4.1.1
P h a n to m design
A synthetic p hantom was constructed using a P V C tube, two acrylic tubes, two p lastic plates and la te x m aterial to em ulate the colon wall, polyps and folds. T he external P V C tu b e is 230mm long w ith a diam eter of 300mm. A c rylic tu bes are 235mm long and th e dimensions of th e inner and outer diam eters are 40mm and 50mm respectively. Hounsfield Unit (HU) values of the P V C tu b e, acrylic tubes and plastic plates are 1500, 100, 90 respectively. T h e construction of the synthetic phantom is illu strated in F igu re 4.2.
(a) (b)
Figu re 4.2: S yn th etic colon phantom , (a) Longitu din al view , (b) Transversal view.
T h e p olyp inserts for phantom were m ade b y la tex m aterial h avin g a H U value of -95. W e have chosen to use la tex as this m aterial allows us to generate very realistic shapes (pedunculated, sessile, flat, flat-depressed) for polyps and folds as illustrated
in F i g u r e 4 .3 . I n a d d i t i o n t h e H U v a lu e s a s s o c i a t e d w i t h t h e l a t e x m a t e r i a l a p p r o x i m a t e w e ll t h e H U v a lu e s o f t h e c o lo n w a ll ( ~ 1 0 H U ) . I n C T C t h e la r g e d iffe re n c e b e tw e e n t h e H U v a lu e s a s s o c ia te d w i t h t h e a i r v o x e ls (-1 0 0 0 H U ) a n d t h e H U v a lu e s o f t h e c o lo n t i s s u e is e v a l u a t e d t o i d e n t i f y t h e s u r f a c e o f t h e c o lo n w a ll. T h e m o d e l f o r p o ly p s w a s m a d e f r o m c la y a n d l i q u id l a t e x w a s p o u r e d o n t o t h e m o d e l t o c r e a te t h e l a t e x p o ly p i n s e r t s (se e F i g u r e 4 .3 ) . T o m a k e t h e s u r f a c e o f t h e l a t e x s h e e t m o r e r e a l i s t i c t h e t h ic k n e s s o f t h e s h e e t w a s m a d e u n e v e n . W e h a v e c r e a t e d tw o s h e e t s o f l a t e x c o n t a i n i n g 4 8 p o ly p s h a v i n g d if f e r e n t siz e s (7 f la t p o ly p s , 2 d e p r e s s e d f la t p o ly p s , 15 n o n - s p h e r i c a l p o ly p s , 2 p e d u n c u l a t e d p o ly p s , 22 s p h e r i c a l / e l l i p t i c a l p o ly p s ) a n d 6 h a u s t r a l fo ld s. I n F i g u r e 4 .4 s e v e r a l 3 D v ie w s o f s o m e r e p r e s e n t a t i v e s y n t h e t i c p o ly p s a r e d e p ic te d .
4.1.2
Im age a cq u isitio n
T h e d e v e lo p e d p h a n t o m d e s c r ib e d i n S e c tio n 2.1 w a s s c a n n e d u s in g a 1 6 -slic e S ie m e n s S o m a t o m S e n s a t i o n C T s c a n n e r in t h e M a t e r H o s p ita l, D u b lin , I r e l a n d . T h e p h a n t o m h a s b e e n s c a n n e d in l o n g i t u d i n a l ( p h a n t o m w a s p la c e d p a r a l l e l t o t h e C T s c a n n e r b e d ) a n d t r a n s v e r s a l d ir e c tio n s , w h e r e t h e s c a n n in g p a r a m e t e r s (s lic e th ic k n e s s , fie ld o f v ie w , t a b l e s p e e d , r e c o n s t r u c t i o n i n t e r v a l a n d m A s ) w e re v a r ie d . A ll s c a n s w e r e p e r f o r m e d a t 1 2 0 k V p a n d 1 .5 m m x 16 c o llim a tio n . I t is u s e f u l t o n o t e t h a t t h e e f fe c tiv e r a d i a t i o n d o s e is in f lu e n c e d b y t h e v a lu e o f t h e t u b e v o lta g e b u t its r e l a t i o n s h i p w i t h im a g e q u a lity , t i s s u e c o n t r a s t a n d im a g e n o is e is c o m p le x a n d t h e e ffe c t o f t h i s p a r a m e t e r w o u ld b e d if f ic u lt t o b e e v a lu a te d . T h e r e f o r e , in t h i s e x p e r im e n t s t h e v a lu e o f t h i s p a r a m e t e r is m a i n