Computed TomographyIts History and Technology

2 CT Basics

The initial use of computed tomography (CT) for applications

in radiological diagnostics during the seventies sparked a

revolution in the field of medical engineering. And even

throughout the eighties, a CT examination lost little if any of

its special and exclusive character. In the meantime, however,

times have changed. Today computed tomography represents

a perfectly natural and established technology which has

advanced to become an indispensable and integral component

of routine work in clinics and medical practices.

The following brochure will give you insights into the history of

computed tomography as well as its present status. Nevertheless,

this is an on-going development process. And Siemens will

continue to consistently extend its leading position in CT

technology. With you. For you. And for your patients.

Innovations For People

3

Historical Outline 4

State of the Art 12

Sequential CT 12Spiral CT 13

Setup of a CT System 14

Scanning unit (gantry) 15X-ray components 15Data acquisition components 16

Scanner Parameters 18

Collimation 18Increment 19Pitch 20Rotation time 21mAs 22

Image Generation 23

A CT image is produced 23What does a CT image show? 24Windowing 24

Image Evaluation and Image Processing 25

Two-dimensional displays 26Three-dimensional displays 27

Clinical Use of CT 32

CT in general clinical use 32Advantages of spiral CT in clinical use 32CT-Angiography (CTA) 33When is a CT examination indicated? 33

The invention of computed tomography is considered to be thegreatest innovation in the field of radiology since the discoveryof X-rays. This cross-sectional imaging technique provideddiagnostic radiology with better insight into the pathogenesisof the body, thereby increasing the chances of recovery. In1979, G.N. Hounsfield and A.M. Cormack were awarded theNobel Prize in medicine for the invention of CT.

Today, CT is one of the most important methods of radiologicaldiagnosis. It delivers non-superimposed, cross-sectional imagesof the body, which can show smaller contrast differences thanconventional X-ray images. This allows better visualization ofspecific differently structured soft-tissue regions, for example,which could otherwise not be visualized satisfactorily.

Since the introduction of spiral CT in the nineties, computedtomography has seen a constant succession of innovations.The development of slipring technology allowed for acontinuously rotating gantry – the prerequisite for spiral CT.The first spiral CT scanner was a Siemens SOMATOM Plus system.Today this technology is widely used.

Historical Outline

4 CT Basics

11/08/1895

Discoverer: The physicistand later nobel prizewinner Wilhelm ConradRoentgen (1845–1923),dean at the JuliusMaximilian University ofWuerzburg and holder ofthe chair of physics

Discovery of X-rayradiation

1896

F.H. Williams

F.H. Williams succeeds intaking the first chest X-rayin Boston, and CarlSchleussner develops thefirst silver bromide coatedphotographic X-ray platesin Frankfurt a. Main,Germany

Diagnostic results cannow be archived; untilthen, fluorescent screensresulting in high radiationexposure had been used

M i l e s t o n e s i n t h e H i s t o r y o f C T

1903

E.A.O. Pasche builds acollimator for suppressingscattered radiation

Radiation protection forall parties involved for thefirst time; until then, bothpatients and physiciansalways had to face the“bare“ X-ray tube

E.A.O. Pasche

1913

Gustav Bucky develops thescattered radiation grid inBerlin, Germany

The engineer William D.Coolidge builds the first high-vacuum hot-cathode tube inMassachusetts, USA

Image quality is improvedby eliminating scatteredradiation before it strikesthe X-ray film. Performanceand durability of X-ray tubesgreatly enhanced

Gustav Bucky

William D. Coolidge

5

X-ray image ofhis wife’s hand

1930/31

Allesandro Vallebona developsstratigraphy, thus paving the wayfor tomography

Not long after, Bernard Ziedsesdes Plantes develops planigraphy,thereby further refining thetomographic technique andcreating geometrically perfectimages for the first time

To some extent this helpedovercome the disadvantageof superimposition in the two-dimensional display of three-dimensional objects whererelevant structural informationcan be lost; the requiredplanigraphic depth had tobe guessed

