perfusion is very low as in the case of resting skeletal muscle with tighter than in the

liver.

16.4.2 Plasma Compartment

The concentration of tracer in the plasma is an identified input function which drives

the system, and the metabolite-amended arterial plasma curve forms the input to the

compartment model. If the intravascular activity is considered in the calculation, one

should use the whole blood concentration, having metabolites. If metabolite-

corrected plasma curve is employed to correct for vascular blood volume fraction,

blood contribution at later intervals could be undervalued resulting in the false

inclusion of an apparent additional tissue compartment.

16.4.3 Retention Model

Our experience of the world is not of a series of unconnected moments. Indeed, one

could not experience the world if we are not aware of a sense of temporality (which

is traditionally the linear progression of past, present, and future). Perception we

have is an impression to our minds which solely is determined by upon both

retention and protention.

Retention is a progression by which a phase of a perceptual act is recalled in our

consciousness. It is a demonstration of that which does not exist before us any longer

and is different from immediate experience. For example, if we watch an object

being thrown, we would retain where the object was in our minds to comprehend its

momentum as we observe it in the immediate present. Retention (Geng and Regnier

1984) is certainly different from representation or memory and is simply a presenta-

tion of a temporally long-drawn-out present which spreads beyond the few short

milliseconds that are recorded in a moment of sense perception. Protention is our

expectation of the ensuing moment—the moment that has yet to be perceived.

Again, relating to the example of the thrown object, our focus shifts along the

expected path the object would take.

Husserl defined three temporal aspects of perception, viz., retention, the immedi-

ate present, and protention along with the flow through which each moment of

protention gradually turns into the retention of the next. The main benefit of the

retention model for clinical measurement of MBF is the easy PET protocol and

minimized computing demands for reconstructing and processing the image as

compared with compartmental modeling.

The retention model crafts flexibility for simplicity and efficiency and hence

addressing the integral technological limitations for former generations of clinical

PET scanners, which had made it very difficult to employ dynamic PET and full

compartmental modeling for routine purposes. Another significant potential advan-

tage of this model is to decrease variability of the MBF estimates at the expense of

increased bias due to the use of approximations and fixed correction elements. In

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K. Tankeshwar and S. Srivastava