t a i n e d c o n s t a n t (1 2 0 k V p ) a n d a n o t h e r r e a s o n is t h e f a c t t h a t t h i s is t h e s t a n d a r d v a lu e o f t h e t u b e v o lta g e u s e d i n c lin ic a l e x a m i n a t i o n s . T h e s c a n n e r u s e d in t o g e n e r a t e t h e C T d a t a a llo w s t h e p o s s ib ility t o a d j u s t t h e v a lu e o f c o llim a tio n t o 0 .7 5 m m b u t t h e v a lu e o f c o llim a tio n w a s fix e d a t1,5 m m in o r d e r t o r e d u c e t h e r a d i a t i o n d o s e . I n t h i s r e g a r d , a C T s c a n p e r f o r m e d w i t h 1 .5 m m c o l l i m a t i o n a n d 3 m m s lic e th i c k n e s s w ill r e s u l t in a n e n e r g y i m p a r t e d o f 7 .0 m S v w h ile t h e e n e r g y i m p a r t e d fo r a C T s c a n w i t h 0 .7 5 m m c o llim a tio n a n d 3 m m s lic e th i c k n e s s is 7 .8 m S v w h ic h is t o h ig h t o b e u s e d s a fe ly i n c lin ic a l s tu d ie s . T h e s m o o t h i n g r e c o n s t r u c t i o n f ilte r u s e d w a s t h e B 3 0 f ilte r [131] a n d t h i s f ilte r h a s b e e n e m p lo y e d b a s e d o n i t s o p t i m a l p e r f o r m a n c e in d a t a s m o o t h i n g a n d n o is e r e m o v a l ( t h i s is t h e f ilte r u s e d in m o s t c lin ic a l s t u d i e s fo r a b d o m i n a l C T s c a n s ) .
(a) 9 # » • Sphsrical —^ \ Flat / P»dunculjtad • C » Spherical aM 4--- Flat Flal / \ _ Non ^ Spherical (b)
Figu re 4.3: L a te x sheet w ith various typ es of polyps and folds.
the following spread of param eters: field of view: 325 and 360mm, tab le speed: 20 to 30 m m /rotation, slice thickness of 2 and 3mm and m As: 100, 80, 70, 60, 50, 40, 30, 20 and 13 (13 m A s is th e m inim um value th a t can be set for Siemens Som atom Sensation C T scanner used in these experim ents). T hese scanning param eters have been divided into six protocols as follows:
• P rotoco l 1: C ollim ation 1.5 x 16m m, slice thickness 3mm, reconstruction in terval 1.5m m , field o f view 325mm, tab le speed 30m m /rotation, m As: 100, 80, 70, 60, 50, 40, 30, 20 and 13. T h is protocol was used to identify the effect of radiation dose and scan orientation (longitudinal and transversal scans) on
(c) (d)
Figu re 4.4: 3D longitudinal view s of the syn th etic polyps m ade from latex.
the perform ance of th e developed au tom atic C A D - C T C system .
• P ro to co l 2: C ollim atio n 1.5 x 16m m, slice thickness 3mm, reconstruction in terval 1.5m m , field of view 360mm, tab le speed 30m m /rotation, m As: 50, 30, 20 and 13. T h is protocol was em ployed to evalu ate the influence of the field of view and th e variation of the radiation dose.
• P ro toco l 3: C ollim ation 1.5 x 16m m, slice thickness 3mm, reconstruction in terval 1m m , field of view 325mm, table speed 30m m /rotation, m As: 100, 80, 70, 60, 50, 40, 30, 20 and 13. T h is protocol was used to analyse th e effect of the recon struction interval and the radiation dose.
• P ro toco l 4: C ollim atio n 1.5 x 16m m, slice thickness 2mm, reconstruction in terval 1m m , field of view 325mm, table speed 30m m /rotation, m As: 100, 50, 40, 30, 20 and 13. T h is protocol was used to generate C T d ata where the effect of the slice thickness and the radiation dose is analysed.
• P rotoco l 5: C ollim ation 1.5 x 16mm, slice thickness 2mm, reconstruction in terval 0.8mm, field of view 325mm, tab le speed 30m m /rotation, mAs: 100, 50, 40, 30, 20 and 13. T h is p rotocol was em ployed to analyse the joint effect of the slice thickness, reconstruction interval and radiation dose.