Allesandro Vallebona

Bernard Ziedsesdes Plantes

1972

Godfrey N. Hounsfield

In London, Godfrey N.Hounsfield’s developmentof computed tomographymarks the beginning ofa new era in diagnosticimaging

With the aid of computedtomography it waspossible for the firsttime to produce non-superimposed imagesof an object slice. Usingso-called convolvers,digitized scan data arereconstructed into animage of a slice plane

1974

First CT systemfrom a medicalequipmentmanufacturer

SIRETOM

1976

OPTI X-raytube with highheat capacity:1.0 MHU

5 s scan time

Whole body CT

SOMATOM

Instant image reconstruction

1978

ECG-synchronizedCT image

512 x 512 displaymatrix

SOMATOM 2

SIRETOM CT system

6

1981/82

512 x 512 reconstructionmatrix

50 cm FOV (display field);graphically selectable andcontinuously variable

3 s scan time for a full360° rotation

1 mm slice width

0.5 mm high-contrastresolution

OPTI X-ray tube with1.35 MHU

Automatic scanning(AutoScan)

Graphically specified scanrange

SOMATOM DR3

1983/84

± 25° gantry tilt angle

70 cm gantry opening

OPTI X-ray tube with1.75 MHU

Integrated MPRfunction (Multi PlanarReformatting)

SOMATOM DRH

1985/86

Modern applications:DYNAMIC CT and 3Dwith MPR and SSD(Shaded SurfaceDisplay)

3D Shaded SurfaceDisplay (SSD)

SOMATOM CR

3D OberflächendarstellungSchlüsselbeinfraktur,

SOMATOM DRH

Coronales MPR,Femurkopf/Azetabulum,

SOMATOM DRH

7

1987/88

Shorter examinationtimes

Increased patient comfort

DURA X-ray tube with4.3 MHU

40 kW DURA generator

Integrated gantry cooling:Heat exchanger

1024 x 1024 display anddocumentation matrix

Real-time MPR

Selection of five slicethicknesses (1–10 mm)

SOMATOM Plus

1 s/360° slipringtechnology

Continuous rotation oftube and detector

1989

Continuous volumemeasurement

Data acquisition duringone breath hold

24 s/24 cm continuousvolume acquisition

First spiral CT scanner in routine operation

1990

32 s continuousSPIRAL scan

QUANTILLARC 3detector

SOMATOM Plus-S

Spiral CT acquisition withcontinuous tabletop feed

Left: 3D Shaded SurfaceDisplay of the lung, high-resolution, spiral CT

Right: Spiral CT, lung,high-resolution, 2 mm,SOMATOM Plus

1991

Compact slipring scanner

Monitor with 4-quadrant display(5122 ea.) for multitasking

High-efficiency QUANTILLARC 4detector

SOMATOM AR

IntuitiveWindows™user interface,mouse-controlled

SOMATOM AR

8 CT Basics

Intuitive, mouse-controlled Windows™ user interface

More volume inthe same time

1994

Larger volumes acquired faster,shorter breath hold, increased patientcomfort, improved thin-sliceresolution

0.75 s scan time for a full 360°rotation, 0.5 s for a quick scan

100 s/130 cm multiple spiralacquisition with 1:1 pitch

Selection of six-slice thicknesses(1–10 mm)

± 30° remote-controlled gantry tilt

QUANTILLARC 6 inert gas detector

40–55 kW modularly upgradeableDURA generator

DURA X-ray tube with 5.3 MHU

SOMATOM Plus 4

Subsecond spiral CT

Advantages of subsecondSPIRAL scans

1992

Integrated CT-Angiography

DURA-S X-ray tube with 4.3 MHU

Prospective and retrospectiveoverlapping SPIRAL reconstruction

SPIRAL XP: 40 s (pitch variable from1:1 to 2:1)

Maximum Intensity Projection (MIP)and Minimum Intensity Projection(MinIP) incl. editing function for CTA

