Streaks and dark bands

Streaks and Dark Bands.

In very heterogeneous cross sections, dark bands or streaks can appear between two dense objects in an image. They occur because the portion of the beam that passes through one of the objects at certain tube positions is hardened less than when it passes through both objects at other tube positions. This type of artifact can occur both in bony regions of the body and in scans where a contrast medium has been used. In the chest scan shown, the contrast medium has caused artifacts that might be mistaken for disease in nearby anatomy.

Scalloping Artefacts

Scalloping

Scalloping Artifact is due to the fact that the slice sensitivity profile (SSP) is increased in spiral CT so that partial volume artifacts also become stronger. Scalloping can occur in skull CTs, particularly in slice positions in which the skull diameter quickly changes its axial direction. This image error can be corrected by reducing the pitch factors.

Artefacts

An x-ray beam is composed of individual photons with a range of energies. As the beam passes through an object, it becomes "harder," that is to say its mean energy increases, because the lower-energy photons are absorbed more rapidly than the higher-energy photons .Two types of artifact can result from this effect: so-called cupping artifacts and the appearance of dark bands or streaks between dense objects in the image.

Cupping Artifacts.

X rays passing through the middle portion of a uniform cylindrical phantom are hardened more than those passing though the edges because they are passing though more material. As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened. Therefore, the resultant attenuation profile differs from the ideal profile that would be obtained without beam hardening . A profile of the CT numbers across the phantom displays a characteristic cupped shape

SEGMENTATION

Segmentation can be performed manually or
(semi)automatically. Segmentation algorithms are
often based on the principle of region growing
. Placing one or more seed points initiates the
segmentation of the target structure. From these
seed points, more and more neighboring voxels
that fulfill predefined criteria are included in the
segmentation . The technique can be applied
in two ways: segmentation of the desired tissue or
segmentation of the undesired tissue with subsequent
removal from the data. The latter method
removes only interfering tissue (bone or densely
enhanced veins) from the CT angiography data
and retains soft tissue as well as contrast-enhanced
vessels for further evaluation. To refine
the boundary of the segmented structures, morphologic
dilation operations may be applied.
A particular problem in threshold-based segmentation
algorithms are areas with close contact
of two tissue types with comparable attenuation,
such as bone and contrast-enhanced vessels
(course of the ICA through the skull base; intraforaminal
sections of the vertebral artery)
. Although the process of segmentation is
semiautomatic, user interaction is necessary to set
additional seeding points or to intervene in cases
of inclusion of neighboring structures due to leakage
of the region-growing algorithm. These procedures
can be time-consuming and may exceed
practical limits in routine clinical work flow.

MIP

MIP images are created by displaying only the
highest attenuation value from the data encountered
by a ray cast through an object to the viewer’s
eye (5,6). The depth information along the
projection ray is lost; to visualize the spatial relationship
of various structures, the volume has to
be rotated and viewed from different angles. If
bone or calcifications are within the projection
ray, these structures are represented on the MIP
image instead of the contrast-enhanced vessel
because of higher attenuation values. Therefore,
bone elimination techniques are essential for pro-
cessing vascular MIP images. Superimposition of
vessels leads to artificially altered lumen margins,
and pathologic conditions may be hidden. To
cope with this problem, a modification of MIP
called closest vessel projection has been proposed
(7). Thin-slab MIP images viewed interactively
may be an alternative, as the necessity for bone
elimination is limited. MIP is not suitable
for the evaluation of stenosis in cases of dense
calcification or stents, but thin-slab MIP can provide
an excellent “road map” of the vessel course
for further evaluation with MPR.

Bolus tracking

What is the purpose of bolus tracking in contrast administration, and how this method works?
Individual timing of contrast material
injection (bolus tracking or test bolus injection)
is mandatory to take advantage of phaseresolved
image acquisition.
To individualize the timing of contrast material
injection, automatic bolus tracking techniques
(Smart Prep, CARE Bolus, and Sure Start) can
be employed (2). These techniques are fast and
easy to use and require only a single contrast material
injection. The disadvantage is that a large
target vessel for monitoring the contrast material

arrival is required, and an additional delay for
table movement and patient instruction is necessary.
Test bolus injection is the alternative to assess
the individual circulation time. Its major advantage
is that it provides information about the timing
of both arterial and venous enhancement in
the vessels of interest. The individual start
delay can be optimized by placing the scan between
the arterial peak and venous contrast material
upslope. Table movement and patient instructions
can be performed prior to the optimal
image acquisition window. The disadvantage is
the necessity for an additional injection of about
10 mL of contrast agent (10%–20% increase of
total amount).