referring to fig1 system 10 for thawing a bag 14 of frozen blood ( e . g ., glycerolized red blood cells ) 15 includes a portable thawing unit 12 within which bag 14 is supported . bag 14 is made of a microwave transmissive material , such as plastic , and holds approximately 680 ml of blood ( i . e ., one unit ). thawing unit 12 includes a housing 13 having a pair of hinged door panels 20 , 22 which together define an internal pressure infusion chamber 24 for receiving bag 14 . pressure infusion chamber 24 is formed within the interior and central portions of door panels 20 , 22 by a pair of inflatable bladders 26 , one in each door panel , that are connected by a hose 30 to a compressor 32 . positioned behind bladders 26 are antenna arrays 34 , 35 ( only array 34 is shown ), comprising waveguide heating elements 36 directed toward broad surfaces 14a , 14b of bag 14 . antenna arrays 34 , 35 receive microwave energy at a frequency of 2 . 45 ghz from an external microwave power source 18 ( available from microwave medical systems , littleton , mass .) connected to thawing unit 12 through a power cable 16 . housing 13 includes metal shielding for preventing microwave energy from escaping from the enclosure during operation of thawing unit 12 . thawing unit 12 is portable , and includes a handle 37 for easy transport . a computer 38 , operating in response to a stored program , controls the activation of power source 18 and compressor 32 and synchronizes their operation . as described in detail below , computer 38 activates compressor 32 to inflate bladders 26 to a selected pressure against bag 14 , and then triggers power source 18 to apply microwave thawing energy to arrays 34 , 35 . computer 38 also records and stores operational and statistical data , and includes a display screen 39 for displaying the monitored blood temperature and the time remaining in the thaw cycle . referring to fig2 a front view of antenna array 34 is shown . array 34 includes waveguide heating elements 36 arranged in two rows 41a , 41b of four elements each . ( antenna array 35 , supported within door panel 22 , has an identical arrangement .) each waveguide heating element 36 includes a transmit / receive antenna 40 having a rectangularly shaped aperture 42 facing broad surface 14a of bag 14 . heating elements 36 are spaced closely ( about 0 . 35 inches apart ) so that array 34 is relatively compact in size ( 8 . 6 &# 34 ;× 5 . 0 &# 34 ;). arrays 34 , 35 are positioned adjacent to only the central region of bag 14 so that the periphery of bag 14 is subjected to substantially less microwave energy than the central region of bag 14 . transmit / receive antennas 40 simultaneously transmit microwave warming energy to bag 14 for heating blood 15 and receive electromagnetic radiant energy from blood 15 , such as the emissivity ( i . e ., passive electromagnetic noise energy ) radiated by blood 15 . referring also to fig3 each heating element 36 of array 34 includes a variable gain amplifier 44 for applying microwave energy to transmit / receive antenna 40 , and a radiometer 46 for controlling the gain of amplifier 44 in response to the electromagnetic radiant energy received by transmit / receive antenna 44 . the emissivity of blood 15 increases as its temperature increases , and thus radiometers 46 generate gain control signals which cause the gains of amplifiers 44 to be reduced as the temperature of blood 15 rises during thawing . as a result , transmit / receive antennas 40 and radiometers 46 passively and non - invasively detect the emissivity of blood 15 , and accurately control the level of transmitted power as blood 15 gradually thaws from a solid frozen body , to a semi - liquid slushy condition , and finally to its liquid condition . among other advantages , this allows the power level applied by transmit / receive antennas 40 to blood 15 to be relatively high when the blood temperature is low ( e . g ., when blood 15 is frozen solid or is a &# 34 ; slush &# 34 ; of ice and fluid ), and then gradually decreased as blood 15 changes to its liquid state and is warmed . the gain of each amplifier 44 is variable between 0 db and 20 db in response to the control signal applied by the associated radiometer 46 . this allows the level of microwave energy radiated by each transmit / receive antenna 40 to be adjusted between 375 milliwatts and approximately 38 watts . as mentioned above , the transmitted microwave energy has a frequency of 2 . 45 ghz . radiometers 46 ( available from microwave medical systems , littleton , mass .) detect the radiant energy from blood 15 at a different frequency -- such as 4 . 0 ghz . as discussed below , transmit / receive antennas 40 are configured to support energy propagation over this frequency range , while isolating signals applied to radiometers 46 from the transmitted energy . in order to reduce the cost and complexity of system 10 , the gain control signals generated by radiometers 46 of heating elements 36 of antenna array 34 are applied via lines 47 to variable gain amplifiers 44 of corresponding heating elements 36 of array 35 . thus , a single set of radiometers 46 controls the microwave energy level transmitted by both antenna arrays 34 , 35 in response to the detected radiant energy . the microwave energy produced by source 18 is applied equally to antenna arrays 34 , 35 by 3 db coupler 60 . moreover , each antenna array 34 , 35 includes a microwave power distribution network 48 for equally dividing the applied microwave energy among the eight heating elements 36 of each array 34 , 35 . a 3 db coupler 58 at the input of distribution network 48 divides the applied microwave energy equally between rows 41a , 41b of heating elements 36 . and each row 41a , 41b includes three series - connected wilkinson couplers 50 , 52 , 54 with coupling values of 6 db , 4 . 