Methods of Data Acquisition

General Methods of Data Acquisition

• Localizer scanning

• Step-and-shoot (axial) scanning

• Helical scanning

Localizer Scans

• Most CT studies begin with one or more localizer images

• They are very similar to images acquired with conventional radiographic projection techniques

– Compared with general x-ray images, CT localizer images are of slightly poorer image quality and deliver an approximately equal radiation dose

• The position of the tube determines the orientation of the image

Localizer Scans (cont’d)

• The optimal localizer scan includes all areas to be scanned– Anatomy to be imaged must be placed within the

scannable range (z direction)– The patient must also be centered appropriately in

the gantry in both x and y directions• Miscentering can result in out-of-field artifacts

– Proper centering is also important when automatic exposure control techniques are used

Localizer Scans (cont’d)

• On all CT systems it is imperative that the technologist input the correct directional instructions before data acquisition is initiated– Head first vs feet first– Supine, prone, or decubitus

• Incorrectly inputting any directional instruction into the CT scanner can result in images that have been mislabeled and can result in misdiagnosis and serious medical errors

Localizer Scans: Z-Axis Coverage

• The localizer scan is use to prescribe the location of cross-sectional slices

• The anatomic area included is controlled by the operator and is dependent on the type of study

• Most procedures rely on beginning and ending landmarks that can be readily identified on the localizer image

Localizer Scans: DFOV and Image Center

• Localizer images are also used to select the optimal DFOV and image center

• Although these parameters can be changed retrospectively, departmental efficiency is improved if they are set correctly from the onset

• DFOV and image center selection is often improved by including both AP and lateral localizer scans

Review

What is the risk associated with incorrectly inputting the patient position (supine versus prone) for the localizer scan?

a. It will be more difficult to select the optimal DFOV; to correct, images must be retrospectively reconstructed after the patient leaves

b. Cross-sectional images will be incorrectly marked and could result in incorrect interpretation

c. It will be more difficult to set the z-axis extent for the cross-sectional slices

Answer

b. Cross-sectional images will be incorrectly annotated and could result in incorrect interpretation– There are reported instances of surgery being

performed on the wrong side as a result of a technologist incorrectly inputting patient position

Step-and-Shoot Scanning

• Earlier scanners operated exclusively in this way

• Also called axial scanning, conventional scanning, serial scanning, or sequence scanning– All are imprecise terms that

tend to create confusion in the field

• The two major limitations of this method are that it is relatively slow in covering a body section and the slices (position and thickness) are fixed at the time of the scan and cannot be changed later.

Step-and-Shoot Scanning (cont’d)• Key aspects

– CT table moves to desired location– Table remains stationary while the x-ray tube rotates within the gantry– Slight pause in scanning as the table moves to the next location

• Referred to as the interscan delay• Early systems, which contained only a single row of detectors in the z axis, obtained

data for one slice with each rotation

http://htm.wikia.com/wiki/Computed_Tomography

Step-and-Shoot Scanning (cont’d)

• In early scanners the time for a complete cycle was relatively long (>6 seconds) and allowed only a single scan to be acquired each time the patient held her breath

• Newer scanners are much faster and allow axial scans to be “clustered.” That is, more than one scan can be taken in a single breath-hold

Step-and-Shoot Scanning (cont’d)

• Scans produced with the step-and-shoot method result in images that are perpendicular to the z axis and parallel to every other slice, like slices of a sausage

Step-and-Shoot Scanning (cont’d)

• Advantages– On phantoms, step-and-shoot methods result

in the highest image quality– Axial scans can be contiguous or

noncontiguous– Axial scans can be programmed to repeat

scans at the same slice location• Called cine or dynamic methods

Step-and-Shoot Scanning (cont’d)

• Disadvantages– The cumulative effect of the pauses between

each data acquisition adds to the total examination time

• The interscan delay is particularly problematic for CT angiography, because blood vessels remain contrast-filled only briefly

– Slice misregistration that occurs when the patient breathes differently with each scan acquisition

Single-Detector Row Systems

• Before 1990 all scanners contained detector elements aligned in a single row

Single-Detector Row Systems

• Each detector element is quite wide in the z direction

• Opening or closing the collimator controls the slice thickness by controlling the portion of the detector’s width that is exposed

Single-Detector Row Systems (cont’d)

• Calculating the area of patient anatomy to be covered– Simple process of multiplying the slice

increment selected by the number of slices acquired

Review

An examination protocol of the chest calls for contiguous, 4-mm slices to be taken from the level of the sternal notch to the lung base; 60 slices are planned. How much anatomy (in the z direction) will be covered with cross-sectional slices?a. 15 mmb. 64 mmc. 120 mmd. 240 mm

Answer

d. 240 mm

4 mm x 60 = 240 mm

Multidetector Row Systems (MDCT)

• Newer CT systems continue to use many detector elements situated in a row

• However, they may contain from 4 to 64 parallel rows

• MDCT provides longer and faster z axis coverage per gantry rotation

• Slice thickness is determined by a combination of the x-ray beam width and the detector configuration

Multidetector Row Systems• The multiple parallel rows of detector elements can be

combined in various ways to yield different slice thicknesses

• Consider the possibilities with a specific four-slice system

Applications for Axial Scanning

• Axial scans are used in protocols in which the acquisition speed is not a major concern and optimal resolution is required