CT-Angio (MIP)of the renalarteries,SOMATOMPlus-S

CT-Angio (MIP)of the circleof Willis,SOMATOMPlus-S

9

Improved thin-sliceresolution

The same volumein less time

Subsecond spiral CT, long MPR,abdomen/pelvis, SOMATOM Plus 4

40 cm/30 s

30 cm/30 s

0,75 s SPIRAL 1,0 s SPIRAL

0.75 s SPIRAL 1.0 s SPIRAL

22 s 0.75 s SPIRAL

30 s 1.0 s SPIRAL

1996/97

UFC Ultra Fast CeramicDetector

Same image quality withsignificantly reducedradiation dose

Improved MPR – coronal/sagittal/oblique

Real-time image display(1 image/rotation)

Volume Rendering Technique(VRT)

Lightning UFC™

1998

Multislice spiral scanningwith 4 slices per rotation

Fastest rotation timeof 0.5 s

Virtually isotropicresolution

HeartView CT: First useof Cardio CT in routineoperation, temporalresolution of up to 125 ms

SureView™:Reconstruction algorithmfor multislice spiralscanning

SOMATOM Volume Zoom

1999

Intuitive user interface forall medical applications andproducts

syngo software

10

2001

CARE Dose

11

2000

Designed to be thesmallest and most cost-effective spiral CT scanner

Plug-and-play installation

Do-it-yourself applicationtraining and serviceconcept supported byinteractive CD-ROMs

SOMATOM Smile

2002

Multislice scanning with 16 slicesper rotation

Isotropic resolution of up to0.6 mm

Routine submillimeter slices(16 x 0.75 mm)

Cardio CT with extremely highimage quality – temporalresolution of up to 105 ms withHeartView CT

Reconstruction algorithm withcone beam correction

SOMATOM Sensation 16

Dose modulation forreduced radiation

Sequential CT

A cross-sectional image is produced by scanning atransverse slice of the body from different angularpositions while the tube and detector rotate 360°around the patient with the table being stationary.The image is reconstructed from the resultingprojection data.

If the patient moves during the acquisition, the dataobtained from the different angular positions are nolonger consistent. The image is degraded by motionartifacts and may be of limited diagnostic value. Thetomographic technique is suitable only to a limitedextent for the diagnosis of anatomical regions withautomatism functions such as the heart or the lung.

State of the Art

12 CT Basics

Patienttable

Gantry

Detectors

Tubes

0

0

Spiral CT

Spiral CT is often referred to as “volume scanning“. This impliesa clear differentiation from conventional CT and the tomographictechnique used there. Spiral CT uses a different scanningprinciple. Unlike in sequential CT, the patient on the table ismoved continuously through the scan field in the z directionwhile the gantry performs multiple 360° rotations in the samedirection. The X-ray thus traces a spiral around the body andproduces a data volume. This volume is created from a multitudeof three-dimensional picture elements, i.e. voxels.

The table movement in the z direction during the acquisitionwill naturally generate inconsistent sets of data, causing everyimage reconstructed directly from a volume data set to bedegraded by artifacts. However, special reconstruction principles– interpolation techniques which generate a planar set of datafor each table position – produce artifact-free images. Thus itis possible to reconstruct individual slices from a large datavolume by overlapping reconstructions as often as required.

Software applications enable the clinical use of spiral CTeven for regions which are subject to involuntarymovements.

13

Direction of patienttransport

Path of rotation tubeand detector

z, mm

t, s

A CT system comprises several components.

These basically include:

• The scanning unit, i.e. the gantry, with tubeand detector system

• The patient table

• The image processor for image reconstruction

• The console

The console represents the man-machine interfaceand is designed to be multifunctional. It is the controlunit for all examination procedures, and is also usedto evaluate the examination results. To enhance theworkflow, Siemens has developed a double consolecapable of performing both functions at the sametime.

Setup of a CT System

14 CT Basics

Scanning unit (gantry)

A CT scanning system consists of an X-ray unit, whichfunctions as a transmitter, and a data acquisition unit,which functions as a receiver. In commercial CTsystems these two components are housed in a ring-shaped unit called the gantry.

