Mid Term Revision Directed Study 1

Dr Mohamed El Safwany, MD.

Advanced tumor detectionand characterizationTaking vascularity and perfusion type into account,lesions such as hepatic adenomas, focal nodularhyperplasia and less-differentiated hepatocellularcarcinomas, as well as endocrine metastases andsarcomas, will result in hyperattenuation. Metastasesof other origins will show hypoattenuation withvarious temporal characteristics in the early arterialphase [6]. If a monophase and monoslice CT techniqueis applied, many of the hypervascular hepaticlesions will be completely invisible, but up to 30 %more lesions are detected in the early arterial phasecompared with the portal venous phase

Acquisition of multiple perfusion phases

• slice thickness 3.2 mm• reconstruction interval 1.6• pitch 1.2• gantry rotation 0.5 s• field of view 350–450 mm• 150–200 mAs

As the scanning process is usually initiated simultaneouslywith the beginning of an intravenouscontrast injection of 120 ml of low osmolar, nonioniccontrast agent at an injection rate of 5 ml/s,no bolus tracking techniques are necessary. Contrastagents with higher iodine concentrations(370–400 mg I/ml) may be advantageous in CThepatic imaging, especially in the visual evaluationof the arterial phase detectability of hepatocellularcarcinomas

The first spiral scan is acquired simultaneouslywith the beginning of the contrast injection, andtherefore without any hepatic contrast enhancement

The second spiral liver scan, approximately10 seconds after contrast initiation, usually showsmoderate contrast enhancement of the abdominalaorta and the hepatic artery, without admixtureof enhanced portal venous blood

The late arterial phase, acquired approximately20 seconds after contrast initiation, leads to a cleardepiction of the hepatic artery and its branches,due to a distinctive contrast enhancement

CT Angiogram• Quickly becoming the test of choice for initial evaluation of a

suspected PE.• CT unlikely to miss any lesion.• CT has better sensitivity, specificity and can be used directly to

screen for PE.• CT can be used to follow up “non diagnostic V/Q scans.

Pulmonary angiogram

• Gold Standard.• Positive angiogram provides 100% certainty

that an obstruction exists in the pulmonary artery.

• Negative angiogram provides > 90% certainty in the exclusion of PE.

Optimization Of CT Scan Protocol In Acute Abdomen

Scan Protocols

• core of every CT examination.• protocols should be appropriate for the

clinical indication• should include all aspects of the exam such• positioning,• nursing instructions,• scan parameters( including radiation dose)• reconstruction/reformatting instructions,

Scanning parameters

• multislice CT is better than single slice• MSCT :• –High quality• –Wider range of examination• –Thinner slices• –Shorter scan time• –Multiphases protocol• –Better reconstruction ( isotropic voxel)

• Slice thickness: Acquire thins, reconstruct thick: Less noise

• Scan coverage: scan length• Rotation speed: Keep fastest…for most regions

to allow breath hold tech and more coverage

Increment

• is the distance between the reconstructed images in the Z direction.

• When the chosen increment is smaller than the slice thickness, the images are created with an overlap.

Increment

• is useful to reduce partial volume effect, giving you better detail of the anatomy and high quality 2D and 3D post-processing .

• can be freely adapted from 0.1 - 10 mm.

General Hints

• Topogram : AP, 512 or 768 mm.• Patient positioning: Patient lying in supine

position, arms positioned comfortably above the head in the head-arm rest lower legs supported.

• Patient respiratory instructions: inspiration• Scout : AP and lateral

General Hints

• Limit scan to intended anatomic area to cut dose by 10%

• –Abdomen:• Just above diaphragm – Inferior pubic symphysis• –Chest:• Routine: Apex to adrenals• PE or benign clinical reasons: Apex to lung bases

CT -HCC pre contrast

Arterial enhancement (central and early)

Washout on portal venousindicates fast flow

HCC Summary

• US - usually heterogeneous Usually HepB +ve with raised alpha FP

• CT – C- low density C+A – central early contrast (high flow rate) C+PV – washout cf with liver

– may have a capsule

• MR – intracellular fat on T1 out of phase - similar perfusion characteristics to CT

CT COLONOGRAPHY

DissectionStrip, anus to caecum

Endoluminal(for fun only)

800/40 windowAxial to loops

OrientationOverview

Advantages / disadvantages• Sensitivity and specificity is of the order of 90 % for 10

mm polyps.• Easy, quick and well tolerated.• Beats barium enema hands down.• Safer than optical colonoscopy • Approx. half the price of optical colonoscopy• No intervention possible as in optical Cy• At present for “Ba enema” indications, but is likely to

be used for screening in future.• Radiology manpower training required.• Radiation dose equivalent to Ba Enema

Incomplete air column -Excess fluid

Supine Prone

Can rotate image volume to view as a Ba enema in 3D

Diverticular disease

CT ENTEROCLYSIS

Jejunum often thick-walled

Can evaluate bowel wall due to negative contrast in lumen and IV contrast in wall.

Evaluates stomach well also

Plus standard CT

Reserved for older patients due to radiation dose

Renal Vasculature Evaluation Using A Multidetector CTScanner

The technique consists of image acquisition, imageprocessing and finally image display. As regards the image acquisition the following was our protocol: 100cc of iodinated contrast was injected at 2.5 ml/sec, using automated techniques e.g.: care bolus (for beginning of acquisition). Images that were obtained were of 1.25 mm slice thickness with 1mm slice collimation.

Scanning is done from the twelfth dorsal or the first lumbar vertebral level to the level of the pubic symphysis. After the arterialphase, a venous phase is followed using same imageacquisition parameters (60 cc after contrast). Furtherwhich a delayed acquisition (12/15 min after contrastinjection) is done with 5mm slice and 5mm collimation toimage the pelvicalyceal system, ureter and bladder. Nooral contrast is used. Acquired images were axiallyreconstructed with overlapping slices and transferred toan imaging workstation

MIP reconstruction is the technique of choice for imagepresentation because it is able to produce angiographylike images

REQUIREMENTS FOR CTA

• PATIENT PREPARATION• ACQUSITION PARAMETERS• CONTRAST MEDIUM ADMINISTRATION• POSTPROCESSING TECHNIQUES

PARAMETERS

• USUALLY ROUTINE CT PRECEDES A CTA EXAM. THE ROUTINE EXAM IS USED AS A REFERENCE SCAN HELPING TO DETERMING THE SCANNING RANGE IN CTA.

SLICE THICKNESS

• SLICE THICKNESS

• SPATIAL RESOLUTION

• CEREBRAL CTA

• ABDOMINAL CTA

• THORACIC CTA

• 1MM (LOWER mA)

• 3MM

• 3MM

SLICE THICKNESS

SPIRAL PITCH

• PITCH • SPATIAL RESOLUTION

TWO TECHNIQUES TO REDUCE MOTION ARTIFACTS IN CARDIAC CT

• PROSPECTIVE TRIGGERING

• RETROSPECTIVE GATING

3-D VISUALIZATION TOOLS IN CTA

• MPR• MIP• SSD• VR• CINE

Good Luck