Submillimeter-resolution magnetic resonance imaging at the Earth’s magnetic field with an atomic magnetometer

Magnetic resonance imaging in the Earth’s magnetic field is achieved using a sensitive atomic magnetometer for detection. We demonstrate images with a submillimeter resolution by recording the flow of two water paths meeting at a $T$-shaped mixer. The high homogeneity of the Earth’s field allows the use of weak gradient fields which circumvent the concomitant-field effect. To distinguish the two input channels, we employed selective polarization, which is a unique and noninvasive labeling method for low-field magnetic resonance imaging. Our technique imposes minimal physical constraints on the object under study, in contrast to conventional high-field magnetic resonance imaging. This technique is applicable for microfluidic imaging in laboratory-on-a-chip devices.

Figure 1

(Color online) (a) Schematic of the experimental setup. Three regions are shown: prepolarization, encoding, and detection. Blue dots represent the analyte. Blue lines indicate the tubes in which water flows from one region to another. (b) Earth’s field MRI encoding system, approximately cubic in shape, 15-cm side length. The compensating coils are mounted on three pairs of aluminum rings. The gradient coils are wound around a Teflon tube. A second Teflon tube with a smaller diameter, on which a saddle coil for excitation is mounted, is inserted into the gradient-coil holder. The bore size for this imaging system is 2.5 cm diameter. (c) Imaging phantom. $T$-shaped mixer, consisting of two input channels and one output channel.

Figure 2

(Color online) Time-resolved flow images. The color scale indicates relative intensity. The time indicated underneath each image shows the travel time required for the encoded water sample to reach the detectors after an encoding sequence. The connection volume between the encoding and detection stages is negligible compared to the encoding volume and detection volumes. The images are obtained from signals averaged over 30 measurements.

Figure 3

(Color online) Time-resolved flow images of selectively polarized water. In this case, only water in the left input channel is polarized by the 0.3-T magnetic field. Note the scale of color mapping for imaging intensity is one-half of the scale used in Fig. 2. Other conditions remain the same as those for previous imaging experiments.

Figure 4

(Color) High-resolution images of mixing in the output channel. Gradient field is $0.2\mu \text{T}/\text{mm}$ for the $z$ axis and $0.4\mu \text{T}/\text{mm}$ for the $y$ axis. All other parameters remain the same as the ones for Fig. 3.