Indian Journal of Radiology Indian Journal of Radiology  

   Login   | Users online: 2214

Home Bookmark this page Print this page Email this page Small font sizeDefault font size Increase font size     

 

NEW TECHNIQUES AND PHYSICS Table of Contents   
Year : 2004  |  Volume : 14  |  Issue : 2  |  Page : 209-216
Multislice CT : A quantum leap in whole body imaging


Medical Journal Armed A.F.M. College, Pune-411040, India

Click here for correspondence address and email
 

   Abstract 

Multislice CT (MSCT) is a specialized CT system equipped with a multiple detector array that simultaneously obtains tomographic data at different siice locations. MSCT provides unparalleled capabilities for detailed analysis of normal and abnormal anatomy and pathology. MSCT facilitates a wide range of clinical applications ranging from 3D imaging to perfusion imaging to CT Fluoroscopy. Having the capability of rapidly scanning large longitudinal volume of patient with high z-axis resolution and true isotropic voxel generation, it offers new challenges and applications, not available hitherto with single-slice CT scanning. This article provides insight into the fundamental physical principles of multidetector CT and glimpse of its powerful capabilities in established and novel clinical applications at this present point of time.

Keywords: Multislice CT; Multidetector CT, 3D Imaging

How to cite this article:
Jatava S K. Multislice CT : A quantum leap in whole body imaging. Indian J Radiol Imaging 2004;14:209-16

How to cite this URL:
Jatava S K. Multislice CT : A quantum leap in whole body imaging. Indian J Radiol Imaging [serial online] 2004 [cited 2019 Nov 12];14:209-16. Available from: http://www.ijri.org/text.asp?2004/14/2/209/28591

   Introduction Top


Multislice CT represents the latest breakthrough in CT Technology. Fundamentally, MSCT scanner [Figure - 1] is equipped with a multiple detector array that concurrently collects data at different slice locations[1], a defining feature that brings in numerous advantageous spin-offs like rapid scanning, large patient coverage volume, high z-axis resolution, generation of true isotropic data sets which in combination facilitates 3D imaging, Perfusion imaging, CT Fluoroscopy and so on [3] MSCT provides a huge gain in performance that can be used to reduce scan times, reduce scan collimation or to increase scan length substantially.

Historical Aspects in Multislice CT Scanner Development

Looking back to 1972, the earliest CT machine typically performed a brain scan of 10 cm coverage in 40 minutes, imposing overnight image reconstruction of the final image. Specifically, each slice of 180-degree rotation took 4 minutes to perform . Reflecting the rapid developments during the last five years, a 16-slice MSCT scanner having advanced features such as 0.42 seconds per rotation with millisecond image reconstruction scanned the entire body with 1 mm slices in 19 seconds, generating instant images. Tracing this staggering transformation, MSCT had its humble beginning in late 1988, with developments in slip ring technology, which removed the need for repeated "rotate-translate" or "step-and-shoot" type of Xray tube-patient scanning mechanism. By this, a continuous spiral run was possible with continuous scanning during continuous patient transport and continuous data acquisition and image generation[3].

Truly, the onset of MSCT technology was in 1992, when Dual slice CT scanner was introduced by Elscint. Surprisingly, the global market was not enthusiastic about it in this period. In the second half of 1998 at the RSNA, (a premier Radiology event held each year at Chicago), four manufacturers announced the launch of MSCT Scanners (GE Lightspeed, Picker MX 8000. Toshiba Aquilon Multi, Siemens Volume Zoom), catalyzing an

Aquilon Multi, Siemens Volume Zoom), catalyzing an unprecedented quantum leap in clinical performance [4]. Depicting the chronological developments, [Table - 1] shows the continuum in technology from Helical CT to Multislice CT.

