CT Imaging

Jermendy Ádám, Lénárd Zsuzsanna, Magyar Péter

Semmelweis University, Dpt. Medical Imaging

• use of X-rays

• attenuation coefficients

What does CT measure? DENSITY

Structure of a CT machineX-ray source detector panel

table is performing longitudinal movement, with the patient lying on it.

Helical scanning:Table movement and imaging is simultaneous.Higher radiation dose.

Sequential scanning:An axial slice is made and then the table moves.

lung HRCT, head CT, intervention applicationsProspective ECG gating at cardio CT

The majority of CT examination

MultiSlice Computer Tomography

multiple slices simultaneously.Better spatial and temporal resolution, lower radiation dose

The Present I.: 256- and 320-detector-row MSCT

In 256- and 320-detector row CT, the entire heart is imaged with temporal uniformity (ie. at the same time point without temporal delay from the base to apex).

It is able to scan an entire heart in one gantry rotation of 300 ms.

High-Risk Plaque Regression and Stabilization: Hybrid Noninvasive Morphological and Hemodynamic Assessment.Giannopoulos AA, Mitsouras D, Bartykowszki A, Merkely B, Chatzizisis YS, Buechel RR, Kaufmann PA, GaemperliO, Maurovich-Horvat P.

The Present II.: Dual-source CTallows differentiation of materials and tissues based on CT density values derived from two synchronous CT acquisitions at different tube potentials (140 kVp and 80 kVp, usually)

image is created based on two different attenuation datasets

can now be used routinely in abdominal imaging

provides evaluation of e.g. renal masses, liver lesions, urinary calculi, small bowel, pancreas, and adrenal glands

allows differentiation of contrast agent from calcifying thrombus in patients with endovascular stents

Graser et al., Dual energy CT: Preliminary observations and potential clinical applications in the abdomen. European Radiology 2008, 19(1):13-23

140 kVp

80 kVp

What is white on CT image? What is dark on CT image?

electron-dense structures, such as metal and bone, stop a large number of x-rays and therefore are bright on computed tomography (CT)

hyperdens

lower electron-density regions, such as air or fat, stop very few x-rays and are rendered as dark

hypodens

Large hard candy. Contrast-enhanced axial CT demonstrates a perfectly round, attenuated mass in the left lower gingivo-buccal sulcus. MecDermott et al, American Journal of Neuroradiology September 2008, 29 (8) 1552-1555;

The Hounsfield-units (HU)

• The attenuation coefficient

Hounsfield-units

water

grey scale.

Windowing (postprocessing)

window

level width

level: -600, width: 1500level: 400, width: 1800level: 50, width: 400

www.radiopaedia.org

CT Contrast Agents I: intravenous administration

Noncontrast CT : foreign bodykidney stonelung HRCT / interstitialis pulm. diseaseto exclude hemorrhage in strokeadrenal gland adenomabone structure (e.g. in trauma)fat/ lipoma / steatosisatherosclerosis (e.g. calcium score)

Contrast-enhanced CT : angiographyto study vascularization (e.g. ischemia, infarction)tumor differentiationinflammatory processes, abscesses

noncontrast phase

www.radiologyassistant.nl: CT contrast injections and protocols

www.radiologyassistant.nl: CT contrast injections and protocols

early arterial phase

www.radiologyassistant.nl: CT contrast injections and protocols

late arterial phase

www.radiologyassistant.nl: CT contrast injections and protocols

hepatic phase

www.radiologyassistant.nl: CT contrast injections and protocols

nephrogenic phase

www.radiologyassistant.nl: CT contrast injections and protocols

late phase

artériás, 15-20 sec

vénás, 70-80 sec késői, 3-10 min

natív

CT Contrast Agents I: intravenous administration

iodinated

non-contrast images: e.g.

orally or at given cases e.g. directly into fistulas

contrast material leaked from the small bowel into the laparotomy wound

CT Contrast Agents II: oral administration

jódtartalmú kontrasztanyagoknak nefrotoxikus hatása van

Post-contrast acute kidney injury (PC-AKI):

(GFR) < 30 ml/min/1.73 m2

kidney function GFR > 30 GFR < 30

ESUR guidelines 10.0 – renal adverse reaction

Role of GFR Emergency situation:

Non-vital indication:

ESUR guidelines 10.0 – non-renal adverse reactionsacute: allergic reactions from mild urticaria to anaphylactic schock

patients at risk: pusage of high-osmolality ionic contrast media

late: skin reactions, headache

very late: thyreotoxicosis patients at risk:

miscallenous:

iodine- based contrast media may be given to the pregnant female

breast feeding may be continued normally

pheochromocytoma and paraganglioma

Note: allergic reaktions to iodine basedsuperficial antiseptics are NOT equal tocontrast allergies.

Metformin, a medicine in type II diabetes

danger for lactate acidosis

patients with impaired renal function are at risk

patients with an eGFR above 45 ml/min/1.73 m2 can continue to take Metformin normally

patients with an eGFR below 45 ml/min/1.73 m2should stop Metformin 48 h before contrast administration

patients with an eGFR below 30 ml/min/1.73 m2 contrast media should be avoided

in emergency patients Metformin should be stopped and after contrast administration the patient should be carefully monitored for the signs of lactate acidosis

Emergency exam (vital indication):

• Consider an alternative imaging method not using iodine-based contrast media.

• Start volume expansion as early as possible before contrast medium administration (see elective examination).

CT can be used in a wide variety• Acute abdominal and chest diseases• Acute cerebral diseases• Oncology imaging, staging exams• Lung disease assessment (HRCT)• CT angiography (CTA)• Traumatology• Cardio CT• Gastrointestinal exams• ...

H R CT

Fields:

csontablak

csontablak

tüdőablak

acute chronic

50-80 HU 0 HU

larger hematomas can be detected on noncontrast CT

small punctuate bleeding can be depicted on MR with higher sensitivity

dense, metal foreign bodies can be detected on noncontrast CT

MR is contraindicated in this clinical situation

density: approx. 1550 HU

bowel infarction, no enhancement

abrupt filling defect in mesenteric superior artery on CT angiography

cyst no enhancement renal cell carcinoma intensive enhancement

sclerotic plaquesNormal coronary

retrospective ECG gating:

prospective ECG gating:

CT enteroclysis / enterography

Indications:

negative oral contrast agent

CT-colonography / virtual colonoscopy – colon carcinoma

Indications:

Pelvic Imaging?MRI is superior to CT in both male and female pelvic soft tissue imaging

axial T2w MRI axial noncontrast CT

Strengths and Weaknesses of CT

high ionizing dose

high cost of equipment

imaging in axial plane (with sagittal and

coronal reconstructions)

its soft tissue resolution is inferior to MRI’s

bony arterfacts (e.g. posterior fossa)

primarily axial images

contraindications (contrast allergy, kidney

failure, pregnancy, youngsters)

3D imaging

tissues are not superimposed, not magnified

good spatial resolution

good soft tissue resolution (primarily after iv.

contrast administration)

usually available

rapid imaging (more regions in some minutes)

can image calcified plaques, small kidney

stones, foreign bodies, bone fragments

can image lung tissue