X-ray components

• Tube

Manufacturers of CT systems use X-ray tubes withvariable focal spot sizes. This makes sense, becausevolumes for which good low-contrast resolution isessential need to be scanned with a large focal spotand high power, whereas high resolution imageswith thin slices require a small focal spot. Tubesused in modern CT scanners have a power ratingof 20–60 kW at voltages of 80 to 140 kV. Thesystems can, however, be operated at maximumpower for a limited time only. These limits aredefined by the properties of the anode and thegenerator. To prevent overloading of the X-ray tube,the power must be reduced for long scans. Thedevelopment of multi-row detector systems haspractically excluded this limitation, since thesedetector systems make much more efficient useof the available tube power.

• Shielding

Each CT scanner is equipped with grids, collimatorsand filters to provide shielding against scatteredradiation, to define the scan slice and to absorb thelow-energy portion of the X-ray spectrum. In thisway, both the patient and the examiner areprotected.

15

•••••••

Data acquisition components

• Detector

The detector system plays a special role in the interaction ofthe CT components. It converts the incident X-rays of varyingintensity to electric signals. These analog signals are amplifiedby downstream electronic components and converted todigital pulses. Over time, certain materials have proven veryeffective in the utilization of X-rays. For example, Siemensuses UFC (Ultra Fast Ceramic) Detectors which, due to theirexcellent material properties, dramatically improve imagequality.

16 CT Basics

Scintillation crystal

X-ray radiation

v

Light

UFC Detector

Photodiode

• Multi-row detector

Multi-row detectors utilize radiation delivered from theX-ray tube more efficiently than single-row detectors. Bysimultaneously scanning several slices, the scan time can bereduced significantly or the smallest details can be scannedwithin practicable scan times. In the adaptive array detectorsused by Siemens, the rows inside the detector are very narrow,becoming wider as you move toward its outer edges in thez direction (longitudinal axis of the body). A combinationof collimation and electronic interconnection providesconsiderable flexibility in the selection of slice thicknesses.At the same time the space required by the detector septa,and therefore the unused space, is minimized.

17

Adaptive array detector

Ultra Fast Ceramic Detector (UFC)

5 mm 2.5 mm 1.5 1 5 mm2.5 mm1 1.5

2 x 0.5 mm 4 x 1 mm 4 x 2.5 mm 4 x 5 mm

Variable slice collimation through primary collimation

and electronic signal combination

18 CT Basics

Collimation

The radiation beam emitted by the X-ray tube is shapedusing special diaphragms also referred to as ”collimators”.A distinction can be made between two types of collimators.The source collimator is located directly in front of the radiationsource – i.e. the X-ray tube. It reduces the radiation beam toform the maximum required fan beam, thus also determiningthe emitted dose.

The detector collimator, which is positioned directly in frontof the detectors, is primarily used to shield the detector againstscattered radiation, thus preventing image artifacts. Thecollimation and focal size determine the quality of the sliceprofile.

From the data volume from a multislice scanner images can bereconstructed with slice thickness equal to or larger than thedetector collimation. For example, a 5 mm collimation allowsimages to be reconstructed with a slice thickness of 5 mmor more. The widest range of possibilities in the selection ofcollimation and reconstructed slice thicknesses is offered inspiral CT using multidetector systems.

Scanner Parameters

19

Increment

The increment determines the distance between imagesreconstructed from a data volume. If an appropriate incrementis used, overlapping images can be reconstructed. In sequentialCT, overlapping images are obtained only if the table feedbetween two sequences is smaller than the collimated slicethickness. This, however, increases the patient dose.

In spiral CT the increment is freely selectable as a reconstructionparameter, i.e. by selecting the increment the user canretrospectively and freely determine the degree of overlapwithout increasing the dose. Overlapping reconstructions offerthe advantage of better image quality due to lower noise andeasier and more accurate diagnosis of small structures.

An illustrative example: A 100 mm range was acquired in thespiral mode with 10 mm collimation. After the acquisition,slices of 10 mm thickness can be reconstructed at any point ofthis range. If an increment of 10 mm is used, contiguous slicesof 10 mm thickness are reconstructed every 10 mm.(see figure 1).