8 db , and 3 . 0 db , respectively , for applying the microwave energy to inputs 56 of variable gain amplifiers 44 of heating elements 36 . the microwave signal generated by source 18 has a power level of approximately 3 watts . thus , it is seen that the coupling values discussed above result in approximately 380 milliwatts of microwave energy being provided to each variable gain amplifier 44 . a coaxial rotary joint 62 ( fig1 ), serving as one of the hinges between door panels 20 , 22 , is connected between coupler 60 and power distribution network 48 ( not shown ) of antenna array 35 . rotary joint 62 ( available from sage laboratories , natick , mass .) allows door panels 20 , 22 to pivot relative to each other without disrupting the electrical connections to array 35 . the maximum power level of 38 watts generated by each heating element 36 is based on the energy required to thaw a standard bag 14 holding 682 ml of glycerolized red blood cells 15 stored at a temperature of - 65 ° c . the following relationship is used to determine the required energy ( q ) under these circumstances : the specific gravity and specific heat of blood 15 are approximately those of water ( i . e ., 1 gm / cm 3 and 1 cal / gm ° c ., respectively ). the use of the above relationship for approximating the required power and time for thawing assumes that any changes in the loss tangent and density as the blood changes from solid to liquid are immaterial . substituting the above values of specific gravity and specific heat into the above formula , the energy ( q ) required to thaw the blood is 44 . 3 kilocalories . the electromagnetic power ( p ) needed to provide that amount of energy for a period of five minutes ( the desired thawing period ) is : ## equ1 ## dividing this power level equally among the sixteen individual waveguide heating elements 36 of arrays 34 , 35 results in a power level of 38 . 75 watts for each heating element 36 . referring to fig4 plot 70 shows the distribution of power from a pair of opposing antenna elements as a function of distance across the thickness of a bag 14 of 0 . 9 % saline , a solution whose microwave characteristics closely approximate those of fresh frozen plasma ( fpp ). the measurement was performed using a test fixture which supported bag 14 between the pair of opposing antennas , each providing approximately 38 watts of power at 2 . 45 ghz . curve 72 demonstrates that the level of power applied to the left broad surface 14b of bag 14 from the adjacent antenna decreases as the energy penetrates the saline , due to transmission loss through the saline . curve 74 provides a similar profile for the opposing antenna . curve 76 represents the sum of the power levels from each antenna at each depth position and shows that , with 38 watts transmitted from each antenna , approximately 30 watts of power is delivered to the centermost region of bag 14 , while about 43 watts are applied near surfaces 14a , 14b of bag 14 . the heating profile may be altered by offsetting the position of opposing antennas with respect to each other . referring to fig5 one of bladders 26 is shown . each bladder 26 is fabricated from a strong but expandable material , such as rubber . bladder includes a matrix of rounded pads or cushions 29 formed on a front surface 27 for ensuring good contact between bladder 26 and bag 14 , even when blood 15 is frozen within bag 14 in an irregular shape . a plate 31 , made of a microwave transmissive material , is positioned on the rear surface of bladder 26 to provide rigidity and support to bladder 26 when inflated . bladder 26 , when inflated , conforms to the shape of bag 14 and applies pressure to bag 14 for substantially maintaining the distribution of blood 15 within bag 14 during thawing . in other words , compression applied by bladder 26 helps prevent blood 15 ( whether thawed or frozen ) from settling into the bottom of bag 14 as the blood thaws . instead , the applied pressure moves the thawed liquid blood portions from the central heated region of bag 14 toward peripheral regions of bag 14 while maintaining a uniform distribution of blood 15 within bag 14 . bladder 26 is inflated with de - ionized water from compressor 32 , which has low transmission loss , a high dielectric constant , and provides a well - matched impedance to the array elements at microwave frequencies . moreover , de - ionized water is a coolant which draws heat away from blood near bag surfaces 14a , 14b , where the level of applied microwave energy is higher than in the interior of bag 14 , as shown in fig4 . providing cooling to surfaces 14a , 14b of bag 14 reduces the