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ORIGINAL / RESEARCH
Role of Multislice CT in Cerebral Perfusion Studies
Anagha R Joshi, Suleman A Merchant, Malini A Lawande, Paritosh C Khanna
Owing to the limited therapeutic time window of four to six hours after onset of symptoms, early detection of acute cerebral ischaemia is extremely beneficial. Xenon CT, SPECT, PET, Perfusion and diffusion weighted MRI are useful methods to evaluate acute strokes. These methods however have disadvantages and are not widely available in all hospitals. Perfusion CT of the brain is the fastest modality for the early detection of acute cerebral ischaemia. Multislice CT, a recent innovation in CT technology, has the potential to revolutionize the role of CT in perfusion imaging of acute strokes.
INTRODUCTION
The diagnosis and treatment of acute stroke can be improved significantly if cerebral
perfusion is measured to identify the tissue at risk. Patients with acute cerebral ischaemia may be treated by thrombolytic therapy within four to six hours of symptoms onset. This necessitates clinical examination and diagnostic imaging within a short period of time. As almost all patients with suspected acute stroke undergo CT to exclude haematoma, and existing infarcts, valuable time can be gained by measuring cerebral perfusion during the initial CT. Cerebral Blood Flow (CBF), Cerebral Blood Volume (CBV) and Time to Peak (TTP) are the parameters, which are calculated to evaluate cerebral perfusion. We present two such cases in which perfusion CT performed on a Multislice CT scanner (MSCT) helped in the detection of early cerebral ischaemia.
MATERIAL AND METHODS
A 55-year-old male (Figs. 1a, b) presented with a history of hemiparesis with contralateral upper motor neuron facial palsy, since 2 hours. On examination, the power on the affected side was zero and the tone was increased, with an up going plantar reflex. His blood pressure was 130/90 mm of Hg. Another 45-year-old male patient (Fig. 2) came with a three-hour history of right-sided hemiplegia and deviation of the angle of the mouth to the left. Plain axial CT scans of the brain were performed on an MSCT scanner (Volume Zoom, Siemens Corp., Forchheim, Germany), which did not reveal any significant abnormality.
A perfusion CT scan of the brain was done by injecting 40 ml of non-ionic water-soluble iodinated contrast media (Ultravist, 370 mg/ml, Schering Diagnostics, Germany) via a 16 gauge cannula in the left antecubital vein, using a pressure injector (Vistron CT, Medrad), at the rate of 6 ml/sec. Forty scans were acquired, each of 10 mm slice width at 5 mm collimation and 120 kV, 110 mAS with a scan time of 0.5 sec. The scans were started at a delay of four seconds after the start of injection of contrast. The reconstruction of images was done using the H30f medium smooth kernel. The sections included the basal ganglia and were adjusted to be perpendicular to the posterior segment of the superior sagittal sinus.
Data in the form of DICOM images were transferred to a workstation (MV300, Siemens Corp., Germany) networked to the CT scanner. Perfusion CT software (Magic View, Siemens Corp., Germany) was used to evaluate cerebral perfusion. This allows the calculation of a number of functional maps of cerebral perfusion. The processing of the entire study takes about two to three minutes. The parameters of CBF, CBV and TTP were calculated and depicted as functional maps. In both the patients, the perfusion CT showed a decrease in CBF and CBV in the lentiform nucleus on the affected side. Also, TTP was increased in the affected area. Depending on the color scale, these areas are assigned specific colors on the CBF, CBV and TTP maps. A follow-up CT done after 12 hours in both patients showed a well-defined hypodensity involving the lentiform nucleus on the affected side, confirming the presence of an infarct.
DISCUSSION
In view of the widely accepted therapeutic window of three to six hours after symptom onset, the search has always been on for a modality, which can detect acute cerebral ischaemia quickly, and is also widely and easily available. At present, PET is not widely available. SPECT is useful in the assessment of cerebral blood flow but substantial motion artifacts occur due to longer acquisition times. Diffusion and perfusion weighted MRI are very sensitive in detecting ischaemic tissue. However echo-planar technology is required which is not widely available. Also, MRI within the first few hours of stroke is limited as adequate monitoring of vital parameters may be a challenge when the patient is placed in the magnet. Xenon CT has the disadvantages of the side effects of xenon, even in subnarcotic doses, and the unpleasant experience of a tightly placed facemask in acute stroke patients.