If an increment of 5 mm is used, slices of 10 mm thickness arereconstructed every 5 mm. The slices overlap by 50%. With anappropriate increment an overlap of 90% can be achieved.Modern CT systems allow the reconstruction of slices witharbitrary increments (see figure 2). A clinical useful overlapis 30%–50%.

Increment

Overlapping slices

Slice thickness

Figure 2

Figure 1

20 CT Basics

Pitch

An important factor in spiral scanning is the table feed perrotation. The larger the table feed, the faster (i.e. with fewerrotations) a body region can be scanned. However, if the tablefeed is too large, image quality will be impaired. In this contextthe term “pitch“ is used. For single-row systems the definition

pitch = table feed per rotation/collimation

is generally accepted. Experience has shown that a good imagequality can be obtained with a pitch between 1 and 2. It shouldalso be noted that the dose can be significantly reduced insingle-row systems if a pitch factor > 1 is applied. In the contextof multi-row systems, pitch cannot yet be defined clearly. Theambiguity involved becomes apparent in the following example(SOMATOM Sensation 4):

Collimation 4 x 2.5 mm, table feed 10 mm

First possibility: pitch = 10 mm/4 x 2.5 mm = 1

Second possibility: pitch = 10 mm/2.5 mm = 4

To avoid misunderstandings, we use “feed per rotation“ ratherthan “pitch“ on the user interface.

15 s 30 s

The first pitch scans the same volume in less time

21

Rotation time

Rotation time is the time interval needed for a complete 360°rotation of the tube-detector system around the patient. Itaffects the spiral scan length and thus the coverage of the scanrange during a certain period of time. Ultra modern CT systemsrequire only 0.4 seconds for one rotation. A short rotation timehas the following advantages:

• A longer spiral scan can be acquired in the same scan time

• The same volume and the same slice thickness can be scannedin less time

• Motion artifacts are eliminated

• Savings on contrast media through shorter examination times

• Reduced patient discomfort, since less contrast mediumis required

For shorter examination times or fast acquisition of largeanatomical regions a subsecond rotation time is recommended.This applies especially, for instance, to constantly moving organssuch as the heart.

22 CT Basics

mAs

The mAs value (e.g. 100 mAs) is the product of thetube current (e.g. 200 mA) and the rotation time(e.g. 0.5 s).

In multi-row CT systems we simplify this equation byusing what is commonly called “effective mAs”. Thisis the product of the tube current and the exposuretime for one slice (rotation x collimation/feed perrotation).

The selected mAs and tube voltage determine thedose. The mAs value selected depends on the typeof examination. Higher mAs values reduce the imagenoise, thus improving the detectability of lowercontrasts. For visualizations of soft tissue, i.e. regionsof low contrast, a higher dose and larger slice thicknessare required. The abdomen and brain are typicalregions of soft-tissue contrast. Visualizations of bonesor the lungs, i.e. regions of high contrast, as well ascontrast studies of vessels require lower doses andthinner slices.

Siemens CT scanners also feature the CARE Dosetechnical measures package (CARE = CombinedApplications to Reduce Exposure), which wasdeveloped to reduce patient exposure to radiation.This package guarantees shorter examination times,the lowest possible exposure to radiation, and imagesof excellent quality.

Ultra modern computer technology “monitors” thepatient during the entire examination period. Duringeach rotation, the radiation is continuously measuredand modulated according to the current attenuationlevel. CARE Dose thus makes it possible to vary theradiation dose depending on the patient’s anatomyand thus reduce it by as much as 56%.

Scanner parameters determine the image quality.Optimal performance of spiral CT systems can beachieved only with an optimal combination ofparameters.

23

A CT image is produced

Acquisition:

In the simplest case, the object (here a round cylinder) is linearlyscanned by a thin, needle-like beam. This produces a sortof shadow image (referred to as ”attenuation profile” or”projection”), which is recorded by the detector and the imageprocessor. Following further rotation of the tube and the detectorby a small angle, the object is once again linearly scanned fromanother direction, thus producing a second shadow image. Thisprocedure is repeated several times until the object has beenscanned for a 180° rotation.