• Axial scans are used when slices are gapped, or when exposure will be interrupted

Helical Scanning

• Introduced in the late 1980s

• Also called spiral or volumetric scanning

http://htm.wikia.com/wiki/Computed_Tomography?file=HelicalCT.jpg

Helical Scanning

• Introduced in the late 1980s

• Also called spiral or volumetric scanning

• Key aspects

– Continually rotating x-ray tube

– Constant x-ray output

– Uninterrupted table movement

Helical Scanning (cont’d)

• Eliminates the interscan delay

• Advantages– Ability to optimize iodinated contrast agent

administration– Reduces respiratory misregistration– Reduces motion artifacts from organs

Helical Scanning (cont’d)

• The major improvements leading to the development of helical scanning– X-ray gantries with a slip ring design– More efficient tube cooling– Higher x-ray output (i.e., increased mA)– Smoother table movement– Software that adjusts for table motion– Improved raw data management– More efficient detectors

Helical Scanning (cont’d)

• Helical scanning produces slices in which the beginning point and the end point are not in the same z axis plane.

• Slices are produced at a slight tilt, similar to the rungs in a spring

Helical Interpolation

• To take the slant and blur out of the helical image

• Complex statistical methods result in images that closely resemble those acquired in a traditional axial mode

• Different interpolation techniques are used

• Interpolation is associated with some loss of image resolution

Helical Interpolation (cont’d)

• The steeper the angle of the slice, the more interpolation required– More interpolation increases the loss of image

resolution

• Interpolation methods can result in a scan that is wider than that selected by the operator. This is referred to as– Slice thickness blooming– Degradation of the slice-sensitivity profile

Helical Pitch• Pitch is a parameter that describes

the CT table movement during a helical scan acquisition

• Most commonly defined as the travel distance of the CT scan table per 360° rotation of the x-ray tube, divided by the x-ray beam collimation width– When table feed and beam

collimation are identical, pitch is 1

– When table feed is less than beam collimation, pitch is less than 1 and scan overlap occurs

Pitch in SDCT

• The table speed varies according to the slice thickness and gantry rotation speed selected

• For example– 5-mm contiguous slices are desired– Gantry rotation is 1 second per slice– Pitch is set at 1 – Therefore, the table must travel 5 mm each

second

Pitch in SDCT (cont’d)

• If a 10-mm slice is selected, and all other parameters are to be held constant, the table must move 10 mm each second

• When the table moves a distance that is equal to the slice thickness during each gantry rotation, the pitch is described as 1

Pitch in SDCT (cont’d)

• Consider what happens if the slice thickness is kept at 5 mm but the table speeds up so that it travels 10 mm each second– This is a pitch of 2– The slant is more pronounced, like pulling the

ends of a spring

• Hence, as pitch increases, so does the slice angle. More interpolation is required to straighten the image

Pitch in SDCT (cont’d)

• Increasing the pitch will result in a scan covering more anatomy lengthwise for a given total acquisition time– It will reduce the radiation dose to the patient (if

other scan parameters are held constant)

• Decreasing the pitch slows down the table speed and decreases the anatomy covered– It will increase the radiation dose to the patient

Pitch in MDCT

• The concept of pitch must be expanded on for MDCT systems

• Although pitch is still the relationship between slice thickness and table travel, with MDCT the terms collimation and slice thickness are no longer synonymous

• In MDCT, pitch is defined as table movement per rotation divided by beam width– Beam width is determined by multiplying the number

of slices by slice thickness

Review

• Consider the following– 16-slice scanner– 0.5-mm slice thickness– Table movement 12 mm per rotation

What is the pitch?a. 0.8b. 1.0c. 1.5d. 1.8

Answer

c. 1.5

12/(16 x 0.5) = 12/8 = 1.5

Helical Scan Coverage

• SDCT scan coverage– Number of images acquired

Pitch x total acquisition time x 1/rotation time = number of images

– Distance coveredPitch x total acquisition time x 1/rotation time x slice thickness =

amount of anatomy covered

Helical Scan Coverage (cont’d)

• MDCT scan coverage– Distance covered

Pitch x total acquisition time x 1/rotation time x (slice thickness x slices per rotation) = amount of anatomy covered

• Example– 20-s acquisition, 0.5-s rotation, 2.5-mm slice,

4 slices per rotation, pitch of 1.21.2 x 20 s x 1/0.5 s x (2.5 mm x 4) = 480 mm of anatomy

covered

Changing Slice Incrementation

• Helical data allow the slice incrementation to be changed retrospectively– Allows the creation of overlapping slices,

without increasing the radiation dose to the patient

– In some instances, changing the slice incrementation can reduce the partial volume effect

Changing Slice Incrementation in MDCT

• Additional options exist when data are from MDCT– Thin slices can be added together to create a

thicker slice for viewing– In some situations, thicker slices can be

divided to produce thinner slices

Dual-Source CT

• This design uses two sets of x-ray tubes and two corresponding detector arrays in a single CT gantry

• The primary goal of this design is increased scan speed

• A second potential advantage arises from the fact that the two x-ray tubes can use different kVp settings– This allows additional information to be collected