difference in temperature between those regions of blood 15 near surfaces 14a , 14b and in the interior of bag 14 . front surface 27 of bladder 26 may be fabricated or coated with a thermally conductive material for drawing heat away from surfaces 14a , 14b of bag 14 during thawing and into the coolant used to inflate bag 14 . referring to fig6 - 9 , a bag 14 of frozen blood 15 is thawed using the system described above in conjunction with fig1 as follows . as shown in fig6 frozen bag 14 of blood ( which typically has outer surfaces with a number of furrows and wrinkles ) is positioned within one side of pressure infusion chamber 24 . then , doors 20 , 22 are closed and locked so that arrays 34 , 35 face the broad surfaces 14a , 14b of bag 14 . note that arrays 34 , 35 are positioned adjacent the central region , rather than the periphery , of bag 14 . as shown by fig6 air spaces are typically present between bag 14 and bladders 26 due to the wrinkles on the surface of bag 14 . when triggered by a command from the user , computer 38 activates compressor 32 to begin inflating bladders 26 . bladders 26 are inflated to a preselected pressure level sufficient to cause both bladders 26 to fully contact broad surfaces 14a , 14b of bag 14 with no air spaces between bag 14 and bladders 26 bag 14 ( fig7 ) and to maintain the distribution of blood 15 within bag 14 during thawing . when the preselected pressure level is reached , computer 38 energizes microwave power source 18 , thereby causing microwave energy to be applied to heating elements 36 of arrays 34 , 35 . initially , the emissivity of blood 15 is relatively low , due to its low temperature . thus , the gains of amplifiers 44 are adjusted to their maximum levels by the control signals produced by radiometers 46 . as a result , the maximum level of microwave power ( e . g ., 38 watts ) is transmitted from each waveguide heating element 36 to heat that portion of frozen blood 15 located in the central region of bag 14 . as blood 15 begins to thaw , it first turns to slush and then very gradually becomes liquid . it is only when blood 15 begins to change to its liquid state that its temperature -- and thus its emissivity -- begins to rise appreciably . the increase in emissivity is detected by radiometers 46 , which respond by decreasing the gains of variable gain amplifier 44 thereby lowering the level of applied microwave energy to bag 14 . adjusting the gains of amplifiers 44 individually with control signals derived from the corresponding transmit / receive antenna 40 allows more ( or less ) microwave power to be applied to localized areas of blood 15 , as needed . the pressure applied by bladders 26 maintains a uniform distribution of blood 15 within bag 14 and causes thawed liquid portions 71 to be squeezed out of the central region of bag 14 to the periphery of bag 14 ( in the direction of arrows 70 of fig8 ), while retaining slush and frozen portions 73 of blood 15 in the central region . as a result , arrays 34 , 35 can continue to direct thawing energy where it is needed -- to slush / frozen portions 73 -- while sparing liquid portions 71 from excessive heating . inflated bladders 26 are in continuous contact with bag 14 and pressure is maintained so that bag 14 has a substantially uniform thickness throughout the thawing operation . bag 14 is centrally and vertically positioned between antenna arrays 34 , 35 to allow any air within bag 14 to rise to its top end . when the monitored temperature indicates that blood 15 has completely thawed , computer 38 deactivates source 18 , and microwave power to antenna arrays 34 , 35 is removed ( fig9 ). upon opening doors 20 , 22 bladders 26 are automatically deflated . referring to fig1 - 12 , transmit / receive antenna 40 includes a rectangular receive waveguide 82 coupled to a rectangular transmit waveguide 80 through an opening 81 in one wall 81a of transmit waveguide 80 . transmit and receive waveguides 80 , 82 share a common axis 83 and a common aperture 42 ( the open face of waveguide 80 ). transmit and receive waveguides 80 , 82 have dimensions ( width and height ) selected so that the 4 . 0 ghz radiant energy received by radiometer 46 is isolated from the transmit microwave energy at 2 . 45 ghz . for the te 10 mode of transmission ( the dominant mode of transmission in rectangular waveguide ), a rectangular waveguide acts as a high pass filter with a cutoff frequency ( f c ) determined by the following relationship : ## equ2 ## where c = velocity of light in free space ; d = the dimension of the waveguide transverse to the orientation of the electric field for the the signal of interest ; and the te xy ( transverse electric ) designation means that for this mode of propagation , the direction of the electric field is transverse to the direction of propagation ( the subscripted numerals indicate the number of half sine wave variations of the electric field components in the x and y directions , respectively ). thus , for the te 10 mode , only one half sine wave variation propagates in the x direction with none propagating in the y direction . referring to fig1 , a dimension ( a 1 ) of transmit waveguide 80 is substituted in the above relationship to provide a cutoff frequency for transmit waveguide 80 below the frequency of the transmitted microwave energy ( 2 . 