Display:

The various attenuation profiles are further processed in theimage processor. In the case of simple backprojection, eachattenuation profile in the scanning direction is added up inthe image memory. This results in a blurred image due to thedisadvantage of this simple backprojection, i.e. each objectnot only contributes to its own display, but also influencesthe image as a whole. This already becomes visible after 3projections. To avoid this problem, each attenuation profile issubjected to a mathematical high-pass filter (also referred toas ”kernel”) prior to the backprojection. This produces overshootand undershoot at the edges of the object. The mathematicaloperation is referred to as ”convolution”. The convolvedattenuation profiles are then added up in the image memoryto produce a sharp image.

Image Generation

X-raytube

Translation

Rotation

Detector andmeasuringelectronicsAttenuation

profile(simplified)

Backprojection

without convolution with convolution

0 projections

1 projection

3 projections

N° projections

N° projections

Profilesection

Collimator

Ka 2000

24 CT Basics

What does a CT image show?

The CT image does not show these µ values directly,but the CT numbers according to Hounsfield:

CT number =1000 (µ – µwater) / µwater,

CT numbers are measured in HU = Hounsfield units. The CTnumber of water and air is defined as 0 HU and –1000 HUrespectively; this scale has no limit in the positive rangeof values. Medical scanners typically work in a range of–1024 HU to +3071 HU.

Windowing

In the CT image, density values are represented as gray scalevalues. However, since the human eye can discern only approx.80 gray scale values, not all possible density values can bedisplayed in discernible shades of gray. For this reason, thedensity range of diagnostic relevance is assigned the wholerange of discernible gray values. This process is calledwindowing.

To set the window, it is first defined which CT number thecentral gray scale value is to be assigned to. By setting thewindow width, it is then defined which CT numbers above andbelow the central gray value can still be discriminated by varyingshades of gray, with black representing tissue of the lowestdensity and white representing tissue of the highest density.

Lung window Soft-tissue window

The Hounsfield scale

CT no.HU8070605040302010

0

Kidneys

Blood

Pancreas

4020

5030

Liver

6050

7050

CT no.HU

1000800600400200

0-200-400-600-800

-1000

Water

Air

Lungs

Fat

Compact bone

Spongybone

+4-4

20050

-990-1000

-550-950

-80-100

250

25

The first obvious results of any CT examination arethe axial cross-sectional images. Since these imagesare already available in digital form on a storagemedium, they can be processed immediately by theprocessor. The evaluation of geometrical parameterssuch as distance, area, angle and volume as well asdensity measurements are part of clinical routinetoday. The tissue density is determined using the CTnumber averaged over a defined evaluation area,the so-called Region Of Interest (ROI). Geometricalparameters can be defined more accurately than inconventional radiography, since the problems ofsuperimposition and distortion do not exist in CT.

Image Evaluation and Image Processing

Examples of 2D post processing capabilities offeredby modern CT systems:

• Display of the CT numbers of arbitrary pixels inthe image

• Display of the CT number profiles along arbitrary intervals in the image

• Zoom and shift of image segments

• Filtering of images

• Addition, subtraction or other possibilities of imageoverlapping

The terms “two-dimensional“ (2D) and “three-dimensional“ (3D) refer to the image content.Views showing entire volumes are referred to as“3D displays”.

26 CT Basics

Two-dimensional displays

CT mainly uses the transverse plane as the imagingplane. Therefore, views of other orientations usuallyhave to be reconstructed from the original images.This is done by Multi Planar Reformation (MPR).

With MPR, a series of axial images are combined toform a stack. By aligning the same columns and rowsof all images of a series, the computer reconstructscontiguous images for any arbitrary plane. Theexaminer can then interactively page through theseimages, using the mouse for example, and evaluatethem (iMPR). By paging forwards and backwards,he finds the image which most clearly shows theanatomical and pathological detail of interest.4-quadrant displays with axial, sagittal, coronary,and oblique slice orientation are standard todayand provide a good overview.

