Neuro-radiology

Neuroradiology (1992) 34:112-116

9 Springer-Verlag 1992

Pediatric craniocervical spiral CT R. A. Zimmerman, D. A. Gusnard, and L. T. Bilaniuk

Department of Radiology, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania, USA

Summary. Spiral CT, which consists of rapid volumetric

data acquisition and planar image display, was performed on 100 children undergoing examinations of the brain and head and neck for a variety of clinical indications. The technique was evaluated for image quality and impact on clinical practice. Images of the brain were generally of diagnostic quality in infants and young children but not in older children, due to their larger heads and present Xray tube current limitations. The lower contrast resolution demands in imaging head and neck structures permitted acquisition of high quality images in all age groups. The rapidity of the technique allowed vascular phase imaging with one-sixth to one quarter the amount of contrast medium injected routinely, reduced the number of artifacts related to patient motion and occasionally permitted scanning of young patients without sedation. As a consequence, the spiral technique appears to have the potential for improving the efficiency and safety of diagnostic CT in investigation of the brain, head and neck in children. Key words: Spiral CT - Pediatric brain - Pediatric head

and neck

In standard computed tomography (CT), a volume is scanned in steps in which one section is made after another. Iuterscan delays are necessary for table feed and tube positioning, which results in a total examination time longer than the actual scan time. It may be desirable to scan continuously, particularly when demanded by the patient's condition, when it is necessary to look at blood vessel enhancement, and when we wish to avoid heavy sedation. Spiral CT allows the patient and table to be moved at a low but accurately controlled speed ranging from 1-10 mm/s, during continuous i s scanning, resulting in a "spiral scanning" geometry [1]. Special reconstruction software calculates relatively artifact-free planar images from the volumetric data [2]. Application of this CT technique in 100 children is the subject of this communication.

Materials and methods

The spiral CT examinations were performed on a scanner specifically modified to allow up to 24 s data acquisition time, a table speed of 1-10 ram/s, and slice thicknesses of 1, 2, 5, 8, or 10 ram. A distance of up to 23 cm can be scanned in 24 s, with a tube current of 120-165 m A and a kV of 120 or 137. For a smaller volume and a scan time of up to 12 s, a higher tube current of 250 m A can be utilized. A stepper motor has been added to the table feed mechanism, while a separate microprocessor-based control unit is used to select the table feed parameters and to start the multirotational scan in exact synchrony with patient transport. The X-ray tube used is rated at 3.5 million heat units storage capacity. The first 12 patients were examined under an Investigational Review Board approved protocol as the spiral CT attachment and software were then considered investigational. The following 88 patients were examined following F D A approval of the attachment and software. Depending upon the clinical indication, the studies were performed for the brain, neck, orbit, or ear. Contrast medium for vascular phase imaging was given in from onequarter to one-sixth of the amount given for routine brain imaging. Contrast medium was delivered by hand injection as rapidly as possible through the scalp vein needle Table 1. Indications in patients examined by spiral CT

Hydrocephalus Coma (hypoxic, etc.) Hearing loss Head injury Orbital mass Neck mass Sinus disease Stroke Brain tumor Congenital brain anomaly Infection Other Total

33 10 9 8 7 7 6 5 5 4 4 2 100

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Fig. la, b. CT of a 1-month-old with a possible episode of asphyxia. a Axial 5 mm section, 165 mA 120 kV, shows normal anatomy. Grade 3 image quality b Higher section at the vertex shows normal gray-white matter differentiation. Grade 3 image quality

Fig.2a, b. Comparison of axial conventional and spiral CT in a 2-year-old with possible hypoxia following seizures. a Conventional unenhanced CT, 2 s scan at 210 mA and 120 kV, 8 mm section, shows normal brain anatomy b Spiral contrast-enhanced CT, 1 s scan at 165 mA and 120 kV, 5 mm section, shows normal contrast enhancement. Grade 4 image quality (24 g in infants). N e i t h e r a special line n o r machine injection were used. Following injection of contrast m e d i u m , a 12 s delay was gave the contrast m e d i u m time to pass t h r o u g h the heart into the carotid circulation. T h e decision to inject contrast m e d i u m d e p e n d e d u p o n the clinical indications (Table 1). In all patients undergoing a before and after contrast m e d i u m e x a m i n a t i o n or axial and coronal sections, one p o r t i o n of the study was carried out using the spiral technique and the other b y conventional CT, in o r d e r to m a k e a direct comparison, the conventional C T serving as the "gold standard". Results O n e h u n d r e d children, 53 males and 47 females, were e x a m i n e d by spiral C T as p a r t o r whole of their C T examination; 120 separate parts were e x a m i n e d by spiral CT.