45 ghz ). thus , transmit waveguide 80 propagates the transmit microwave energy at 2 . 45 ghz to blood 15 and also supports the propagation of the radiant energy from blood 15 at 4 . 0 ghz . on the other hand , dimension ( a 2 ) of receive waveguide 82 is selected to provide a cutoff frequency above the 2 . 45 ghz transmit energy , but below the 4 . 0 ghz radiant energy so that radiant energy at 4 . 0 ghz propagates , and the transmit energy at 2 . 45 ghz is attenuated , within receive waveguide 82 . receive waveguide 82 is oriented orthogonally with respect to transmit waveguide 80 . in other words , receive waveguide 82 is rotated 90 ° with respect to transmit waveguide 80 so that dimension ( a 1 ) of transmit waveguide 80 is perpendicular to dimension ( a 2 ) of receive waveguide 82 . in this embodiment , dimension ( a 2 ) of receive waveguide 82 is selected to be equivalent to the height of transmit waveguide 80 . with this configuration , the orientation of the te 10 mode electric field of the 2 . 45 ghz transmit energy propagating in transmit waveguide 80 is orthogonal to the orientation of the te 10 mode electric field of the 4 . 0 ghz radiant energy propagating in receive waveguide 82 . this orthogonal configuration of waveguides 80 , 82 provides further isolation between the transmit energy at 2 . 45 ghz and the received radiant energy at 4 . 0 ghz . in addition , the larger aperture of transmit waveguide 80 serves as an antenna horn for the radiant energy at 4 . 0 ghz propagating to receive waveguide 82 . dimension ( b 1 ), which defines the width of opening 81 , is also selected to provide a cutoff frequency above 4 . 9 ghz , the second harmonic of the transmit energy at 2 . 45 ghz , so that the second harmonic is substantially attenuated within receive waveguide 82 . dimension ( b 1 ) also limits propagation of both the transmit energy ( 2 . 45 ghz ) and radiant energy ( 4 . 0 ghz ) oriented transverse to radiant energy propagating in receive waveguide in the te 10 mode , ( i . e ., the te 01 mode ). a transmit port 86 , in the form of a coaxial connector , is mounted on a wall 88 of transmit waveguide 80 , and includes a probe 90 extending vertically into waveguide 80 for applying the microwave transmitted energy within transmit waveguide 80 in the te 10 mode . a receive port 92 ( also in the form of a coaxial connector ) is mounted to a side wall 84 of orthogonally coupled receive waveguide 82 , and includes a probe 94 that extends horizontally into waveguide 82 to receive the radiant energy in the te 10 mode . probes 90 , 94 are both positioned a distance of about one quarter wavelength , at the transmit and receive frequencies , from walls 81a , 85 , respectively , so that the transmitted and radiant energy is maximally coupled between transmit and receive ports 86 , 92 and respective waveguides 80 , 82 . it is important to note that the amplitude of the transmitted microwave signal is generally several orders of magnitude higher than the received radiant energy , and a significant amount of attenuation is required to reduce the transmitted signal to a level below that of the radiant energy . although much of the attenuation is provided by the frequency cutoff effect associated with the width of opening 81 , the orthogonal arrangement of waveguides 80 , 82 provides additional isolation between transmit port 86 and receive port 92 and , as a result , the sensitivity of radiometer 46 in receiving the radiant energy is increased . the interior regions of both waveguides 80 , 82 are filled with a dielectric material having a relatively high dielectric constant ( e . g ., ε r = 2 . 55 ) for allowing the dimensions of waveguides 80 , 82 to be decreased relative to the dimensions that would be needed if air were to be used as the dielectric . the result is a highly more compact ( i . e ., low profile ) antenna 40 . dielectrically loading waveguide 80 also provides a smoother surface for contacting inflation bladders 26 . for example , as shown in fig1 , output power control of heating array elements 36 may be achieved through the measurement of reflection coefficient , rather than emissivity . with this approach , the dual mode frequency operation ( i . e ., transmitting at one frequency and receiving at a different frequency ) as described above need not be used . instead , the level of transmitted warming energy from variable gain amplifier 44 is based on the amount of the transmitted energy reflected from blood 15 during the thawing process . in this embodiment , a waveguide directional coupler 100 is used to transmit the energy from amplifier 44 to bag 14 and to couple a relative portion of the reflected energy ( at the same frequency as the transmitted energy ) to a microwave detector 102 . the ratio of the reflected power to the power incident on the bag , commonly referred to as the reflection coefficient , changes dramatically as the impedance of blood 15 changes during thawing . with this approach , the waveguide impedance of coupler 100 is initially matched to the intrinsic impedance of blood 15 in its frozen state . as blood 15 begins to thaw , the impedance becomes