A pixel is a picture element (pix, abbreviation of pictures + element). Tomographic images are composed of several pixels.
The corresponding size of the pixel may be smaller than the actual spatial resolution. Pixels do not have a fixed size; their diameters are generally measured in micrometers (microns). Although the pixel is not a unit of measurement itself, pixels are often used to measure the resolution (or sharpness) of images. As a hypothetical example, a 600 x 1000 pixel image has 4 times the pixel density and is thus 4 times sharper than a 300 x 500 pixel image, assuming the two images have the same physical size.

An RF array coil consisting of 2 - 6 several separately resonant elements, 2 or more of which can be selected as the receiver coil at a particular time. Coils not in use are decoupled. The spatial information related to the coils of a receiver array are utilized, reference images are required, obtained with each array element and with a body coil. Fast parallel imaging techniques using surface multicoils, such as sensitivity encoding (SENSE) or simultaneous acquisition of spatial harmonics (SMASH), have recently raised interest in the field of MR imaging.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

(SENSE) A MRI technique for relevant scan time reduction. The spatial information related to the coils of a receiver array are utilized for reducing conventional Fourier encoding. In principle, SENSE can be applied to any imaging sequence and k-space trajectories. However, it is particularly feasible for Cartesian sampling schemes. In 2D Fourier imaging with common Cartesian sampling of k-space sensitivity encoding by means of a receiver array enables to reduce the number of Fourier encoding steps.
SENSE reconstruction without artifacts relies on accurate knowledge of the individual coil sensitivities. For sensitivity assessment, low-resolution, fully Fourier-encoded reference images are required, obtained with each array element and with a body coil.
The major negative point of parallel imaging techniques is that they diminish SNR in proportion to the numbers of reduction factors. R is the factor by which the number of k-space samples is reduced. In standard Fourier imaging reducing the sampling density results in the reduction of the FOV, causing aliasing. In fact, SENSE reconstruction in the Cartesian case is efficiently performed by first creating one such aliased image for each array element using discrete Fourier transformation (DFT).
The next step then is to create a full-FOV image from the set of intermediate images. To achieve this one must undo the signal superposition underlying the fold-over effect. That is, for each pixel in the reduced FOV the signal contributions from a number of positions in the full FOV need to be separated. These positions form a Cartesian grid corresponding to the size of the reduced FOV.
The advantages are especially true for contrast-enhanced MR imaging such as dynamic liver MRI (liver imaging) , 3 dimensional magnetic resonance angiography (3D MRA), and magnetic resonance cholangiopancreaticography (MRCP).
The excellent scan speed of SENSE allows for acquisition of two separate sets of hepatic MR images within the time regarded as the hepatic arterial-phase (double arterial-phase technique) as well as that of multidetector CT.
SENSE can also increase the time efficiency of spatial signal encoding in 3D MRA. With SENSE, even ultrafast (sub second) 4D MRA can be realized.
For MRCP acquisition, high-resolution 3D MRCP images can be constantly provided by SENSE. This is because SENSE resolves the presence of the severe motion artifacts due to longer acquisition time. Longer acquisition time, which results in diminishing image quality, is the greatest problem for 3D MRCP imaging.
In addition, SENSE reduces the train of gradient echoes in combination with a faster k-space traversal per unit time, thereby dramatically improving the image quality of single shot echo planar imaging (i.e. T2 weighted, diffusion weighted imaging).

(SMASH) Several lines of data are acquired for each phase encoding step, which is also referred to as a k-space trajectory.
SMASH imaging with a four-element array coil is four times faster and can be used to achieve almost real-time imaging. The maximum reduction in acquisition time is determined by the number of array coil elements. Thus, the heart can be scanned with higher temporal resolution and increased spatial resolution.
SMASH and SENSE differ from other techniques in which only one line of k-space data is acquired for each phase encoding gradient step.

See Sensitivity encoding.

An RF array coil consisting of several separately resonant elements, any one of which can be selected as the receiver coil at a particular time. Coils not in use are decoupled. Applications of switchable coils include imaging the whole spine without patient repositioning (where the coil elements may collectively be known as a ladder coil), imaging of bilateral structures such as TMJ or orbit using separate coils, or imaging using a coil with selectable field of view.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'