Heralding the thirtieth anniversary of clinical CT scanning in 2002, the widespread advent of Multislice CT (MSCT) scanners is considered as the single most important development in Computed Tomography since the introduction of CT itself. Today many terms have been used to describe this new technology. Multislice helical CT Multirow helical CT, Multidetector helical row CT, Multisectional helical CT and Multichannel hetical CT [5]. Multislice and multisection implies that for each gantry rotation more than one image slice or image section is created. Multidetector row describes that the detector is composed of several rows of detectors. Multichannel expresses that several channels of information are collected during each gantry rotation. However, the term multislice and multidetector CT are generally used in technical and clinical literature to describe this new technology.


   The physics of detection design Top


The fundamental difference between MSCT scanners and its predecessors i.e., single slice spiral or helical scanners, lies in the detector array design. The beneficial strategy in redesigning CT scanner detectors of replacing a single detector row by four or more rows allows a dramatic increase in data acquisition capability besides numerous other spin-offs [Table - 2] Traditionally, a single slice, spiral CT scanner has a single tube source that irradiates one row of detectors measuring about 20 mm in length (along Z or long axis)[6]. Regardless of the X-ray beam collimation, there is only one row of detectors. In MSCT this is replaced by a multiple row of detectors called a detector array that enables simultaneous acquisition of 4 or 16 slices during one gantry rotation[7].

Three types of detector array matrices are available presently : fixed adaptive and mixed types. A fixed detector array has all matrix elements of same size (as in GE Lightspeed versions), while an adaptive matrix has elements wider away from the center (Philips/Siemens). A mixed array has all matrix of same size with exception of a number of thinner ones at center (Toshiba) [Figure - 2] A & B. The array design is an important entity in MSCT scanner [Figure - 3]A & B since it directly influences a) the minimum slice width capabilities, b) the number of slices possible at minimum width, c) the range of slices available and d) the maximum length imaged in one direction. The detector array is configured to operate with a slip ring that allows it to continuously rotate. Both these result in a significant reduction of rotation time of tube detector assembly from 1 s to 0.5 s. The net result of doubling the rotation speed and providing four times the detector rows is an eight fold increase in speed, on average. A comparison of salient features in currently available 4 and 16 slice scanners is presented in [Table - 3].


   Advantages Top


A list of advantages derived from MSCT technology is displayed in [Table - 4]. In clinical applications however a combination of these advantages are utilize for obtaining useful imaging information. Two beneficial areas of MSCT technology are a) an improved scan speed and b) availability of isotropic imaging. Improved scan speed enables not only better coverage in a single breath hold, but provides a significant reduction in patient movement artifacts and encourages better use of contrast media [8]. The faster speed of the MSCT scanner also translates into several dramatic changes in routine practice. In scanning pediatric applications the improved scan speed results in lesser need for sedation[9]. It is also possible to freeze the motion of a patient, while acquiring a volume of information rather than simply a set of slices. The other feature of MSCT that provides spectacularly useful images is "isotropic" imaging[10]. It is defined as identical resolution of a structure in all dimensions. This valuable feature is of importance in 3D imaging [Figure - 4], where stair step artifacts are virtually eliminated and the anatomical edges are well defined [11].


   Clinical Applications Top


Broadly, MSCT technology has improved existing clinical applications while bringing in many novel clinical applications. These are summarized below.

a) Existing applications improved by Multislice CT

i. CT Angiography : MSCT augments CT angiography by a) allowing multiphasic studies of vascular system (i.e. arterial and venous phases) due to faster scan times and optimal use of contrast, b) accurate delineation of narrow vessels due to the improved z axis resolution and c) peripheral runoff angiography due to longer scan and volume coverage [12]. Overall, vascular protocols greatly benefit from the high speed of MSCT. Consequently, the amount of contrast material is substantially reduced for imaging the diseased thoracic aorta or emboli in pulmonary circulation.

ii. 3D Imaging : The term 3D imaging is defined as "all processing operations that are applied to acquire multidimensional image date to facilitate visualization, manipulation, and analysis of the information captured in the image data "MSCT has introduced staggering advances in 3D imaging due to a combination of factors such as isotropic imaging that result in no significant loss of resolution in z axis [13].

