The transmitter side

On the transmitter side, the rf excitation is modulated to form pulses of given shape and phase. These pulses are amplified and fed into the resonator or rf oscillator of the probe, which is usually shared with the receiver. For material applications, most of these components are required in duplicate if rare nuclei such as C and Si are imaged. 

Double irradiation (cf. Section 7.2.12) of rare and abundant ( H) nuclei is necessary for sensitivity enhancement as well as for decoupling of the heteronuclear dipole-dipole interaction. 

The transmitter power needs to be adjusted to provide pulses short enough to obtain nonselective 180° pulses. In applications requiring large coils, resonators can be constructed which are fed with the sine and the cosine component of the rf signal [Hou2]. By using such circularly polarized excitation a factor of two is saved in the rf power in comparison to linearly polarized excitation [Glol]. 

The rf power P is proportional to the square of the NMR frequency wo and the fifth power of the sample radius. For a cylindrical sample of length I parallel to Bo. diameter and conductivity cr the rf power is approximated by [Krel, Mori] 

Typical values for medical imaging are d = I = 0.4 m, a = 0.5/i2m (physiological saline solution), a selective 180° pulse of duration 1 ms, and an NMR frequency of cuo = 27T42.6 MHz, resulting in a calculated power of 2.8 kW. This is by a factor of about 2 too large compared to experimental values [Krel]. 

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When the same surface coil is used for excitation and for reception, B xy R) enters into the detected signal from the transmitter side as well as from the receiver side. As a result the sensitive volume changes in size compared to excitation or reception only. For a homogeneous sample excited by a single pulse, the distribution of signal amplitude is described by... 

This is a transcendental equation, which is not easily solved by ordinary methods. Nowadays, however, computers make the solution of such equations by successive approximations easy. In this case, again using EXCEL , we find that the value of T that makes the left-hand side of equation (42-50) become zero, which thus gives the value corresponding to the transmittance corresponding to minimum relative error, is 0.32994, rather than the previously accepted value of 0.368... 

Each neuron usually releases only one type of neurotransmitter. Neurons that release dopamine are referred to as dopaminergic, for example, while those that release acetylcholine are cholinergic, etc. The transmitters that are released diffuse through the synaptic cleft and bind on the other side to receptors on the postsynaptic membrane. These receptors are integral membrane proteins that have binding sites for neurotransmitters on their exterior (see p. 224). 

Initially, the control units were quite clumsy and offered uncountable manipulation options. It was often the case that only physicists were able to handle and use them. With the Introduction of PCs the requirements In regard to the control units became ever greater. At first, they were fitted with Interfaces for linkage to the computer. Attempts were made later to equip a PC with an additional measurement circuit board for sensor operation. Today s sensors are In fact transmitters equipped with an electrical power supply unit attached direct at the atmosphere side communication with a PC from that point Is via the standard computer ports (RS 232, RS 485). Operating convenience Is achieved by the software which runs on the PC. 

Concentric-orifice devices can be easily installed in high-pressure lenses. The high-end- and the low-end connection of such devices could be coupled with differential pressure transmitters, for example, with the before-mentioned devices of [54]. The devices are mounted in different ways. Mostly, the open pipe technique is used. Furthermore, both connected sides of the transmitter are fed with an inert gas, for example, nitrogen. For cases where the systems must separated, membrane devices can be arranged between them. All the mentioned orifice devices are technically proved and are applied in the high-pressure area. 

The differential-pressure transmitters are only available for moderate pressures, up to 400 bar. Membrane systems give the possibility of choosing corrosion-resistent materials for the parts of a device (wet system), or to protect the inside of the device by using an additional membrane which divides the instrument side from corrosive media (dry system). 

A few transmembrane molecular rods having different lengths and terminal shapes and moving freely by lateral diffusion may bind to a given receptor-transmitter on one side (left) of the membrane in a configuration in the membrane plane and in a displacement perpendicular to the membrane that will depend on the disposition and depth of the binding sites on the transmitter. This will impose on the... 

Recording Colorimeter. The flowing stream which now has attained its appropriate color development is passed into a recording colorimeter. The air is removed in a small precell attachment feeding the flow cell. The colorimeter employs the dual beam principle and compares the electrical output of the photocell from the reference side with that from the flow cell. This difference is then traced as per cent transmittance on a conventional chart recorder. The solution which has been read and... 

The tank is open to the atmosphere therefore, it is necessary to use only the high pressure (HP) connection on the AP transmitter. The low pressure (LP) side is vented to the atmosphere therefore, the pressure differential is the hydrostatic head, or weight, of the liquid in the tank. The maximum level that can be measured by the AP transmitter is determined by the maximum height of liquid above the transmitter. The minimum level that can be measured is determined by the point where the transmitter is connected to the tank. 

The high pressure connection is connected to the tank at or below the lower range value to be measured. The low pressure side is connected to a "reference leg" that is connected at or above the upper range value to be measured. The reference leg is pressurized by the gas or vapor pressure, but no liquid is permitted to remain in the reference leg. The reference leg must be maintained dry so that there is no liquid head pressure on the low pressure side of the transmitter. The high pressure side is exposed to the hydrostatic head of the liquid plus the gas or vapor pressure exerted on the liquid s surface. The gas or vapor pressure is equally applied to the low and high pressure sides. Therefore, the output of the AP transmitter is directly proportional to the hydrostatic head pressure, that is, the level in the tank. 

The filled reference leg applies a hydrostatic pressure to the high pressure side of the transmitter, which is equal to the maximum level to be measured. The AP transmitter is exposed to equal pressure on the high and low pressure sides when the liquid level is at its maximum therefore, the differential pressure is zero. As the tank level goes down, the pressure applied to the low pressure side goes down also, and the differential pressure increases. As a result, the differential pressure and the transmitter output are inversely proportional to the tank level. 

A condensing pot at the top of the reference leg is incorporated to condense the steam and maintain the reference leg filled. As previously stated, the effect of the steam vapor pressure is cancelled at the AP transmitter due to the fact that this pressure is equally applied to both the low and high pressure sides of the transmitter. The differential pressure to the transmitter is due only to hydrostatic head pressure, as stated in Equation 3-3. 

A loss of differential pressure integrity of the secondary element, the DP transmitter, will introduce an error into the indicated flow. This loss of integrity implies an impaired or degraded pressure boundary between the high-pressure and low-pressure sides of the transmitter. A loss of differential pressure boundary is caused by anything that results in the high- and low-pressure sides of the DP transmitter being allowed to equalize pressure. 

Action of the neurotransmitter is terminated by several means. For many neurotransmitters, the bulk is recycled back into the presynaptic terminal via an active uptake process to be reused. Alternatively, some of the neurotransmitter may simply diffuse away and be enzymatically destroyed elsewhere. In other cases enzymes may be located on the postsynaptic side of the cleft, in the vicinity of the receptor, which serves to rapidly break the transmitter down into inactive metabolites. 

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