B1 Homogeneity

High resolution NMR spectroscopists spend a great deal of time shimming the magnet to ensure that the static magnetic field, B_o is homogeneous. This is because the transverse magnetization precesses about B_o. The Larmor equation implies that if there is a distribution in B_o across the volume of the sample, there will be a distribution of frequencies for each resonance line (i.e. the NMR resonances will be broad). The more homogeneous B_o across the sample volume, the sharper the NMR lines.

There is another field we must consider when doing NMR experiments - the magnetic field due to the RF pulse in the rotating frame of reference. In the rotating frame of reference, during the application of a pulse, an "on resonance" NMR line experiences an effective field, B_eff equal to the magnetic field due to the pulse, B₁. B₁ is a static magnetic field in the rotating frame of reference. Due to the finite dimensions of the coil in the probe with respect to the sample, the B₁ field will not be homogeneous across the entire volume of the sample. For example, a 90° pulse for the sample in the center of the coil will not be equal to a 90° pulse for the sample near the edges of the coil. While an x phased pulse is being applied to an equilibrium magnetization vector, the magnetization will precess about the x' axis in the rotating frame in the z-y' plane exactly like transverse magnetization precesses about the z axis in the x-y plane. While the magnetization precesses in the z-y' plane during the pulse, it is affected by the inhomogeneity in the B₁ field. The inhomogeneity of the B₁ field can be measured by doing a simple pulse calibration, applying longer and longer pulses well beyond that needed for a 360° pulse. After the pulse, the magnetization vectors precess again about B_o, and can be measured. The magnitude of the magnetization for a 90°, (90° + 360°), (90° + 720°) ..... etc. pulse will depend on the B₁ homogeneity. An example of this is shown in the figure below. The figure shows a simple ¹H pulse calibration for the decoupler coil of a 5 mm broadband NMR probe. The B₁ homogeneity is expressed as the ratio of intensity for an 810° pulse compared to that from a 90° pulse. In this case the B₁ homogeneity is 0.43. Much higher B₁ homogeneity would be expected for an inverse detection probe.

Why are MRI Scanners so Loud?

If you have ever had a magnetic resonance image (I prefer the term NMR image) taken in a hospital, you have undoubtedly heard very loud noises while the instrument collected the data. These noises are the result of the application of magnetic field gradients by way of the gradient coils inside the main magnetic field. The magnetic field gradients are applied in short pulses and are used to spatially encode the sample (person) such that the Larmor frequency of the protons in one region of the sample differs from that in other regions. It is this spatial frequency labelling which allows for the generation of the image. The time dependant magnetic field generated by the pulsed magnetic field gradients interacts with the main magnetic field with a force exactly like the one we are all familiar with when we move two magnets in close proximity to one another. Since the pulsed field gradient coils are held in a rigid form within the main magnet, they are unable to move any great distance however the sudden application of a gradient pulse generates a very strong force which physically slams the gradient coils inside the rigid form making a loud noise from the vibration. The larger the main magnetic field or the stronger the gradient pulses, the greater the force generated and the louder the noise.

These noises can sometimes be heard in high resolution NMR probes equipped with pulsed field gradients. Recently, a student approached me concerned that the NMR spectrometer was making a soft "tick" noise for every scan collected in his gradient COSY experiment. Immediately I thought perhaps the probe was arcing so I turned down the RF power. The noise persisted. Upon closer inspection, I found that the gradient strength was set unnecessarily high for the measurement. When the gradient strength was turned down, the "tick" noise ceased.

NMR and Food Chemistry - Popcorn

Both liquid state and solid state NMR have become very important tools in the food industry for both research and quality control. Since starch is a very important biopolymer and a major constituent in many foods, a great deal of work has been done on its chemical and physical characterization. Starches are 1 -4 linked polymers of glucose. Native starches are a mixture of a linear polymer (amylose) and a branched polymer (amylopectin). Furthermore, the starches are of two main types differing in their crystal structures and water content: the A type (cereal starches) and the B type (tuber starches). In addition to A or B starches, the starch granules in plants can have some amorphous starch as well. Corn starch is of the A type.

Popcorn is a snack enjoyed by millions around the world. As a child, I remember being fascinated at watching it pop and wondering what was going on. Much study has been devoted to the physics of popping corn. Essentially, every kernel of corn is a pressure vessel. When cooked, the moisture trapped within the starchy endosperm of the kernel is superheated. When the steam pressure inside the kernel becomes high enough the hull (pericarp) of the kernel explodes and the superheated water in the starch granules suddenly vaporizes, expands and rapidly cools making a solid foam out of the starchy endosperm. Each starch granule is a bubble in the solid foam.

Although much work has been done to understand starch and much work has been done to understand the popping of corn, I was unable to find any efforts directed to the chemical changes in corn starch before and after popping. This prompted me to collect a few spectra to address the issue. The first figure below shows the starch region of the ¹³C CPMAS spectra of unpopped corn (bottom trace), popped corn (middle trace) and cooked but unpopped corn (upper trace). The spectrum of unpopped corn is a mixture of A type starch (with a characteristic three line pattern in the C1 region) and amorphous starch (with a broad distribution of overlapping lines in the C1 region). The spectra of popped corn and cooked but unpopped corn are essentially identical and characteristic of amorphous starch. The data indicate that the heating and dehydration of the corn transform the crystalline A starch into amorphous starch. The observation is consistent with published studies on the hydration of starches. Aside from this change, there is no evidence for any other chemical transformation in the starch.

The second figure shows an expansion of the C1 region highlighting the conversion of crystalline A type starch into amorphous starch.

Probe Arcing

Probe arcing can occur during the application of an rf pulse. It is the passage of a spark between a localized area of high voltage inside the probe to ground. In cases of severe probe arcing, one can hear a "snap" during the application of a pulse. If the probe is removed from the magnet and the cover is removed, one can see the arcing as a small "bolt of lightning" between the high voltage area and ground. When a probe arcs, the integrity of the pulse is affected and one will observe FIDs of irreproducible amplitude and phase. Needless to say, the quality of data collected on an arcing probe will be severely compromised. If a probe is permitted to arc for extended periods of time, some of the electronic components inside the probe can be permanently damaged. The amplifiers can also be damaged as arcing will cause mismatching and high levels of reflected power. Here are a few things you can do to help stop probe arcing.

1. Use lower pulse power levels.
2. If the location of the arcing can be found visually (i.e. you can see the spark), then the high voltage area and the path to ground can be wrapped with teflon tape.
3. Round off any sharp edges inside the probe as these are the areas of highest local voltage. In particular, any solder joints around the coil should be smooth.
4. Keep the coil and capacitors as far away from ground as possible.
5. Purge the inside of the probe body continuously with nitrogen gas.