iii. Virtual Endoscopy : MSCT offers a promising method of endoluminal evaluation of structures like airway, bowel vessels. Sourced from high resolution, two-dimensional axial images and requiring an entry point on the three-dimensional data set, the endoscopy view are essentially 3D fly through images with negligible image editing [Figure - 5]. Virtual Bronchoscopy depicts airways from an endoluminal perspective using shaded surface display or volume rendering with increased opacity settings. It is a minimally invasive, well-tolerated and safe procedure for the evaluation of the endoluminal surface of the tracheobronchial tree. Likewise, Virtual Colonography is another application of volume imaging that utilizes the data generating three-dimensional images of the entire colon, increasing the speed and ease of locating and analyzing polyps. In particular, Virtual Colonography is emerging fast as a screening tool for colonic polyps and cancer, simulating the standard procedure[4].

iv. Virtual labyrinthoscopy : MSCT offers detailed visualization of the tiny and complex structures of the inner ear by using high-quality 3D representations like direct volume rendering. 3D visualization enabled high-resolution images of selected (a) semicircular canals, (b) ampullae and vestibule, (c) cochlea, (d) inner auditory canal, and (e) mastoid and labyrinthine segments of the facial canal. Intuitive manipulation of any of the above structure by clip planes often displays the best possible representation of the pathologic condition affecting inner ear.

v. 3D Dental CT generates panoramic, cross-sections and axial imaging of the teeth and jaws. Dedicated software allows the evaluation of the odontic anatomy by semi-automatic reformatting of panoramic views along jaw bone and paraxial reformats which give the buccal to lingual perspective perpendicular to the arch of the jaw, 3D Dental CT is commonly used for pre-surgical assessment of implant insertion, for diagnosis of tumors and bony lesions in maxilla and mandible, for diagnosis of anomalies of temporomandibular joint and in diagnosis of structural anomalies of the maxilla and mandible.

b) New Applications pioneered by Multislice CT

I Perfusion CT Imaging of Brain is a quick and convenient method of assessing perfusion disturbances in acute stroke patients. Three-color image maps with quantitative results related to patient regional cerebral blood volume (rCBV), mean transit time (MRTT) and regional cerebral blood flow (rCBF) are generated that displays stroke much earlier than the conventional CT images [15]. Areas of less severe CBF reduction, with preserved CBV value represent "ischemic penumbra", a term describing tissue at high risk for infarction but not yet irreversibly infracted. The larger the ischemic penumbra relative to the core, the more likely the patient would benefit from early thrombolytic therapy. If both CBV and CBF are already reduced dramatically, the tissue is considered irreversibly infracted. In the acute stroke setting CTA with Perfusion CT is useful for : 1) quick and reliable identification of stroke signature; 2) improved selection of patients for thrombolytic therapy; 3) identifying the vascular origin of ischemic insult ; and 4) mapping the sequelae of stroke like final infarct size and hemorrhagic risk. Besides Perfusion CT of Brain, Body perfusion imaging is increasingly applied for lesions in kidneys, liver and lungs.

ii. Cardiac Imaging : Fundamentally, MSCT has been utilized in three novel areas of cardiac imaging : a) Cardiac calcification scoring, b) Coronary angiography and c) Assessment of cardiac function. Cardiac calcification scoring is a promising technique utilizing the numerical quantification of calcium in coronary artery as an indicator of coronary heard disease. The calculation of the volume of pixels above a certain threshold (usually 130 HU) enables the assessment of the degree of cardiac calcification. CT Coronary angiography is essentially a non-invasive imaging technique of coronary arteries by retrospectively ECG gated cardiac techniques that enable the detection of stenosis and visualization and differentiation of soft plaque [16]. It is useful in following up of post CABG, particularly in cases with stent placement

iii. CT Fluoroscopy: Introduced by Katada et al, in 1993 using a modified Toshiba 3rd generation CT scanner, this powerful tool uses continuous scanning facilitating real time imaging. It is particularly useful for interventional techniques such as biopsy and drainage of thoracic, abdominal, pelvic and retroperitoneal lesions, drainage and aspiration of intracranial haematomas, rod and seed placement for brachytherapy, motion analysis and bolus tracking in CTA [17]. A 'real-time" information display removes the impact of patient breathing and motion on image quality and permits accurate depth and direction demarcation of the needle during procedures. It also offers increased accuracy of fine needle control thereby reducing complications while reducing the procedure time. It is estimated that the skin entrance dose from this procedure is approximately 4 mGy/s.

