CT

Structure of CT scan machine

Gantry with X-ray tube + row of detectors

Most common CT used

3rd generation CT scanner

Mechanism of CT scan machine

Tube rotation + patient translational movement

Planes in CT scan

• Axial / Transverse,

• Coronal,

• Sagittal

Pitch in CT scan

Parameter	Definition
Pitch	Table movement per rotation ÷ Slice thickness
Pitch ↑	↓ Image quality
Pitch ↑	↓ Radiation dose
Pitch = 1	Balanced quality and dose

Pitch is inversly proportional to image quality and dose

Types of CT scans

Type	Features
Single detector CT	Older technology
Multidetector CT (MDCT)	Higher speed, better 3D/volumetric acquisition
Helical / Spiral CT	Higher speed, lower radiation dose

What is hounsfield units (HU)?

Gives objective measurement of blackness/whiteness on CT

Chart of values of HU as per stucture

Structure	HU Value
Air	−1000
Fat	−10 to −100
Water	0
Soft tissue	20–30
Acute hemorrhage	60–90
Iodinated contrast	100–300
Bone / Calcium	+1000

HU value of air

-1000

HU value of fat

-10 to -100

HU value of water

0

HU value of soft tissue

20-30

HU value of acute hemorrhage

60-90

HU value of ionised contrast

100-300

HU value of bone/calcium

+1000

20-30 HU in CT scan is of

Soft tissue

-1000 HU in CT scan is of

Air

-10 to -100 HU in CT scan is of

Fat

0 HU in CT scan is of

Air

100-300 HU in CT scan is of

Ionised cotrast

60-90 HU in CT scan is of

Acute hemorrhage

+1000 HU in CT scan is of

Bone/calcium

Difference in NCCT and CECT

Parameter	NCCT	CECT
Contrast	Not used	Used
Hemorrhage	Best	Not preferred
Calcification	Best	Poor
Tumors	Limited	Excellent
Stroke (initial)	IOC	Not first line

NCCT is investigation of choice (IOC) for

1} For calcification -

• Intracranial calcification

• Renal and ureteric calculi

• Salivary stones

2} For bone cortex -

• bone cortex fractures except ‘stress fractures’ ( MRI done )

• Osteoid osteoma ( for other bone tumors MRI used)

3} For foreign body -

• 1st investigation - X-ray

• IOC - NCCT

• C/I - MRI

4} For acute hemorrhage -

• Head and spine trauma

• Acute SAH

• 1st investigation in stroke

5} For air -

• Pneumoperitoneum

• Pneumothorax

• Pneumomediastinum

• Pneumocephalus

• Intestinal obstruction

CECT is investigation of choice (IOC) for

• Lung tumors (except Pancoast)

• Renal tumors

• Pancreatic carcinoma (DPCT)

• Liver tumors (TPCT)

• Mediastinal masses (except posterior)

• Acute pancreatitis (48–72 hrs)

CT angiography is investigation of choice for

Condition
1} Aortic dissection
2} Aortic aneurysm
3} Pulmonary embolism
4} Mesenteric ischemia
5} Pulmonary sequestration

What can we see in HRCT?

HRCT is high resolution CT which evaluate ‘lung parenchyma’.

HRCT is IOC for

• Interstitial lung disease

• Bronchiectasis

• Covid 19 evaluation

• Paranasal sinus

• Temporal bone

What is VRT?

VRT is 3D volumetric reconstruction

Where we use Dual Energy CT (DECT)?

To see material decomposition

Where we can use DECT?

• Gout

• Renal/ureteric caliculi

• Perfusion map

How we do CT urography?

• first contrast is injected

• Then after 10-15 min. , we can get CT to visualize enal collecting system

Where we use CT perfusion?

• To check blood flow parameters

• Use to diagnose ‘Penumbra

CT for virtual bronchoscopy

• Non-invasive virtual endoscopy is performed

• For virtual colonoscopy - CO2 used

Limitations of virtual bronchoscopy

No biopsy, No intervention