An extension of the MPR technique allows theinteractive combination of thin slices into slices (slabs)of any thickness. This technique is used for bettervisualization of structures, such as vessels, whichextend across several slices. This technique hasbecome known as Sliding-Thin-Slab (STS).

Advantages of two-dimensional displays

• Direct, accurate display of CT numbers

• Easy orientation in the volume

• Unambiguous interpretation of image values

• Interactive evaluation on the monitor

• Basis for 3D images

sagittal coronary transversal

27

Three-dimensional displays

For 3D visualizations the position and viewing directionwith respect to the volume of interest must beindicated. Along this viewing direction through thedata volume a spatial image is reconstructed fromthe CT numbers pixel by pixel. Such virtual views areespecially suitable for structures which clearly standout against their surrounding area, such as the skeletalsystem or contrast-filled vessels.

Shaded SurfaceDisplay (SSD)

Maximum IntensityProjection (MIP)

28 CT Basics

• Shaded Surface Display (SSD)

For threshold based surface displays a CT number,e.g. 150 HU, is predefined as a threshold. All pixels,i.e. voxels, which exceed this threshold valuecontribute to the result image. From the viewer‘sposition these are all those pixels along each beamwhich first exceed the threshold. The surface isthen reconstructed from the totality of pixels andilluminated by an artificial light source to create ashading effect (Surface Shaded Display, SSD). Theshading effect intensifies the depth impressionfor the viewer. With this technique, however, theoriginal density information from the CT numbersis lost.

With SSD it must be noted that the gray scale valuesare no longer related to the original density of thestructures. For example, if several structures whichexceed the threshold value are superimposed inthe viewing direction, only the structure closestto the front surface of the monitor is displayed.This also applies if the structures behind it haveconsiderably higher CT numbers.

It also needs to be taken into account that SSDdisplays always depend on the threshold valueselected. For example, the display of vascularstenoses can be distorted by selecting aninappropriate threshold. If the threshold value isincreased, a higher degree of stenosis results, if itis decreased, stenoses may be obscured. In this waycalcifications and contrast medium in the vesselscan no longer be differentiated. Therefore, SSDimages are hardly suitable for diagnosis. They are,however, used to document results or for 3Ddisplays.

CT numberHU

600

400

200

55 10

Lumen filledwith CM

Calcification

CT numberprofile alongthe search beam

160 HUthreshold Soft tissue

y, cm

Observer’s position

Virtual lightsource

29

• Maximum Intensity Projection (MIP)

Maximum Intensity Projections (MIP) are based onthe voxels with the highest density, i.e. CT number.Along a virtual beam extending from the viewerthrough the 3D image volume, the voxel with thehighest density value is displayed in the resultingMIP image. Each MIP image is therefore a 2Dprojection image. Running several MIP imagesin fast sequence can create a realistic spatialimpression. For this, an image series is createdby varying the viewing angle in small steps.

In contrast to SSD, with this technique a minimumof density information is retained. Moreover, theprojection is always a combination of those voxelsfrom the entire volume that have the highestdensity, irrespective of whether these voxels arelocated further towards the front or the back of theimage. Unlike in SSD images, calcifications andcontrast medium are clearly differentiated in MIPimages.

Alternative to the MIP image, it is also possibleto display pixels with the lowest intensity in theprojection image. These images are called MinIPimages. They are used to display structures suchas the bronchial tree.

CT numberHU

600

400

200

55 10

Lumen filledwith CM

Calcification

CT numberprofile alongthe search beam

160 HUthreshold Soft tissue

y, cm

Observer’s position

30 CT Basics

• Volume Rendering Technique (VRT)

Volume Rendering Technique (VRT) refers to theprocess of reconstructing a 3D model from a 2Dimage stack. VR techniques go beyond SSD and MIPtechniques in their basic approach and performance.They are not limited to a certain threshold ormaximum density value. Instead, all densityvalues along a virtual beam which have a suitableweighting can contribute to the result image. Incontrast to MIP and SSD, the entire Hounsfield scalecan be included in VRT. Each CT number is assignedan opacity and color via freely selectable andinteractively modifiable transfer functions. Thismakes it possible to simultaneously display anextremely wide variety of tissue structures of variousdensity or HU value in a single volume data set.