iv. Fusion Medicine : Fusion imaging essentially combines anatomical and physiological mapping of lesions, making it a powerful tool for clinicians and radiologists in the better understanding of disease [18]. MSCT is a core modality in Fusion Imaging since it provide accurate spatial and density information. It is combined with Nuclear Scans, PET or MRI Scans and operates at either system levels or data acquisition or processing level. Such a use of "two-in-one" systems that allow fusion of the high spatial resolution CT images with functional information provided by Nuclear Scan, PET or MRI Scan images is currently being evaluated in specialized centers. Its role in 3D radiation treatment planning (3DRTP) enables accurate radiation dose calculations to deliver appropriate dose to the planning target volume (PTV) and sparing of Organs at Risk (OAR)

v. Screening CT : This is an emerging concept targeting early detection of disease entities like lung cancer, colon cancer and coronary artery disease [19]. Low Dose CT lung cancer screening features computer-aided detection of nodules, which automatically matches and compare areas of suspicion while detecting new small lesions. Low-Dose CT lung cancer screening of long-time smokers often reveals suspicious nodules. The imaging strategy in the screening of these cases aims to identify stage IA cancers. It has been estimated that the overall yield in screening lung cancers is better when screening is limited to high risk patients over 60 years with at least 10 pack-years of smoking history. Coronary artery calcium screening test focuses on coronary calcification and its relationship to coronary atherosclerotic plaques and the likelihood of occlusive disease. Screening Colonography is emerging as a screening test of five-minute duration used to detect colorectal cancer in people over 50. In clinical trials, it comes close to matching the performance of conventional colonoscopy [20],[21].


   Issues of Concern Top


The three big issues of concern in a MSCT environment are a) Radiation b) Data overload and c) the overall costing.

i. Radiation: High end CT applications such as Perfusion CT, CT Angiography, CT Colonography, CT Fluoroscopy and Screening CT entails more radiation dose to a potentially large number of individuals. A recent study comparing radiation exposure in coronary calcium quantification low dose protocols showed that radiation exposure between 0.5 and 1.3 mSv are being used (exposure by 4 slice trigger 2.5 mm score, 16 slice trigger 3 mm score, 4 slice gating 3 mm score and 16 slice gating 3 mm score were 0.7 mSv, 0.6 mSv, 1.3 mSv, 1.1 mSv respectively) [22]. High-resolution protocols for cardiac and coronary CT angiography necessitate 3 to 5 mSv radiation exposure (exposure by slice gating 1.25 mm Coronary CTA and 16 slice gating 0.75 mmCTA were was 3.5 mSv and 4.3 mSv). In comparison, the same study deliberated that the exposure in calcium scoring by EBCT, Diagnostic Coronary Angiography and Background radiation per annum were 0.9mSv, 2-6 mSv and 2-5 mSv respectively [22]. Addressing this serious problem, dose reducing measures have been aggressively undertaken which include a) Use of optimized CT settings; b) Reduction of tube current; c) use of tube-current settings based on patient weight / diameter / anatomical region of interest; d) Increasing pitch; e) Reducing the number of repeat scans with contrast material and f) Purging inappropriate referrals for CT

ii. Data Overload : MSCT scans generate an alarming number of images in each study, which can slow down the work flow in a busy Radiology Department For instance, a scan of chest and abdomen of 60 cm coverage with 4x1 mm collimation over 50 seconds generates 500 to 600 images, depending on the degree of overlap. This data explosion is further compounded, by the numerous combinations possible using various rendering techniques in orthogonal and non-orthogonal planes and in multiple window settings. To tame the deluge of MSCT imaging data, remedial measures have come to the forefront, that includes: a) a mandatory need for Dicom 3.0 compatible workstations capable of high end 3D processing; b) increased processing power at workstations with optimized software package: c) presence of high-speed network for image transfer from the CT system to workstation; d) shift in emphasis of analyzing and filming 2D axial images to 3D rendered images [23].

