Neurosurgery

are thallium-activated sodium iodide crystal (NaI(TL)) and cadmium telluride (CdTe). The NaI(TL) detector uses a scintillation crystal that converts the energy of the incoming photons into visible light and is coupled to a photomul-tiplier tube, which then converts these scintillations into electrical pulses. The crystal thickness is usually 6 mm, to provide maximum counting efficacy, which is about 90-99% for 133Xe. The crystal is mounted behind a lead collimator that is 25 mm thick and has a tapered opening from 30 mm at the surface of the crystal to 22 mm at the front end. This detector assembly, consisting of the collimator, crystal, photomultiplier tube and pre-amplifier, is mounted on a stand that can be moved up to the patient's head. The CdTe crystal is 16 mm in diameter x 2 mm thick, mounted inside a lead collimator that is packaged in a metal cylinder. This unit is attached to the patient's head by means of a strap. The counting efficiency of this unit with 133Xe is 96%. The detector is lighter in weight and operates at a lower voltage than the NaI(TL) system, but is more expensive.

Signal processing of the electrical pulses generated by the detector assembly requires a pre-amplifier, amplifier, pulse height analyzer and count-rate meter. The pre-amplifier and amplifier function to match the impedance level between the scintillation detector and the pre-amplifier, and amplify the low-voltage pulses from the pre-amplifier to a sufficient level to drive the pulse height analyzer. The pulse height analyzer is used to select only those pulses that coincide to the energy level of 133Xe (81 keV) and discriminate against background noise and scattered radiation outside the selected energy range (75-200 keV). The count-rate meter is used to determine the average number of counts per unit time and is recorded on a strip chart recorder.

Ultimately, the measurements obtained by the scintillation detectors and processed as described above yield clearance curves representing the washout of 133Xe (Figs 17.6 and 17.7). These clearance curves can be analyzed by one of several methods for curve analysis to calculate the value of CBF: initial slope index, determined from the slope of the first minute of the clearance curve; stochastic method; and two-compartmental analysis to yield both gray-and white-matter CBF values. The initial slope index is most commonly used, while the other

Fig. 17.6. Cerebral blood flow curve following intra-arterial 133Xe injection into the internal carotid artery. Cerebral blood flow in ml/100 g/minute is calculated by dividing 3,600 by the half-peak value in seconds, in this example 55.5 ml/100 g/ minute. Cpm, counts per minute (by permission of Mayo Foundation. Sundt TM Jr, editor. Occlusive cerebrovascular disease: diagnosis and surgical management. W.B. Saunders, 1987; 182-90).

Fig. 17.6. Cerebral blood flow curve following intra-arterial 133Xe injection into the internal carotid artery. Cerebral blood flow in ml/100 g/minute is calculated by dividing 3,600 by the half-peak value in seconds, in this example 55.5 ml/100 g/ minute. Cpm, counts per minute (by permission of Mayo Foundation. Sundt TM Jr, editor. Occlusive cerebrovascular disease: diagnosis and surgical management. W.B. Saunders, 1987; 182-90).

Fig. 17.7. Sequential CBF curves during carotid endarterectomy. Top graph depicts baseline CBF value of 46 ml/100 g/min. Middle graph shows initial occlusion CBF of less than 5 ml/ 100 g/min, increasing to 44 ml/100 g/min with shunt placement. Lower graph shows CBF value on restoration of flow of 67 ml/100 g/min. CBF, cerebral blood flow (by permission of Mayo Foundation. Sundt TM Jr, editor. Occlusive cerebrovascular disease: diagnosis and surgical management. W.B. Saunders, 1987; 182-90).

Fig. 17.7. Sequential CBF curves during carotid endarterectomy. Top graph depicts baseline CBF value of 46 ml/100 g/min. Middle graph shows initial occlusion CBF of less than 5 ml/ 100 g/min, increasing to 44 ml/100 g/min with shunt placement. Lower graph shows CBF value on restoration of flow of 67 ml/100 g/min. CBF, cerebral blood flow (by permission of Mayo Foundation. Sundt TM Jr, editor. Occlusive cerebrovascular disease: diagnosis and surgical management. W.B. Saunders, 1987; 182-90).

CEREBRAL BLOOD FLOW: PHYSIOLOGY AND MEASUREMENT TECHNIQUES

two analytic methods require an online computer to determine results.

A primary limitation of CBF measurement using 133Xe is the "look-through" phenomenon. This is the failure to indicate areas of low-or no-flow regions because the detector can see only areas of perfused tissue. In other words, the detector sees the under and/or overlying tissue and also tissue peripheral to the area in the field of view. A second limitation is that performing fast serial measurements introduces error into the measurement. The time interval between CBF measurements at 5 and 10 minutes will result in overestimation of CBF by 10 and 5%, respectively. Therefore, a wait period is recommended between measurements to minimize errors due to a greater-than-normal background. A third limitation is Compton scattering, which may introduce considerable error in the determination of the volume and severity of focal ischemia. Compton-scattered photons can be minimized by setting the lower level of the pulse height analyzer to 75 keV.

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