Basics of CT scan

by

Chakradhar

Neurology resident

HISTORY

Computed tomography (CT) scan machines uses X-rays, a powerful form of electromagnetic energy.

Sir Godfrey hounsfield-1972 Nobel prize in 1979 with cormack six generation of scanners Latest 128 multidetector ct

G.N.HOUNSFIELD ALLAN M. CORMACK

PRINCIPLE Internal structure of an object can be reconstructed from multiple

projections of the object. Uses x rays applied in sequence of slices across the organ Images reconstructed from x-ray absorption data Xray beam moves around the patient in a circular path

Beam of light projected in two direction's, detecting two different shadows

Terminology Hounsfield Unit (HU)- mean attenuation of x-rays by

different tissues.

PIXEL & VOXEL

Each square of the image matrix is called pixel

Thickness of block of tissue called voxel

Linear attenuation coefficient The linear attenuation coefficient () of each pixel is

determined by :

1. Composition of the voxel

2. Thickness of the voxel

3. Quality of the radiation beam

Hounsfield units represent logarithmic scale of CT density.

Pure water has an HU value of ‘0’. Current CT scans measure from – 1204 to + 3407.

DESCRIPTION Approx. HU DENSITY

Calcium > 1000 Hyperdense

Acute blood 60-80 Hyperdense

Grey matter 38 (32-42) Hyperdense

White matter 30 (22-32) Hyperdense

CSF 0-10 ISODENSE

Fat -30 to - 100 Hypodense

Air - 1000 Hypodense

PARTS 1)xray tube-akin to that in a x ray machine.

2)detectors

3)gantry- which houses xray apparatus

4)patient couch

5)viewing console

1.X-ray tube & collimator

2.Detector assembly

3.Tube controller

4.High freq. generator

5.Onboard computer

6.Stationary computer

X ray tube Internal structure of gantry

FILTERS

Compensation filter is being used

To absorb low energy x rays

To reduce patient dose

To provide a more uniform beam

COLLIMATORS To improve image quality Collimator width

determines the slice thickness

FIRST GENERATION

Narrow pencil beam Single detector per slice Translate –Rotate movements

of Tube- detector combination Scan time-5min Designed only for evaluation

of brainFIRST GENERATION

ONE DETECTOR, TRANSLATE ROTATERY, PARALLEL BEAM

SECOND GENERATION

Narrow fan beam(30-100)

Linear detector array(30)

Translate-Rotate movements of Tube-Detector combination

Fewer linear movements are needed as there are more detectors to gather the data.

Between linear movements, the gantry rotated 30o

Only 6 times the linear movements got repeated

Scan time~20secs

THIRD GENERATION

Rotate(tube)-Rotate(detectors) Translatory motion is completely eliminated

Pulsed wide fan beam(500-550) Arc of detectors(600-900) Detectors are perfectly aligned

with the X-Ray tube Both Xenon and scintillation

crystal detectors can be used Scan time< 5secs

FOURTH GENERATION

Continuous wide fan beam(500-550) Ring of detectors(> 2000) Rotate(tube)-Fixed(detector) X-ray tube rotates in a circle inside

the detector ring When the tube is at predescribed

angles, the exposed detectors are read.

Scan time< 2 secs

TYPES

Spiral ct-

uses principle of volumetric acquisiton.

no respiratory misregistration

HRCT

CT cisternography and myelography

OTHER SCAN CONFIGURATIONS

Interest in faster scan times evolves from a desire to image moving structures such as the wall of the heart and contrast material in blood vessel and heart chambers and to overcome motion artifacts due to cardiac rhythm and patient breathing .

Dynamic Spatial Reconstructor(DSR)

Electron beam computed tomography

DYNAMIC SPATIAL RECONSTRUCTOR

28 X-ray tubes

X-ray tubes are aligned with 28 light amplifiers and TV cameras that are placed behind a single curved fluorescent screen

The gantry rotates about the patient at a rate of 50 RPM

Data for an image acquired in about 16 ms.