Table 2. Grading of image quality of cerebral spiral CT Grade" 1

2

3

4

Total

Factors 160 mA 120 KV 0 145 mA 137 KV 0 Total 0 Grade Description

1 3 4

14 29 43

1 2 3

16 34 50

Uninterpretable Anatomic delineation of ventricles and brain clear. Detail of gray-white matter obscured by grain or streak artifacts Anatomy clear, gray-white matter definition seen, but image slightly grainy Image the same as conventional CT. No graininess

114 Table 3. Grading of image quality of ear and orbit spiral CT

Gradea 1 2

3

4

Total

Factors 160 mA, 120 KV 0 1 1 1 3 145 mA, 137 KV 0 0 7 15 22 Total 0 1 8 16 25 Grade Description Uninterpretable Interpretable, but artifact or graininess degrades image quality Interpretable, average quality CI', slight graininess Above average quality CT, comparable to best conventional CT Table 4. Total surface radiation dose for 24 5 mm sections (cGy)a Radiographic factors 120 KV 165 mA 137 KV 145 mA

Spiral CT 24s

Conventional CT 24xls 24x2s

24.35

24.35

48.70

27.14

27.14

54.28

"Information supplied by Siemens Medical Systems, Inc., Iselin, New Jersey, USA

decade were examined. The most frequent indication (in 33 patients) was investigation of hydrocephalus, including complications such as shunt failure; the second was investigation of coma in emergency r o o m patients, m o s t of w h o m had sustained asphyxia (10 patients). Table 1 gives the indications for spiral CT. In most studies of the brain, we used a 5 m m slice thickness, 160 m A and 120 k V or 145 m A and 137 kV. T h e y studies were graded on a scale of 1 to 4 according to image quality (Figs. 1, 2); the results are given in Table 2. Twentyfive studies were carried out for the orbit or ear, with 2 m m slices, at 160 m A and 120 k V or 145 m A and 137 kV; these studies are graded in Table 3. In 3 patients, spiral brain CT was also c o m p a r e d with single conventional i s CT images at similar m A and kV. The conventional CT images were considerably more grainy. With neither the 5 m m brain sections nor the 2 m m orbit and ear sections were there uninterpretable studies: all were diagnostic, with only 6.6 % marred by artifact or excessive graininess. Ninety-six percent of the studies of the orbit and ear and 94 % of those of the brain fell into the two highest grades of image quality.

Discussion

The age of the patients ranged f r o m birth to 16 years. The single largest group, comprising 50 patients, were those up to 2 years of age, the next largest group those between 2 and 10 years (40 patients). Only 10 patients in the second

The advantages the spiral technique brings to CT are rapidity, the ability to reconstruct sections at variable table positions chosen retrospectively by the radiologist , reduced radiation dose, a need for lower doses of contrast m e d i u m for vascular phase imaging, and the possibility of studying infants and young children using less sedation

Hg.3a, b. Axial CT of a 14-month-old with congenital heart disease, hypotension, sepsis and cerebral ischemia, a 5 mm conventional contrast-enhanced CT, (acquisition time 2 s, mA 250, kV 120) shows slight opacification of both middle cerebral arteries (arrowheads). The cerebral hemispheres are hypodense, b Spiral contrast-enhanced CT, 10 mm section 210 mA, 120 kV, after bolus injection, shows opacification of the circle of Willis

Fig.& Hypoxic brain necrosis in a 5-year-old boy. Axial 10 mm spiral CT section after bolus injection of contrast medium shows a uniformly hypodense brain without grey/white matter differentiation; the falx cerebri is slightly denser than the brain. There is no evidence of arterial or venous flow. A linear area of air within the brain proved at autopsy to be due to necrosis of cerebral tissue, with gas formation

