Tx Machines I

Grenz ray therapy energy and depth

very soft x-rays (below 20 Kv)

very low depth of penetration

Grenz ray is used to treat

inflammatory disorders, mycosis fungoides

no longer used in RT

contact therapy energy

x-ray produced at potential from 40 to 50 Kv

beam completely absorbed within 2 cm of soft tissue

contact therapy is used to treat? and why>

superficial skin lesions and superficial rectal tumors

rapidly decreasing depth dose in tissue, so skin is irradiated but underlying tissues are spared with depth

contact therapy is not used for

not good for tumors deeper than 1-2 mm

what is used with contact therapy

0.5-1 mm thick aluminum filter placed in the beam to absorb soft component of the energy spectrum

applicators come in contact with the patient for endocavitary treatments to produce a SSD of 2 cm or less

superficial therapy energy and what is treated

x-rays with energy ranging from 50-150 kVp

skin cancer or tumors no deeper than 5 mm (90% DD at 5 mm) are treated

accessories for superficial therapy

1-6 mm aluminum to harden the beam

applicator or cone to maintain SSD between 15-20 cm

orthovoltage (deep therapy) energy

150-500 Kv

orthovoltage deep is used to treat

skin, mouth, cervical (cones inserted into patient)

orthovoltage where does max dose occur

max dose occurs close to skin

90% DD at 2 cm

orthovoltage what can be used 1

by increasing filtration and combining 2 or more beams from different directions, high dose can be delivered to deeper tumors

orthovoltage therapy filters

filters to achieve HVL between 1 and 4 mm Cu

orthovoltage SSD is usually

set at 50 cm

orthovoltage therapy limitation

skin dose

increased absorbed dose in bone (for tumors behind bone)

supervoltage therapy energy

500 Kv to 1 Mv

KV unit problem

insulating the high voltage transformer

efficiency for xray tube

1%, the other 99% goes to heat

x-ray tube target

reflection target

what is the anode and what does it consist of

positive electrode

thicker copper rod at the end of which is a tungsten target

anode (positive electrode)- diagnostic vs therapy

diagnostic: anode rotates to reduce heat

therapy: the focal spot larger and anode not rotating

cathode what is it and what does it consist of

negative electrode

wire filament, current providing current, negatively charged focusing cup

wire filament in cathode

tungsten filament when heated emits electrons by thermionic emission

cathode cup function

negative charged and directs electrons to anode so they hit target in a well defined area called the focal spot

target-what is used

tungsten is used because of high z and high melting point

what is the focal spot

target region where xrays are emitted from

size of the focal spot

small for diagnostic so images are sharp

large for therapy xray tube

removal of heat

circulate oil, air, or water or rotate the anode in diagnostic xray

target angle and focal spot

a=A sin O

a=apparent focal spot, focal spot size seen on film

A=actual focal spot size

heel effect

xray beam not uniform, more xrays are absorbed in the back end of the target

xray tube efficiency defined

ratio of output energy emitted as xrays to input energy deposited by electrons

Eff= 9 × 10^-10 ZV

 What happens to the rest of energy?  Efficiency depends on what quantities

variation of line voltage with time

since the anode is positive only during half of the voltage cycle, the tube current flows through that half of the cycle

what is a self rectified unit

during the next half cycle, voltage is reversed and current cannot flow in the opp direction

tube current and xray are only made in the next half cycle when the anode is positive-called a self rectified unit

self rectifation

during the inverse cycle, electrons flow from anode to cathode bombarding it and destroying the filament

half wave rectification

voltage rectifiers can solve the problem of tube conduction during the inverse voltage

half wave rectification-what do rectifiers do

rectifiers prevent tube from conducting during inverse voltage cycle

current flows as usual during the cycle when the anode is positive relative to the cathode

full wave rectification

cathode negative, anode positive during both cycle

full wave rectification described

four rectifiers are arranged in the high voltage part of the circuit so that the cathode is negative and anode is positive during both half cycles of voltage

effect of full wave rectification

effective tube current is higher since current flows in both half cycles

beam quality

quality in the range of kvp is specified as energy (kvp) and HVL

HVL

absorber thickness needed to reduce radiation intensity to half the OG value

effect energy of xray tube

energy of the monoenergetic beam that would give the same HVL as the xray beam in question

average energy

average xray energy is about 1/3 the max energy or kvp

the unfiltered xray spectrum shows

continuous distribution for BR

superimposed by characterisitc radiation of discrete energies

added filtration

enriches the beam with higher energy photons by absorbing lower energy components

what does the thoraeus filter contain and how are they oriented

Sn, Cu, Al

Sn faces the xray tube

aluminum faces the patient

Cu sandwiched between

what does the thor filter do

increases HVL of orthovoltage beam without reducing beam intensity to unacceptably low value

diagnostic and superficial xray energy range filters

primarily aluminum (mm Al)

orthovoltage range filter

1-4 mm Cu

cesium and cobalt therapy machine filter

none (monoenergetic)

MV xray beam filters

inherent filtration of transmission target

flattening filter