slightly mismatched , resulting in an increase in reflected energy ; the impedance mismatch becomes significant as blood 15 approaches its liquid state , and thus the level of the reflected energy rises markedly . the reflected energy is rectified by detector 102 and applied to a signal conditioning circuit 104 for generating a gain control signal for amplifier 44 which is inversely proportional to the reflected energy level . that is , the gain of amplifier 44 is decreased as the reflected energy level increases . a three - port ferrite isolator 106 connected between amplifier 44 and the throughput arm of coupler 100 absorbs reflected energy that leaks through coupler 100 . isolator 106 also isolates amplifier 44 from the changing impedance of blood 15 . one important consideration is that the change in reflection coefficient as blood thaws is offset , to some extent , by the fact that the density or specific gravity of the blood becomes smaller as it thaws , resulting in a correspondingly greater amount of energy per degree of temperature change as the blood thaws from a solid to a liquid . referring to fig1 , variable gain amplifier 44 may be replaced with a diode switch modulator 108 , triggered by the control signal produced by radiometer 46 or detector 102 . modulator 108 ( available from microwave medical systems , littleton , mass .) applies the microwave energy from distribution network 48 to antenna element 40 as a series of pulses . modulator 108 adjusts the duty cycle of the pulses ( i . e ., the ratio between the width of the pulses and the time between pulses ) in response to the level of the control signal to modulate the transmitted power level . during the period when modulator 108 is &# 34 ; off &# 34 ; the input power provided by a high power magnetron 107 is reflected back into distribution network 48 where it is absorbed by the terminated ports of couplers 50 , 52 , 54 ( fig3 ). to ensure that each heating element 36 &# 39 ; coupled to distribution network 48 is capable of delivering the necessary 38 watts at 2 . 45 ghz , high power magnetron 107 is required to provide approximately 600 watts of continuous wave power . a magnetron and power supply used in a conventional microwave oven can be modified for use with this approach . in still other embodiments , a vibration mechanism may also be coupled to pressure infusion chamber 24 to agitate and encourage mixing of the blood during thawing . it is generally desirable to maintain the displacement and frequency of the vibration mechanism ( e . g ., less than 1 / 8 &# 34 ; displacement at 400 hz ) below noticeable noise levels . other configurations are also within the scope of the claims . for example , the number of heating elements and the array pattern ( e . g ., the positions of and the spacing between the array elements ) may be varied to provide a desired heating profile . the size and geometry of transmit and receive waveguides 80 , 82 may also be changed to operate at different frequencies within , for example , the microwave range of 300 mhz to 3 . 0 ghz , or in different waveguide modes . the waveguide components including transmit / receive antenna 40 and the couplers of distribution network 48 can also be replaced with ridge waveguide , integrated finline structures or planar printed circuits ( e . g ., suspended substrate circuits ) which will reduce the size and , in some cases , the cost of the thawing apparatus . inflatable bladders 26 may be inflated with a gas , liquid , or powder to compress bag 14 within pressure infusion chamber 24 . it is desirable that the medium used to inflate bladders 26 have a low transmission loss at both the transmit and receive frequencies of operation so that energy applied from or radiated to the antenna arrays 34 , 35 is not absorbed . moreover , the dielectric constant of the medium should be approximately that of blood 15 so that its intrinsic impedance is closely matched to the impedance of the heating elements 36 . referring to fig1 , inflatable bladders 26 may also include a compression plate 110 positioned over front surface 27 and fabricated from a thermally conductive material ( e . g ., beo ceramic ). compression plate 110 provides better cooling of surfaces 14a , 14b of bag 14 and , thus , a more uniform temperature distribution of blood 15 between surfaces 14a , 14b . in certain applications , the heating elements may time - share a common radiometer 46 . on the other hand , to provide greater individual control , heating elements 36 of array 35 may include their own radiometers 46 . a bandpass filter may be used with radiometer 46 to attenuate outside of the band of the radiometer , which includes signals at the transmitted frequency . the system for thawing the red blood cells is equally applicable for thawing other types of blood products , including fresh frozen plasma and frozen platelets . plasma and platelets , unlike red blood cells , do not require the addition of glycerol , and therefore can be stored at higher temperatures (- 18 ° c .). for these types of blood products , a smaller , lower power , thawing unit may be used , perhaps having only a single heating array positioned to provide heating from only one side of the bag of blood product .