CT numberHU

600

400

200

55 10

Lumen filledwith CM

Calcification

CT numberprofile alongthe search beam

160 HUthreshold Soft tissue

y, cm

Observer’s position

• Virtual endoscopy (VE)

A special type of VRT is perspective VolumeRendering (pVR), which is used mainly to generatevirtual endoscopic views. This technique is used toobtain a perspective view of the display region.Virtual endoscopy is mainly used for anatomicalcavities: These include, for example, the bronchialtree, large vessels, the colon and paranasal sinuses.But VE is also used for areas not directly accessiblefor conventional endoscopy, such as the cisternaeof the brain or the gastrointestinal region.

When the endoscope is inserted in a cavity displayedwith the perspective Volume Rendering Technique,this gives the user the impression of “flying through”the displayed region (virtual flight).

Advantages of three-dimensional displays

• Realistic display of volumes

• Presentation of the entire volume in onesingle image

• Improved recognition of diagnosticallyrelevant details

• Helpful for more precise surgical planning

• CT image data as a basis for three-dimensional models

• Possible free rotation of 3D objects

31

CT in general clinical use

The use of spiral CT has significantly shortened scantimes compared to sequential CT. This is of greatadvantage when examining patients who, due to thenature of their illness, are unable to cooperate. Motionartifacts caused by different respiratory conditionsduring the acquisition are reduced considerably,because in spiral CT the entire volume is scannedfaster and without gaps. Multiple scans due tobreathing during the acquisition are no longernecessary. The patient dose is therefore reduced.

Advantages of spiral CT in clinical use

• Complete coverage of organs in a single respiratoryposition

• Short scan times (resulting in fewer motion artifactsand a lower contrast medium requirement)

• Additional diagnostic information due to improvedresolution (thinner slices) and 3D visualization inroutine operation

• Special cost-effective applications based onspiral CT

Clinical Use of CT

32 CT Basics

CT-Angiography (CTA)

CT-Angiography (CTA) enables the display of vascularstructures aided by injections of contrast medium.The introduction of the multislice scanner has madeit possible to display the entire vascular system withmaximum contrast enhancement in extremely shortscan times. Image post processing enables gooddisplay of the entire vascular system. Even smallvascular exits and origins (branches) and embolismsor dissection membranes can be displayed. Thephysician can retrospectively select any projectionand generate three-dimensional images, e.g. forsurgical planning.

33

34 CT Basics

HeadHead, general/brainOrbitaSella turcicaPetrous bonesParanasal sinusesCircle of Willis3D cranial, facial bone

NeckCervical soft tissueCarotids

ThoraxMediastinumThorax high resolutionThoracic vesselsPulmonary vesselsHeart

SpineCervical spineThoracic spineLumbar spine

When is a CT examination indicated?

Here are several examples of CTexaminations:

• Head • Abdomen• Neck • Extremities• Thorax • Spine

35

Abdomen/pelvisLiverCT-Arterioportography (CTAP)PancreasKidneys, biphaseAdrenal glandsRenal arteriesAbdominal vesselsSmall pelvisVessels, pelvic/lower extremity

ExtremitiesShoulder jointHip jointWrist boneKnee jointFoot

Order No. A91100-M2100-A844-1-7600

Printed in Germany

BKW 63844 WS 11025.

Siemens AG, Medical SolutionsComputed TomographySiemensstr. 1, D-91301 ForchheimGermanyTelephone: ++49 9191 18-0

The information in this document contains general

descriptions of technical possibilities which do not

necessarily apply to each individual case. Any

performance features individually required must

therefore be stipulated upon conclusion of the relevant

agreement.

Siemens reserves the right to modify designs and

specifications without prior notice. Please contact your

local Siemens Sales Representative for the most current

information.

Original images always lose a certain amount of detail

when reproduced.

Siemens AG, Medical SolutionsHenkestr. 127, D-91052 ErlangenGermanyTelephone: ++49 9131 84-0www.SiemensMedical.com