iii. Cost of CT Hardware : MSCT scans necessarily requires high heat capacity x-ray tubes Multislice CT X-Ray tubes operate at 7MHU, in contrast to the conventional CT tubes, which are of 1 to 2 MHU capacity. Expectedly they are expensive ranging Rs.30 to 40 lakhs. Besides this, important components such as generators, ceramic detectors, on board computers, all utilize more advanced technology and are inherently expensive [24].


   Future Top


It is generally believed that further exponential increase in number of detectors to 32, 64 and 256, will not expand the clinical applications in MSCT imaging. Rather, newer frontiers will be explored by a) further reduction of rotation time to less than 0.4 seconds: b) improving the detector characteristics; c) reducing the mechanical stress on the gantry stems and d) further developments in cone beam CT. Specifically, faster scanning using "flat panel" detectors is one area of progress. They are based on the same flat-panel technology employed in digital radiography and with promising results in its preliminary applications. Research work with cone beam scanner is underway allowing capture of very high-resolution data sets. It is expected that this machine will perform radiography, fluoroscopy as well as CT, a situation of convergence with endless possibilities.


   Conclusion Top


In conclusion, multislice CT is the current standard in computed tomography imaging. Its advantages in speed and spatial resolution, generation of isotropic voxel (volume element) imaging have augmented many existing applications such as cardiopulmonary, pediatric and angiographic imaging, while ushering in novel applications and newer tools of promise like CT Fluoroscopy, Perfusion CT and Cardiac Imaging MSCT has bought about impressive changes not only in speed of scanning and novel applications, but has dramatically changed radiology workflow and even the way radiologists and clinicians handle CT data. Indeed, this is just the beginning of yet another imaging renaissance and a sense of eager anticipation awaits the medical fraternity at large.

