Division of Physics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, FDA, 10903 New Hampshire Avenue Silver Spring, MD 20993-0002, USA.
Biomed Eng Online. 2009 Dec 15;8:39. doi: 10.1186/1475-925X-8-39.
Magnetic resonance imaging (MRI) of patients with implanted cardiac pacemakers is generally contraindicated but some clinicians condone scanning certain patients. We assessed the risk of inducing unintended cardiac stimulation by measuring electric fields (E) induced near lead tips by a simulated MRI gradient system. The objectives of this study are to map magnetically induced E near distal tips of leads in a saline tank to determine the spatial distribution and magnitude of E and compare them with E induced by a pacemaker pulse generator (PG).
We mapped magnetically induced E with 0.1 mm resolution as close as 1 mm from lead tips. We used probes with two straight electrodes (e.g. wire diameter of 0.2 mm separated by 0.9 mm). We generated magnetic flux density (B) with a Helmholtz coil throughout 0.6% saline in a 24 cm diameter tank with (dB/dt) of 1 T/sec (1 kHz sinusoidal waveform). Separately, we measured E near the tip of leads when connected to a PG set to a unipolar mode. Measurements were non-invasive (not altering the leads or PG under study).
When scaled to 30 T/s (a clinically relevant value), magnetically-induced E exceeded the E produced by a PG. The magnetically-induced E only occurred when B was coincident with or within 15 msec of implantable pacemaker's pulse.
Potentially hazardous situations are possible during an MR scan due to gradient fields. Unintended stimulation can be induced via abandoned leads and leads connected to a pulse generator with loss of hermetic seal at the connector. Also, pacemaker-dependent patients can receive drastically altered pacing pulses.
植入式心脏起搏器患者的磁共振成像（MRI）一般是禁忌的，但有些临床医生允许对某些患者进行扫描。我们通过测量模拟 MRI 梯度系统在导联尖端附近产生的电场（E）来评估引起非预期心脏刺激的风险。本研究的目的是绘制在盐水池中导联远端尖端附近的磁场感应 E 的分布图，以确定 E 的空间分布和幅度，并将其与起搏器脉冲发生器（PG）产生的 E 进行比较。
我们以 0.1 毫米的分辨率绘制了尽可能靠近导联尖端 1 毫米的磁场感应 E。我们使用带有两个直电极的探头（例如，直径为 0.2 毫米的电极，间距为 0.9 毫米）。我们使用亥姆霍兹线圈在直径为 24 厘米的盐水池中产生磁通密度（B），B 值为 0.6%（1 kHz 正弦波的（dB/dt）为 1 T/sec）。另外，我们测量了将导联连接到设置为单极模式的 PG 时尖端附近的 E。这些测量是非侵入性的（不会改变正在研究的导联或 PG）。
当（dB/dt）被缩放为 30 T/s（一个具有临床相关性的值）时，磁场感应 E 超过了 PG 产生的 E。磁场感应 E 仅在 B 与起搏器脉冲同时发生或在起搏器脉冲发生后 15 毫秒内发生。
由于梯度场的存在，在 MRI 扫描期间可能会出现潜在的危险情况。通过废弃的导联和与脉冲发生器连接的导联（连接器失去密封）可以感应到非预期的刺激。此外，依赖起搏器的患者可能会接收到明显改变的起搏脉冲。