Reconstruct 250 C.S. images from each scan data

DSR

The Dynamic Spatial Reconstructor (DSR) is a high-temporal resolution,

studies of cardiovascular structure and function. 3-D dynamic images can be obtained after reconstruction.

DSR is currently used involves studying selected pediatric patients with complex congenital heart disease

advantages

Disadvantages

High Cost

Mechanical motion is not eliminated

This instrument represents a novel concept in the use of x-ray to obtain fast tomographic scanning. In contrast to the DSR and conventional CT, EBCT has no mechanical parts moving around the patients, resulting in lower heat production and enabling fast scanning.

An electron beam, originating from an electron gun located behind the patient is magnetically deflected sequentially onto four tungsten target rings, producing eight fan beams (two from each target ring) of x-ray radiation that pass through the patient.

Eight almost simultaneous renal tomographic sections can thereby be obtained,

Electron Beam CT

Electron Beam Computed Tomography

Electron gun Large Arcs of tungsten targets Detector ring 17 slices per second

EBCT

Why Is It Done?

This test is used to identify calcium buildup in heart arteries, which can be a risk factor for coronary artery disease (CAD). It may be used as a screening tool to detect hardening of the arteries in people who are at high risk of developing atherosclerosis. only CT method which can scan the beating heart.

EBCT in measuring RBF

slices are thicker (8 mm) than those produced by the DSR.

temporal resolution is lower than that offered by the DSR (50 or 100 msec/image),

Disadvantages

Radiation dose from EBT scans compared to other sources of radiation

X ray chest-0.1 mSV

Ct brain-2 mSV

EBCT-0.5 to 0.7 mSV

Environmental radiation per year-0.02 mSV

Image Quality in CT

Image quality is the visibility of diagnostically

important structures in the CT image.

The factors that affect CT image quality are Quantum mottle (noise) Resolution : Spatial and contrast Patient exposure.

The factors are all interrelated

CT ARTIFACTS

Artifacts are distortions or errors in the image that are unrelated to the object scanned .

Most common artifacts in CT are Motion artifacts Streak artifacts Beam hardening artifacts Partial volume averaging artifacts Ring artifacts

STREAK ARTIFACTS

Cause: Presence and movements of objects of very high density(contrast media, metallic implants,surgical clips)

Appearance: StreaksREMEDY:-•Remove the offending object if possible. Use a smoothing algorithm. e.g. Standard algorithm.

DENTURES PRODUCING STREAK ARTIFACT

SURGICAL CLIP IN HEART PRODUCING STREAK ARTIFACT

RING ARTIFACTS

CAUSE : Detector failure or miscalibration of a detector

APPEARANCE:-

Ring Rectification : regular quality assurance checks

RING APPEARANCE

CECT

To detect abnormal disrution caused by tumor,abscess ,infarct etc

Uses ionic or non ionic contrast(6 fold reduction in allergic reactioin 0.04%)

In normal CNS vessels,pituitary choroid and dura enhance

Indications for non ionic contrast

Prior adverse reaction BA Allergy or atopy hx <2yr RF(Cr>2) Cardiac DM Severe debilitation

CT

Advantages – Easy availabilty Fast Better for bone and acute blood,lesions of skull base and calvarium Calcification Less limited by patient factors

Disadvantages- high radiation poor visualisation of posterior fossa lesions

INTERPRETATION OF CT BRAIN

1-GENERAL INFORMATION 2-EXTRACRANIAL TISSUE 3-CRANIAL BONE 4-BLOOD 5-CSF FLOW A-VENTRICULAR SYSTEM B-CISTERNS 6-BRAIN TISSUE A-MASS LESIONS B-SULCI & GYRI C-GRY & WHITE DIFFERENTIATION

Low density High density

Csf Bone

Fluid Calcification

Air Blood

Fat Contrast

Physiologic calcifications

Chorid plexus-rare before 10yrs

Basal ganglia-rare before 40ys

Pineal gland-common after 30 yr rare before 10yr

Falx

Dentate nuclei

INDICATIONS

To diagnose neuro infections and their complications

Stroke to distinguish infarct from hemorrhage

Ct angio before thrombolysis

Ct venogram for cerebral venous thrombosis(cvt)

