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Aim: The aim of this study is to determine the variation in Hounsfield values with single and multi-slice methods using in-house software on fan-beam computed tomography (FCT), linear accelerator (linac) cone-beam computed tomography (CBCT), and Icon-CBCT datasets acquired using Gammex and advanced electron density (AED) phantoms. Materials and Methods: The AED phantom was scanned on a Toshiba computed tomography (CT) scanner, five linac-based CBCT X-ray volumetric imaging systems, and Leksell Gamma Knife Icon. The variation between single and multi-slice methods was assessed by comparing scans acquired using Gammex and AED phantoms. The variation in Hounsfield units (HUs) between seven different clinical protocols was assessed using the AED phantom. A CIRS Model 605 Radiosurgery Head Phantom (TED) phantom was scanned on all three imaging systems to assess the target dosimetric changes due to HU variation. An in-house software was developed in MATLAB to assess the HU statistics and the trend along the longitudinal axis. Results: The FCT dataset showed a minimal variation (central slice ± 3 HU) in HU values along the long axis. A similar trend was also observed between the studied clinical protocols acquired on FCT. Variation among multiple linac CBCTs was insignificant. In the case of the water insert, a maximum HU variation of −7.23 ± 68.67 was observed for Linac 1 towards the inferior end of the phantom. All five linacs appeared to have a similar trend in terms of HU variation from the proximal to the distal end of the phantom, with a few outliers for Linac 5. Among three imaging modalities, the maximum variation was observed in gamma knife CBCTs, whereas FCT showed no appreciable deviation from the central value. In terms of dosimetric comparison, the mean dose in CT and Linac CBCT scans differed by <0.5 Gy, whereas at least a 1 Gy difference was observed between CT and gamma knife CBCT. Conclusion: This study shows a minimal variation with FCT between single, volume-based, and multislice methods, and hence the current approach of determining the CT-electron density curve based on a single-slice method would be sufficient for producing a HU calibrations curve for treatment planning. However, CBCTs acquired on linac, and in particular, gamma knife systems, show noticeable variations along the long axis, which is likely to affect the dose calculations performed on CBCTs. It is highly recommended to assess the Hounsfield values on multiple slices before using the HU curve for dose calculations.

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Two experiments were performed. Experiment 1 had 12 subjects pedal on a stationary bicycle v s a constant load f o r 12 min twice i n each of t h r e e c o n d i t i o n s : a ) s t a n d a r d bicycle s e a t and no toe c l i p s , b) experimental web s e a t and toe clips, and c) experimental web s e a t and no toe clips. The c r i t e r i a were h e a r t r a t e (12 min work + 18 min recovery), Borg vote, and a semanticd i f f e r e n t i a l vote. Heart r a t e was 105.3 for condition a, 110.2 f o r condition b, and 110.5 f o r condition c Condition a was significantly lower than b and c; b and c d i d not d i f f e r significantly. Borg vote and semantic-differential vote gave s i m i l a r results.The 12 conditions were a l l combinations of: a) toe c l i p s v s no toe g l i p s , b) s e a t horizontal vs s e a t t i l t e d 7' down, and c ) center of s e a t a t a 19, 25, or 3 1 a n g l e v s v e r t i c a l and hub centerline. The c r i t e r i a were h e a r t rate, Borg vote, and a semantic-differential vat% Toe c l i p reduced h e a r t r a t e by 3.58 d u r i n % e x e r c i s e . than 19 ; t h e difference between 25 and 31 was non-significant.Although tilt did not a f f e c t h e a r t r a t e , the subjects preferred no tilt. Experiment 2 had 10 subjects pedal f o r 10 min once i n each of 12 conditions. Seat l o c a t i o n was better a t 25 W e became i n t e r e s t e d i n human generated power. The bicycle is q u i t e e f f i c i e n t (Wilson, 1973; Whitt and Wilson, 1982). Yet t h e r e i s r e l a t i v e l y l i t t l e published systematic experimentation on some design variables. For those i n t e r e s t e d i n human power, c o n s u l t W h i t t and W i l s o n (1982). T h e r e a l s o i s B i k e Tech (ttBicycling magazine's newletter f o r the technic a l enthusiast"). ' Two design f e a t u r e s were s e l e c t e d f o r i n v e s t i g a t i o n : a ) t h e use of t o e c l i p s , and b) t h e r e l a t i o n of t h e s e a t and t h e pedal. I n Experiment 1, t h e standard s a d d l e s e a t was replaced by a webbed chair on a metal frame ( a k i n t o lawn c h a i r s b u t w i t h t h e m e t a l c r o s s bar below the webbing so the bar did not contact t h e thigh). It had a webbed back. The second experimental variable was the use (and non-use) of t o e c l i p s .EXPERIMENT 1 Bi2k Each subject s a t on a stationary bicycle i n an environmental chamber (70 F and 50% rh). They p e d a l l e d a t a c o n s t a n t r a t e of 60 rpm (cadence from a 120 b e a t h i n metronome) f o r 12 min against a light constant load (100 W). Each 12 min exercise period was followed by an 18 min rest period, s e a t e d on a chair. There were three conditions on a day: a ) standard bicycle s e a t and no toe clips, b) experimental web s e a t and toe clips, and c) experimental web s e a t and no t o e c l i p s . After a 24 t o 48 h i n t e r v a l , t h e t h r e e c o n d i t i o n s were r e p e a t e d i n a m i r r o r e d order (different order f o r each subject). Subiects e...