• P rotoco l 6: C ollim ation 1.5 x 16mm, slice thickness 3mm, reconstruction in terval 1.5m m , field of view 325mm, tab le speed 20m m /rotation, m As: 100, 50, 40, 30 and 20. T h is protocol was used to find the effect of table speed at different radiation doses.
W ith a m ulti-slice C T scanner, the selection of the (reconstructed) slice w id th is independent of patien t dose, being solely reliant on collim ation selected. Therefore if the collim ation rem ains the same, th e selection of a 5mm slice w idth w ill generate th e same radiation dose as a 3mm slice thickness. It has been found th a t the Siemens Som atom Sensation 16 slice C T scanner shows sim ilar im parted radiation dose for b o th 3mm and 5mm (7m Sv for b o th 3mm and 5m m at lOOmAs). Thus, it has been decided to ignore the 4mm, 5mm slice thickness in this stu d y since these settings w ill have v irtu a lly no effect on the im parted radiation dose received b y the patients.
A s already explained, for Siemens Som atom m ulti-slice C T scanner the variation in the tab le speed is possible, b u t b y increasing th e tab le speed (reduce the duration of the C T scan) does not va ry the patient dose, as this scanner utilises the ’’ effec tiv e tu be current” m odel where the m A s is kept constant throughout (a variation in scanning tim e results in a concom itant variation in m A s). In this study, the ta ble speed has been varied to evaluate the influence of the m otion artefacts on the perform ance of the C A D - C T C . Thus, only the 30m m /rotation and 20m m /rotation tab le speeds were chosen to use in this study.
A s m entioned earlier, a collim ation of 1.5m m was used for two reasons. F irstly because this setting is recognised as adequate to d etect clin ically significant colonic polyps (5mm and greater). Secondly while a 0.75 collim ation was possible w ith the Siemens scanner, this settin g generates a m arkedly increased patient dose. A 1.5m m collim ation, w ith 3mm slice w id th results in a scan tim e o f 10.2sec and an im parted energy of 7.0m Sv. T h e 0.75m m collim ation setting w ith a 3mm slice results in a scan tim e of 20.14sec and an associated im parted energy of 7.8mSv. T his was deemed
to be unacceptable as this radiation dose is too high to be used safely in clinical exam inations.
4.1.3
C h a ra c te risa tio n o f p h a n to m C T d ata
T h e m ethod applied for feature detection for phantom polyp is the m ethod based on th e statistical features th a t is discussed in C h ap ter 3.2. T h e statistical features include the stan dard deviation (SD ) o f surface variation, SD of the three axes of the ellipsoid, SD of the sphere radius, SD of the ellipsoid fittin g error, SD of the sphere fittin g error, G aussian distribution, principal axes o f th e ellipsoid and sphere radius. In this section we evalu ate the statistica l features for phantom polyps, real polys and folds in order to illu strate the fact th at the phantom polyps em ulate closely the p olyps encountered in clinical studies. Figu re 4.5 shows the standard deviation of th e surface variation for 45 phantom polyps (14 polyps > 10m m, 20 polyps between [5 — 10)m m , 5 polyps < 5m m and 6 flat polyps) and 41 real patient polyps and 274 folds w ith different sizes. T h e SD s of surface variation for phantom polyps are placed close to those o f the real p olyps. Sim ilarly, the features: SD of m ajor axis, SD of ellipsoid error, SD of sphere radius and SD of sphere error for phantom exhibit sim ilar characteristics w ith th e real p olyps as illu strated in Figures 4.6 to Figure 4.9.
Standard d e viation o f surface change curve fo r p o ly p and folds
— ♦— Phantom Polyps — ■— Real Polyps ■■■*"■ Fold Class_l Fold Class_2 Fold Class_3 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 Number of candidate surface
F igu re 4.5: Stan d ard d eviation of the surface variation for phantom polyps, real p olyps and folds.
Standard d e viation o f m ajor axis fo r polyps and fo ld s —*— Phantom Polyps —■— Real Polyps Fold Class_l ■ Fold Class_2 ■ ■■*•■• Fold Class_3 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 Number of candidate surface
F igu re 4.6: S tan dard deviation of th e m ajor axis o f ellipsoid fittin g for phantom polyps, real polyps and folds.
Standard deviation o f e llip so id error fo r polyps and folds
♦ Phantom Polyps —■— Real Polyps
*■»**• Fold Class_l Fold Class_2 ... p0]d ciass_3
Number of candidate surface
Figu re 4.7: Standard deviation (SD) of the ellipsoid fittin g error for phantom polyps, real p olyps and folds.