The early changes of sulcal effacement and hypo-attenuation of gray matter are dependent on both the severity and duration of CBF disturbances. They indicate the presence of pronounced or long-standing ischaemia. This appearance on conventional CT is due to ischaemic oedema, which is a secondary step of brain damage; this increases with time and mainly shows tissue that has already been irreversibly damaged. The reduction in CBF can be considered the primary event in cases of embolic occlusion of cerebral vessels. Thus, detection of perfusion abnormalities and, in particular, the depiction of reduced CBF is the decisive factor in the diagnosis of acute cerebral ischaemia.1
Early attempts at perfusion CT suffered from technical limitations of CT scanners such as the insufficient temporal resolution and longer scanning time. Multisection CT presently has four data acquisition systems connected to multidetector arrays, increasing the speed of data collection by a factor of four over conventional helical CT scanners (Quad section CT scan). Also these scanners have gantry rotation speeds of two revolutions/sec, twice that of conventional helical CT scanners. These two improvements have combined to increase the speed of multislice CT scanner of 8 times the speed of conventional helical scanners presently. The benefits of quad section CT are improved temporal and spatial resolution, an increased concentration of intravascular contrast material and decreased image noise and motion artifacts.2
Iodinated contrast is mainly used as a non-diffusible tracer for evaluation of CBF parameters. As long as the blood brain barrier is intact, contrast acts as a pure intravascular tracer. After rapid intravenous injection of a compact contrast bolus, changes in local attenuation during first pass through the brain are detected by rapid sequence scans at a single level. The relationship between the CT number and actual concentration of contrast is linear. High rates of contrast injection are preferable as it is important to use bolus administration techniques that produce arterial peak times as short as possible.
Although identification of some arteries, like the branches of the ACA and MCA is possible on an axial scan through the basal ganglia, arterial contrast enhancement values tend to be markedly reduced due to partial volume effects. Thus, for practical purposes, a good approximation can be achieved by measuring the maximum value in the superior sagittal sinus.
Peak tissue enhancement
CBV = Peak enhancement in
Superior sagittal sinus.
Conventional CT is still the primary imaging modality in the evaluation of stroke to exclude haemorrhage. Perfusion CT adds only a few minutes to the duration of the basic CT examination, thus making it an effective and practical modality so that treatment can be started in the narrow therapeutic window (four to six hours).
Although perfusion CT provides information about CBF from only a single level of the brain, early ischemia is detected with high sensitivity (89 per cent). Even small infarcts are well depicted due to the excellent spatial resolution of this technique.1
CBF measurements can help in the selection of a suitable therapy for the patient. Ueda et al reported that ischaemic tissue with CBF > 55 per cent of cerebellar flow can be salvaged with recanalization by intra-arterial thrombolysis even after six hours. Brain tissue with CBF > 35 per cent of cerebellar flow may be saved within less than five hours while tissue with CBF > 35 per cent of cerebellar flow is at risk for haemorrhage.3
Thus we can conclude that with the advent of Multislice CT (MSCT), perfusion CT is a useful technique for the assessment of stroke as it allows detection of ischaemic brain within one to two hours after symptom onset, i.e. within the critical period for the initiation of potentially effective therapeutic measures. Since all patients with suspected acute stroke undergo conventional CT to exclude haematoma and existing infarcts, another couple of minutes and contrast injection in the form of perfusion CT goes a long way in reducing the long-term neurological deficits in patients with stroke.
REFERENCES
1. Koenig M, Klotz E, Luka B, et al. Perfusion CT of the Brain : Diagnostic Approach for Early Detection of Ischemic Stroke. Radiology 1998; 209 : 85-93.
2. Rydberg J, Buckwalter KA, Caldemeyer KS, et al. Multisection CT : Scanning techniques and clinical applications. Radiographics 2000; 20 : 1787-1806.
3. Mayer T, Hamann G, Baranczyk J, et al. Dynamic CT Perfusion Imaging of Acute Stroke. Am J of Neuroradiol 2000; 21 : 1441-49.
ZINC DEFICIENCY
In 1958, a 21 year old male patient in the Iranian city of Shiraz presented with dwarfism, hypogonadism. In addition he consumed 0.5 kg of clay daily.
The problem has been known for 40 years and a solution is still outstanding. Despite all the evidence practically no attention has been given to the problem of zinc deficiency by the world’s organisations. Growth retardation, increased susceptibility to infectious and cognitive impairment are common in developing countries where nutritional deficiency of zinc is also prevalent. Thus a correction of zinc deficiency is likely to have a great impact on the health of a large population in the developing world and it is imperative that the World Health Organization must include this problem in its top priorities.
Ananda S Prasad, BMJ, February, 2003; 326 : 409-10.
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