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Fig.5a-d. Osteomyelitis of the ramus of the mandible in a 12-yearold girl. Clinically the neck was normal. a Coronal 2 mm contrastenhanced spiral CT (165 mA, 120 kV) shows marked swelling of the facial soft tissues (arrowheads) with some enhancement b The next contiguous section with a wider window and higher window level, shows periosteal reaction (arrowheads)at the site of the soft tissue swelling. Biopsy revealed chronic osteomyelitis e Bony 3D reconstruction of 23 contiguous 2 mm coronal spiral sections shows irregular cortex of the mandible (arrowhead) at the site of the periosteal reaction d Axial 5 mm spiral CT, 165 mA, 120 kV, after bolus injection of contrast medium shows normal opacification of both common carotid arteries and both jugular veins than usual or none at all. I m a g e quality has b e e n a primary concern: if the images are obtained rapidly, but are of p o o r quality, little has b e e n gained. In Tables 2 and 3 the results are given for 75 studies, 50 of the head and 25 of the orbit or ear. The image quality was b e e n good (Figs. 1,2). H o w ever, half the patients were infants in the first two years of life, i. e., at a time when the calvarium is not thick and the head is smaller. Given the present tube current limitations, a m a x i m u m of 165 m A at 120 k V or 145 m A at 137 kV, studies of the brain in adolescents and older children were not routinely performed. Examination of patients in the second decade of life gave images which were diagnostic, but grainy (Grade 2). A very limited n u m b e r of 8 and 10 m m section brain studies have b e e n performed, so that the data for this approach are too limited. Calculations show that radiation doses in spiral CT studies are half those received in conventional studies of the brain using a 2 s scan time on the same system (Table 4). The indications given in Table 1 reflect those generally found in young children referred for CT of the brain, and

head and neck. Tumors are poorly represented, as most are initially investigated by MR. Trauma, though common, was much m o r e prevalent in older children and adolescents, not presently examined by spiral CT because of tube current limitations. Vascular phase imaging proved a useful adjunct in investigating the comatose patient in w h o m hypoxia or vascular occlusion entered into the differential diagnosis. A d e q u a t e visualization of intra- and extracranial arterial and venous anatomy (Figs.2-5) was achieved following hand injection of contrast medium, even through a small needle (24 gauge in infants). The amount of contrast medium used was only 1/6-1/4 of that routinely injected for studies of the brain or head and neck with conventional CT (1 cc/lb). Thus, the cerebral vasculature could be shown to be patent (Fig. 3 b), even when the supratentorial brain was diffusely infarcted. In this patient, routine CT after injection of the remainder of the contrast medium proved less precise than the spiral CT because it left unanswered the question of the patency of the circle of

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Fig.6. Marked unilateral lymphadenopathy due to Hodgkin's disease in a 4-year-old boy. Axial 5 mm spiral CT, 165 mA, 120 kV, at the level of the hyoid bone after injection of 7 cc Angiovist 282. There is a mnitilobulated left-sided mass, medially displacing and compressing the carotid artery (arrow). The compressed jugular vein was not visible

Fig. 7 a, b. Congenital anomaly of the left external and middle ear in a 12-day-old boy. a Conventional axial CT (2 s acquisition, 165 mA, 120 kV, 1 mm section), which passes through the left temporal bone at a higher level than the right, shows absence of the left middle ear cavity and slight anterolateral orientation of the long axis of the left internal auditory canal, b Spiral CT, 1 mm section, 145 mA, 137 kV shows the same anatomy

Willis (Fig. 3 a). By contrast, complete lack of brain perfusion was demonstrated with spiral CT (Fig. 4) in a patient who had severe hypoxic brain swelling and intracranial hypertension, leading to cessation of cerebral blood flow, brain death and necrosis. Vascular phase imaging also demonstrated the patency and position of the cervical carotid arteries and jugular veins, and their compression and displacement by masses (Fig. 6). Some regions of the head and neck, such as the mandible, orbits, paranasal sinuses and ears have inherent high radiographic contrast, unlike cerebral grey and white matter, and therefore lend themselves to study by spiral CT. L o w e r tube current is not a limitation and I and 2 m m thick sections are readily obtained. Scanning can be carried out with various algorithms, such as a soft tissue one in which the images were displayed with a narrowed window and lower window level for soft tissue (Fig. 5 a) and a higher window level and wider window width for b o n e (Fig. 5 b) or they can be scanned for soft tissue, but reconstructed with a b o n e algorithm (Fig.7b). Bone detail in spiral CT is not noticeably different from that achieved

with conventional higher radiation dose CT as shown in a case of congenital ear anomaly (Fig. 7). The rapidity of acquisition decreases the likelihood of patient motion and increases the artifact free quality of 3D reconstructions (Fig.5 c).

References 1. Rigauts H, Marchal G, Baert AL, Hupke R (1990) Technical note: Initial experience with volume CT scanning. J Comput Assist Tomogr 14:675-682 2. Kalender WA, Vock R Seissler W (1990) Spiral CT scanning for fast and continuous volume data acquisition. In: Fuchs WA, ed. Advances in CT, European scientific user conference SOMATOM PLUS Springer-Verlag, Berlin R. A. Zimmerman, M. D. Department of Radiology The Children's Hospital of Philadelphia 34th Street and Civic Center Boulevard Philadelphia, PA 19104 USA