, the frequency components of the EPI pulse, its corresponding single-frequency (with current peak value 600 A) acoustic responses and amplitudes of its Fourier series are displayed in Table I. Using the optimization procedures shown in Fig. 4, after a repeated comparison, SPL0 was designated as 116 dB, and then the bond italic frequency components were removed from the Fourier series. The deviation of the pulse plateau with the remaining sinusoidal components is 1.6%. The standard trapezoidal pulse form and the altered one were plotted and are shown in Fig. 5, demonstrating great similarity. After the alteration, the overall SPL of the exemplified EPI pulse was attenuated from 108.4 dB to 94.6 dB. Thus, a 13.8-dB SPL reduction was acquired. In reality, the effect of the proposed method depends on the frequency components of the pulse. The pulses with a higher energy level at the resonant frequency components experience greater noise reduction compared to those where the main energy is concentrated in a non-resonant frequency band.The gradient pulse alteration method is investigated theoretically on its ability to reduce the noise level of a split MRI system. This method is easily implemented without changing or reassembling the split MRI scanner. Future experimental measurements will be conducted to test the effect of this method and evaluate imaging quality.The proposed gradient pulse alteration method can effectively attenuate the SPL of a split MRI system while keeping the pulse form. In the exemplified case an overall SPL reduction of 13.8dB was achieved when the proposed method was applied. It also kept the plateau deviation at 1.6%.