 
   References Top

1.Rydberg J, Buckwalter KA, Caldemeyer KS et al. Multsiection CT : scanning techniques and clinical applications. Radiographics. 2000;20(6): 1787-806.   Back to cited text no. 1    
2.Kopp AF, Klingenbeck-Regn K, Heuschmid M, Kuttner A et al Multislice Computed Tomography: Basic Principles and Clinical Applications Electromedical 68 (2000) No 2:94-95.   Back to cited text no. 2    
3.Kalender WA, Seissler W, Klotz E, Vock P. Spiral volumetric CT with single-breath hold technique, continuous transport, and continuous scanner rotation. Radiology 1990; 176:181-183.   Back to cited text no. 3  [PUBMED]  
4.Helical and Multi-slice Principles Available at http://www.impactscan.org/impactdayslides.htm Accessed on 01 Sept 2003.   Back to cited text no. 4    
5.Prokop M. General principles of MDCT. Eur J Radiol. 2003; 45 Supple I:S4-10.   Back to cited text no. 5    
6.Kalender WA. Principles and applications of spiral CT. Nucl Med Biol. 1994;21(5): 693-9   Back to cited text no. 6    
7.Horton KM, Sheth S, Corl F, Fishman EK. Multidetector row CT: principles and clinical applications. Crit Rev Comput Tomogr. 2002;43(2):143-81.   Back to cited text no. 7    
8.Hu H, He HD, Foley WD, Fox SH. Four multidetector-row helical CT: image quality and volume coverage speed. Radiology. 2000:215(1): 55-62.   Back to cited text no. 8    
9.Denecke T, Frush DP, Li J. Eight-channel multidetector computed tomography: unique potential for pediatric chest computed tomography angiography. J Thorac Imaginag 2002;17(4): 306-9.   Back to cited text no. 9    
10.Kalender WA Thin-section three-dimensional spiral CT: is isotropic imnaging possible ? Radiology 1995 197:578-580.   Back to cited text no. 10    
11.Fishman EK: Clinical 3D Imaging-Has Its Time Finally Arrived? Available at http:/www.ctisus.org.multidetector/syllabus/Clinical_3D.html. Accessed on 01 Sept. 2003.   Back to cited text no. 11    
12.Rubin GD. Shiau MC, Schmidt AJ, et al. Computed tomographic angiography historical perspective and new state-of-the-art using multi detector row helical computed tomography. J Comput Assist Tomogr 1999;23(supple 10: 83-90.   Back to cited text no. 12    
13.3D Imaging and Virtual Reality Applications of Multidetector CT 5th International Som atom CT Scientific User Conference, Zurich, June 16-17 2000 Conference volume Available at http:/www.radiology-usz.ch/html/institut/3dlabor/pdi/MSCT.pdf; Accessed on 01 Sep 2003.   Back to cited text no. 13    
14.Foley WD. Special focus sessioin: multidetector CT : abdominal visceral imaging. Radiographics. 2002;22(3):701-19.   Back to cited text no. 14    
15.Konig, M., Klotz, E., Luka, B: Perfusion CT of the brain diagnostic approach for early detection of ischemic stroke. Radiology, 209, (1998), 85-93.   Back to cited text no. 15    
16.Kopp AF, Schroeder S, Kuettner A, et al. Coronary arteries: Retrospectively ECG-gated multidetector row CT angiography with selective optimization of the image reconstruction window. Radiology. 2001;221:683-688.   Back to cited text no. 16    
17.New Applications in CT Available at http://www.impactscan.org/impactdayslides.htm Accessed on 01 Sep 2003.  Back to cited text no. 17    
18.Antoch G, Beyer T, Freudenberg LS, Miller SB, PET/CT or CT/PET? A Radiologist's perspective Electromedica 71(2003) No 1:64-69.   Back to cited text no. 18    
19.R. M. Friedenberg The 21st Century: The Age of Screening Radiology, 2002:223(1):1-4.   Back to cited text no. 19    
20.Johnson CD, Dachman AH CT Colonogrpahy: The Next Colon Screening Examination? Radiology 2000;216(2):331-341.   Back to cited text no. 20    
21.T. Ferrucci, Colon Cancer Screening with Virtual Colonoscopy: Promise. Polyps, Politics Am. J Roentgenol 2001;177(5):975-341.   Back to cited text no. 21    
22.Jakobs TF, Becker CR, Ohnesorge B, Flohr T et al Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation. Eur Radiol 2002; 12(5): 1081-6.   Back to cited text no. 22    
23.Rubin GD. Data explosion: the challenge of multidetector-row CT, Eur J Radiol. 2000:36(2): 74-80.   Back to cited text no. 23    
24.Challenges in the evaluation of 8 and 16 slice CT scanners Available at http://www.impactscan org/slides/ukre2002/mdatalk/sld001htm Accessed on 01 Sept 2003.  Back to cited text no. 24    

Top
Correspondence Address:
S K Jatava
Medical Journal Armed A.F.M. College, Pune-411040
India
Login to access the Email id

Source of Support: None, Conflict of Interest: None


Rights and PermissionsRights and Permissions


    Figures

[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5]

    Tables

[Table - 1], [Table - 2], [Table - 3], [Table - 4]

This article has been cited by
1 Periampullary and pancreatic incidentaloma: A single institutionęs experience with an increasingly common diagnosis
Winter, J.M., Cameron, J.L., Lillemoe, K.D., Campbell, K.A., Chang, D., Riall, T.S., Coleman, J., (...), Yeo, C.J.
Annals of Surgery. 2006; 243(5): 673-680
[Pubmed]



 

Top
 
  Search
 
    Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
    Email Alert *
    Add to My List *
* Registration required (free)  


    Abstract
    Introduction
    The physics of d...
    Advantages
    Clinical Applica...
    Issues of Concern
    Future
    Conclusion
    References
    Article Figures
    Article Tables

 Article Access Statistics
    Viewed11092    
    Printed365    
    Emailed17    
    PDF Downloaded0    
    Comments [Add]    
    Cited by others 1    

Recommend this journal