Acute changes in mental status

Focal neurologic findings

Trauma

Suspected SAH

Cns tumors

Peidural hematoma

Convex shape

Subdural hematoma

Cresent shape

Skull fracture

Infarcts

Anterior cerebral artery infarct

Middle cerebral artery infarct

Posterior cerebral artery infarct

Hyper dense MCA sign

Internal cerebral artery infarctACA+MCA

hemorrhage

Intra parenchymal hemorrhage in putamen Sub arachnoid hemorrhage

hyperdensities in sylvian fissure,basal cysterns

Neuro infections bacterial meningitis radiological signs

meningeal enhancement

cerebral edema

complications

abscess stages

sub dural abscess,epidural abscess

Bacterial meningitisIndicatations for ct brain

before lumbar puncture-

to look for obstructive hydrocephalus-to prevent herniation

to conform meningeal involvement—by meningeal enhancement

meningitis complications suggested by seizures, altered sensorium, focal deficits

encephalitis- cerebral edema is seen

others

cerebral abscess

epidural/sub dural empyema

arteritis leading to infarct

hydrocephalus

seen well effaced

Gyri and sulci

Normal parenchyma

cerebral edema

hydrocephalus

Sub dural effusion

arteritis leading to infarct

cerebral abscess

stages

Early cerebritis

early capsule, thin rim

Late capsular, thick rim

Multi loculated

Late cerebritis

d/d for multiple ring enhancing lesions

Tuberculoma

Neurocysticerosis

cns crptococcosis

Metastasis Abscess (also cerebritis)

Glioblastoma, Granuloma Infarct (esp. Basal ganglia) Contusion (rare)

AIDS (Toxoplasmosis, etc.) Lymphoma (common in AIDS )

neurosarcoidosis Demyelination (active) Resolving hematoma, Radiation change (necrosis)

TUBERCULOMA

Non contrast ct normal or may show complications

On contrast basal enhancing exudates,meningeal anhancement, tubeculomas with ring enhancement,ependimitis

Basal exudate enhancement

Tuberculomas with perilesional edema

Coalising tuberculomas

NEUROCYSTICERCOSIS

Stages

vesicular stage- live stage

only hypo dense lesion with out perilesional edema/ring enhancement

colloidal stage- perilesional edema with ring enhancement

granular stage- scolex gets calcified resulting in central hyper density

nodular stage- entire lesion gets calcified nodular stage-

vesicular stage- colloidal stage- granular stage- nodular stage-

Tuberculous granuloma neurocysticercosis

>20 mm size <20mm

large perilesional edema usually small area

irregular margine regular margin

Coalising lesions noncoalising

These findings are not path gnomic,above signs can be seen viceversa

neurocysticercosis

TUBERCULOMAs

central dot sign Stary sky

TOXOPLASMOSISCT - (70-80 % cases ) multiple B/L hypo dense contrast enhancing focal lesions with

predisposition to the basal ganglia and subcortical region.

A double dose contrast with increased delay scan time may increase the sensitivity.

Why not MRI them all???

- MRI is generally preferable to CT for evaluating intracranial neoplasms

- CT is preferred for visualizing tumor calcification or intratumor hemorrhage.

Cns tumors

Commonly Calcified and Hemorrhagic Lesions

Calcified HemorrhagicOligodendroglioma Glioblastoma multiforme

Choroid Plexus tumor Oligodendroglioma

Ependymoma Metastatic:

Central neurocytoma Melanoma

Craniopharyngioma Breast

Teratoma Lung

Chordoma

meningioma

Pilocytic Cerebellar Astrocytoma

Cystic mass with nodular enhancement in the wall

Ependymoma

Enhancing lesion with in 4th ventricle

Glioblastoma Multiforme

Cystic,solid and partially calcified

Image finding irregularly enhancing with necrotic centre

medulloblastoma Premitive neuro ectodermal tumor

Usually arises from roof of 4th ventricle

It is partially enhancing on ct

meningioma Extra axial tumour

imaging- homogenously hyper enhancing